Parasite 2015, 22, 10
J. Lorenzo-Morales et al., published by EDP Sciences, 2015
DOI: 10.1051/parasite/2015010
REVIEW ARTICLE
Available online at:
www.parasite-journal.org
OPEN
ACCESS
An update on Acanthamoeba keratitis: diagnosis, pathogenesis
and treatment
Jacob Lorenzo-Morales1,a,*, Naveed A. Khan2,a, and Julia Walochnik3,a
1
2
3
University Institute of Tropical Diseases and Public Health of the Canary Islands, University of La Laguna, Avda. Astrofísico Fco.
Sánchez, S/N, 38203 La Laguna, Tenerife, Canary Islands, Spain
Department of Biological and Biomedical Sciences, Aga Khan University, Karachi, Pakistan
Institute of Specific Prophylaxis and Tropical Medicine, Medical University of Vienna, Vienna, Austria
Received 30 October 2014, Accepted 9 February 2015, Published online 18 February 2015
Abstract – Free-living amoebae of the genus Acanthamoeba are causal agents of a severe sight-threatening infection
of the cornea known as Acanthamoeba keratitis. Moreover, the number of reported cases worldwide is increasing year
after year, mostly in contact lens wearers, although cases have also been reported in non-contact lens wearers. Interestingly, Acanthamoeba keratitis has remained significant, despite our advances in antimicrobial chemotherapy and
supportive care. In part, this is due to an incomplete understanding of the pathogenesis and pathophysiology of the
disease, diagnostic delays and problems associated with chemotherapeutic interventions. In view of the devastating
nature of this disease, here we present our current understanding of Acanthamoeba keratitis and molecular mechanisms associated with the disease, as well as virulence traits of Acanthamoeba that may be potential targets for
improved diagnosis, therapeutic interventions and/or for the development of preventative measures. Novel molecular
approaches such as proteomics, RNAi and a consensus in the diagnostic approaches for a suspected case of Acanthamoeba keratitis are proposed and reviewed based on data which have been compiled after years of working on this
amoebic organism using many different techniques and listening to many experts in this field at conferences,
workshops and international meetings. Altogether, this review may serve as the milestone for developing an effective
solution for the prevention, control and treatment of Acanthamoeba infections.
Key words: Acanthamoeba, keratitis, diagnosis, therapy, pathogenesis.
Résumé – Mise au point sur la kératite à Acanthamoeba : diagnostic, pathogenèse et traitement. Les amibes à
vie libre du genre Acanthamoeba sont les agents causant une infection sévère de la cornée, dangereuse pour la vue,
appelée kératite à Acanthamoeba. De plus, le nombre de cas signalés à travers le monde est en augmentation année
après année, principalement chez les porteurs de lentilles de contact, bien que des cas de kératite à Acanthamoeba
aient également été signalés chez les non-porteurs de lentilles. Fait intéressant, la kératite à Acanthamoeba est
restée significative, en dépit de nos progrès dans la chimiothérapie antimicrobienne et les soins de soutien.
En partie, cela est dû à une compréhension incomplète de la pathogenèse et la physiopathologie de la maladie,
aux retards du diagnostic et aux problèmes associés aux interventions chimiothérapeutiques. Compte tenu de la
nature dévastatrice de cette maladie, nous présentons ici notre compréhension actuelle de la kératite à
Acanthamoeba et des mécanismes moléculaires associés à la maladie, ainsi que les traits de virulence de
Acanthamoeba qui peuvent être des cibles potentielles pour l’amélioration du diagnostic, les interventions
thérapeutiques et/ou pour l’élaboration de mesures préventives. Des approches moléculaires comme la
protéomique, l’ARNi et des approches consensuelles de diagnostic pour un cas suspecté de kératite à
Acanthamoeba sont proposées et examinées sur la base des données qui ont été compilées après des années de
travail sur cet organisme amibien, utilisant de nombreuses techniques différentes et l’écoute de nombreux experts
sur ce domaine à des conférences, ateliers et réunions internationales. Au total, cette étude peut servir de jalon
pour développer une solution efficace pour la prévention, le contrôle et le traitement des infections à Acanthamoeba.
a
All authors contributed equally to this manuscript.
*Corresponding author: jmlorenz@ull.edu.es
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
1. Introduction – What is Acanthamoeba
keratitis?
Acanthamoeba species are the causative agents of a sightthreatening infection of the cornea known as Acanthamoeba
keratitis (AK) (Fig. 1). Interestingly, AK is increasingly being
recognized as a severe sight-threatening ocular infection,
worldwide. Although contact lens (CL) wear is the leading risk
factor for AK, Acanthamoeba spp. can cause infection in noncontact lens wearers. Patients with AK may experience pain
with photophobia, ring-like stromal infiltrate, epithelial defect
and lid oedema. If AK is not treated adequately and aggressively, it can lead to loss of vision [18, 46, 47, 56, 87, 111,
112, 117].
Diagnosis of AK is challenging, and the available treatments are lengthy and not fully effective against all strains.
Moreover, the pathogenesis of Acanthamoeba keratitis is still
under study, and the identification of the key factors involved
in this process should be useful for the development of fully
effective therapies. The current difficulty in effective treatment
is due to the resistant cyst stage of Acanthamoeba. Together
with common misdiagnosis of AK in most cases and a lack
of a consensus for AK diagnosis, AK has remained significant.
Nevertheless, AK is still considered a rare disease and is
included in the Orphanet database (ORPHA67043) and with
an estimated prevalence of 1–9/100,000.
2. Diagnostics of AK
The most important step in AK diagnosis is to think of it.
Generally, AK should be considered in all contact lens wearers
and in any case of corneal trauma with exposure to soil or contaminated water [20, 23, 36, 54]. Common symptoms are massive pain, photophobia and tearing. The sooner the disease is
diagnosed, the better the outcome [6, 12, 54, 99, 104]. If diagnosis is delayed, the amoebae have already penetrated deeply
into the corneal stroma and successful therapy becomes
exceedingly difficult. AK is usually unilateral and progresses
slowly, from epithelial to stromal disease. At the beginning
of the infection, a diffuse superficial keratopathy is found, later
multifocal infiltrates are almost always observed in the stroma.
Acanthamoeba sclerokeratitis is an uncommon complication of
AK and assumedly has an immune-mediated origin. Tu et al.
[104] established five levels of AK severity based on slit-lamp
biomicroscopy findings: epitheliitis, epitheliitis with radial
neuritis, anterior stromal disease, deep stromal keratitis, or ring
infiltrate. The characteristic ring infiltrate is, however, only
seen in approximately 50% of patients. In the early stage,
AK can easily be confused with Herpes simplex keratitis, while
in the advanced stage, the infection resembles the clinical picture of a fungal keratitis or a corneal ulcer (Table 1).
Contact lens wearers typically seek medical help late,
because they are used to minor irritations in the eye.
The tentative diagnosis of AK can often be made by in vivo
confocal microscopy (IVCM). The Acanthamoeba cysts
appearing as hyper-reflective, spherical structures are usually
well defined because of their double wall; the trophozoites
are difficult to distinguish from leukocytes and keratocyte
Figure 1. (A) Corneal melting and vascularization in a patient with
Acanthamoeba keratitis. (B) Observed corneal damage in AK is
shown after sodium fluorescein application. Original.
nuclei [110]. However, the direct detection of the causative
agent in a corneal scrape specimen is the only reliable diagnostic method for AK. Culture remains the gold standard of Acanthamoeba laboratory diagnosis, but today several PCR-based
techniques are also well established and usually increase sensitivity significantly [41, 59, 84, 90]. In cases of severe infection,
amoeba density is sometimes very high and the amoebae can
already be detected by direct microscopy of the clinical sample, without enrichment. Acanthamoeba trophozoites or cysts
are readily recognizable in phase contrast microscopy, but also
stain well in several stains and cysts exhibit auto-fluorescence
[46, 54]. However, particularly if patients have already been
pre-treated with antibiotics, amoeba density is usually very
low. Moreover, amoebae exhibit altered morphologies – in
these cases, even culture often remains negative and molecular
techniques are indispensable. Reliable identification below the
genus level requires genotyping. Serological techniques are of
no diagnostic value as specific antibodies are also detected in
apparently healthy people due to the ubiquity of
Acanthamoeba.
In contrast to infections with other amoebae, acanthamoebae can form cysts within the tissue. As a single cyst surviving
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
Table 1. Important characteristics for the differential diagnosis of Acanthamoeba keratitis (AK) compared to keratitis due to other infectious
agents.
Specific characteristics of AK
Pseudo-dendritiform epitheliopathy, epithelium defects without terminal knots, perineural
infiltrates, [ring infiltrate]*, endothelium is not involved
Usually restricted to cornea, absence of anterior chamber activity, stromal infiltrates are usually
multifocal (not monofocal), [ring infiltrate]*
Usually restricted to cornea, clear epithelium defects, perineural stromal infiltrates, [ring infiltrate]*
*
When compared to
Herpes simplex keratitis
Bacterial keratitis
Fungal keratitis
The characteristic ring infiltrate is only seen in the advanced stage and even then only in 50% of patients.
in the cornea can lead to reinfection, the progress of therapy
should be checked regularly. An ongoing infection should be
monitored every 1–2 weeks. After clinical recovery, monthly
checks are sufficient, ideally until 6 months after decline of
symptoms.
In most countries, the vast majority of AK cases occur in
CL wearers and AK can be prevented extensively by strict contact lens hygiene. Typically, singular amoebae gain access to
the lens case via tap water or the air, rapidly grow to high densities within the lens case if this is not cleaned properly and
regularly, and then attach to the lenses and infect the eye
[116]. Wearers of soft contact lenses using multipurpose solutions are at particular risk, because acanthamoebae adhere
especially well to the hydrophilic plastic of these lenses, and
soft lenses are more difficult to clean than rigid lenses. Moreover, soft lenses are often over worn (dailies used for several
days, monthlies used for several months) and are also the type
of lenses used by people who do not regularly but only occasionally (e.g. once a week for sports) wear their contacts,
and who are often unaware of proper contact lens hygiene.
For prophylaxis of AK, lens cases should be cleaned manually
and air dried, contact lenses should be cleaned and stored using
an appropriate (best: two-step) contact lens cleaning system,
and both, lenses and lens cases have to be exchanged regularly.
2.1. Material
For confirmation of an AK, sampling and investigation of
the correct material is crucial. Only if amoebae are detected
in corneal scrapings or in corneal biopsies a reliable diagnosis
can be made. Acanthamoebae penetrate the cornea and are
usually not found on the corneal surface, thus superficial swab
samples or tear samples often remain negative, particularly in
the advanced stage of the disease and/or if patients have
already been pretreated with antibiotics. On the contrary, contact lens containers, even those of entirely healthy CL wearers,
are almost always positive for acanthamoebae, at least in PCR.
This means that the detection of Acanthamoeba spp. in the CL
case does not necessarily indicate an AK. When the CL case is
negative, however, it is very unlikely that the patient has an
AK, unless, of course, the CL case was recently changed.
The optimal material for AK diagnosis is a corneal
scraping/biopsy stored in 200 lL of sterile saline (amoeba
saline* or PBS or 0.9% NaCl) in order to prevent
desiccation.
*See Table 2.
Table 2. Neff’s Amoeba Saline (AS) [71]. 10 mL of each stock
solution (10·) are added to 950 mL dH2O, mixed, sterilized by
filtration and aliquoted into needed volumes.
Stocks (10·)
NaCl
MgSO4-7H2O
CaCl2 Æ 2H2O
Na2HPO4
KH2PO4
Grams per 100 mL ddH2O
1.20
0.04
0.04
1.42
1.36
2.2. Sample preparation
A major challenge in AK diagnostics is the many different
types of sample material on the one hand, and sample transport
media and containers on the other. Below, we have attempted
to provide a guideline for sample preparation depending on
the type of material received. A general overview of the diagnostic procedure is given in Figure 2.
When a corneal scraping/biopsy is received, the sample
itself is used for DNA isolation, while the transport medium
(ideally 200 lL of sterile saline) is used for culture. Larger
tissue samples can be cut into two halves, of which one can
be transferred onto an agar plate and the other used for
DNA isolation. When the sample is received in >200 lL of
transport medium, the sample is used for DNA isolation and
the transport medium is shaken well, centrifuged at 700 g/
7 min, resuspended in 200 lL of sterile saline and processed
as described above.
When only liquid is received (e.g. contact lens solution),
samples 200 lL should be mixed and split into two aliquots
directly upon receipt, one aliquot is then used for culture, the
other aliquot is used for DNA isolation. Liquid samples
>200 lL are centrifuged at 700 g/7 min, resuspended in
200 lL of sterile saline and processed as described above.
Contact lenses or swabs are shaken vigorously in the original transport medium (contact lens solution/sterile saline) and
the lens/swab is then inoculated onto an agar plate and the
liquid can be used for DNA isolation. When contact lens cases
are received, the liquid is processed as described above, but a
biofilm swab from the inner surface should also be taken and
inoculated onto a plate culture.
When fixed material is received (swabs/contact lens case
cell pellets/tissue samples fixed either in ethanol or formalin
or embedded in paraffin or as stained sections on microscopic
slides), it is recommended to perform staining (lactophenol
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
Figure 2. Overview of the diagnostic procedure for Acanthamoeba keratitis.
Figure 3. Acanthamoeba trophozoites with the characteristic acanthopodia (A) in phase contrast, (B) in bright field microscopy. Scale bar:
10 lm. Originals.
cotton blue and/or immunostaining) and/or PCR. However, it is
important to isolate the DNA using a suitable protocol for the
respective material and to adapt the PCR protocol for fragmented DNA, particularly when the material is formalin-fixed
(i.e. amplicons should not exceed 300 bp in length).
2.3. Direct microscopy
In severe infections or when highly contaminated contact
lens cases are investigated, the amoebae can usually already
be detected by direct microscopy (200·–400· magnification)
of the original sample. For microscopic investigation of
amoebae, phase contrast or interference contrast are particu-
larly well suited. Nucleated corneal cells of lower cornea layers
may resemble amoebae, but acanthamoebae can be discriminated from other mononuclear cells by their large central
nucleolus, their contractile vacuole and their hyaline pseudopodia with characteristic hyaline protrusions, the so-called acanthopodia. The trophozoites are 15–45 lm in size and have an
oval to elongated outline (Fig. 3). They move slowly by forming usually one or two pseudopodia in the direction of
movement. The cysts are smaller (12–25 lm) and polygonal
or star-shaped (Fig. 4). They have two cyst walls which are
connected at several points. These points of contact between
endocyst and ectocyst are covered by an operculum, which is
removed during excystation.
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
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Figure 4. Acanthamoeba cysts in interference contrast microscopy (A) morphological group I, (B) morphological group II,
(C) morphological group III. Scale bar: 10 lm. Original.
2.4. Stains
Stains are practical for the detection of cysts in fresh
clinical material or in pelleted lens-case solution and for the
investigation of tissue sections. Fast and easy stains are lactophenol-cotton blue or Giemsa, but also calcofluor white
and acridine orange usually give very good results. If morphological details are to be studied, it is recommended to use a
silver stain, which is particularly well-suited for the investigation of the cysts. However, amoebae have to be cultured prior
to staining. A general problem is that other cells, particularly
fungi also stain well in these stains. As a specific staining,
immunostaining using anti-Acanthamoeba antibodies is recommended which is also the stain of choice for tissue sections.
Alternatively, tissue section can be stained with haematoxylin
& eosin (HE) [35].
2.4.1. Lactophenol-cotton blue (LPCB)
The material is mixed with an adequate volume of LPCB
stain (20 g phenol crystals, 20 mL lactic acid, 40 mL glycerol,
0.05 g cotton-blue and 20 mL dH2O; or ready-mixed available
through e.g. Sigma-Aldrich) and investigated by light microscopy [101]. This stain is particularly well suited for Acanthamoeba cysts; the cyst walls and the nucleus appear in an
intensive blue, while the cytoplasm stains light-blue.
2.4.2. Acridine orange
Samples are fixed in 95% methanol for 2 min onto a glass
slide, air dried, covered with acridine orange staining solution
( pH 4) for 2 min, rinsed with H2O and air dried. Cysts appear
bright orange and are easily discernible in fluorescence
microscopy.
2.4.3. Calcofluor white
Samples are transferred to a glass slide, air dried and fixed
for 3 min with methanol. Subsequently, the sample is rinsed in
PBS and stained using 2–3 drops of calcofluor white solution
(0.1%, e.g. Sigma-Aldrich or Thermo Scientific). After 5 min,
the slide is rinsed with PBS and counter-stained with Evan’s
blue (0.05%, e.g. Sigma-Aldrich or Thermo Scientific) for several seconds. It is important to use an embedding solution without auto-fluorescence. Slides are investigated by fluorescence
microscopy (300–440 nm). Acanthamoeba cysts appear in a
light green because the calcofluor white binds to the cellulose
in the cyst walls. Evan’s blue diminishes the background fluorescence making the trophozoites appear reddish-brown.
2.4.4. Silver
The cysts are harvested from a culture plate/flask, suspended in 2 mL amoeba saline (Table 2) and washed three
times in amoeba saline by centrifugation (500 g/10 min).
The sample is fixed for 20 min in 2% formalin and washed
in amoeba saline, the supernatant is removed and the pellet
is transferred to a glass slide using an inoculating loop and
mixed with Mayer’s albumin (glycerine-albumin 1:1, e.g.
Hardy Diagnostics). Then, the cysts are fixed onto the slide
using Clarke’s fixative (95% alcohol-acetic acid 9:1) for 2 h.
The fixative is removed using dH2O and the slides are incubated in 0.5% silver-protein solution in a water bath at
60 C. After 2 h, the slides are transferred to the reducing
agent (1% hydroquinone in 5% Na2SO3) and incubated during
gentle shaking for several seconds up to 5 min. The slides are
washed in dH2O, dehydrated in an alcohol series, cleared with
xylene, mounted and investigated by bright field microscopy.
2.4.5. Immunostain
To the best of our knowledge, no commercial kit is available, but antisera against the three Acanthamoeba groups (I–
III), produced by immunization of a rabbit with Acanthamoeba
whole-cell antigen, are available in many laboratories (including our own) and can be obtained upon request.
2.4.6. Haematoxylin & eosin (HE)
The tissue section is fixed in 10% neutral buffered formalin
solution (e.g. Sigma-Aldrich). Serial sections of 6 lm are pro-
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
Table 3. PYG medium [71]. Compounds are weighted into a 1 L
bottle, filled up to 1 L with dH2O, mixed and sterilized by filtration.
Compound
Proteose-Peptone
Glucose
NaCl
MgSO4-7H2O
CaCl2 Æ 2H2O
Na2HPO4
KH2PO4
Grams
10.00
18.00
1.20
0.04
0.04
1.42
1.36
If the isolated amoebae are needed for further studies (e.g.
genotyping), sub-cultures should be prepared since fungi and
other microorganisms also grow very well in these cultures.
In addition, several physiological properties can be used to further describe and discriminate Acanthamoeba isolates, including growth rate, temperature tolerance, cell culture
pathogenicity and in vivo mouse pathogenicity [22, 24, 34,
115].
Figure 5. Acanthamoeba trophozoites observed in culture in a case
of severe AK infection.
duced, deparaffinized for 1–2 min in xylene, dehydrated in
alcohol and washed with dH2O. Subsequently, the sample is
stained with haematoxylin and eosin, washed, covered with a
cover slip and investigated by bright field microscopy.
2.5. Culture
The gold standard for Acanthamoeba detection is still the
plate culture technique [71, 91]. The material (corneal scrapings/biopsies or transport medium/contact lenses/swabs, etc.)
is applied centrally onto a 90 mm 1.5% non-nutrient (NN) agar
plate covered with a lawn (100 lL) of a 24 h old culture of
non-mucous bacteria (e.g. Escherichia coli). Plates are sealed
with Parafilm, incubated at 30 C and screened daily for
amoebae, optimally by inverted phase contrast microscopy.
In cases of severe infection, amoebae are usually already visible after 24–48 h (Fig. 5). However, samples should be
observed for up to 1 week to reliably prove a negative result.
Alternatively, amoebae can be cultured in tissue culture flasks
in a suspension of bacteria in PBS.
In culture, acanthamoebae form cysts within approximately
1 week (depending on temperature and availability of nutrients). These cysts can be identified at least down to the morphological group level (Acanthamoeba sp. group I–III) based
on size, morphology and number of opercula [80] (Fig. 4).
Most AK cases are caused by representatives of group II
(Fig. 4B), but group III strains have also been described as
causative agents of AK. Group II strains have polygonal cysts
with 3–7 cyst arms, while group III strains are rounded and do
not have clearly visible cyst arms. Group I strains with their
large and beautifully star-shaped cysts have not (yet?) been
described to cause AK. Species identification can be achieved
using the identification key by Page [71]. However, in some
cases, morphological identification is ambiguous and the validity of some described species has been questioned altogether.
2.5.1. Sub-culture
From positive samples, clonal cultures can be prepared by
transferring a small piece (<1 cm2) of agar with only few clean
cysts on it (optimally a single cyst using a micromanipulator)
upside down to a fresh plate. Plates should be sub-cultured
every 2–4 weeks.
Monoxenic plates sealed with Parafilm can be kept for
several months at room temperature. If they do not entirely
dehydrate, cysts remain viable for many years.
2.5.1.2. Temperature tolerance
Sub-cultures can be used for investigating the temperature
tolerance of the respective isolate. Parallel cultures are incubated at 30 C, 37 C and 42 C, respectively. The temperature
of the human eye is approximately 34 C. Usually however, the
ability to grow at 37 C (body temperature) and 42 C (high
fever) is also investigated. Plates are investigated daily by
phase contrast microscopy.
2.5.2. Axenisation
Acanthamoebae can be axenised by harvesting cysts from
the plate cultures and incubating them in 3% HCl overnight in
order to eliminate the bacteria. It is usually sufficient to install
three parallel plate cultures and wait for cyst formation (usually
approximately 2 weeks). It is important that cysts be fully
mature, because otherwise they will not survive the acid treatment. Subsequently, the cysts are washed 2–3· in amoeba saline (700 g/7 min) to remove remaining acid and transferred to
liquid cultures. An easy culture medium for acanthamoebae is
proteose peptone-yeast extract-glucose (PYG) [71] (Table 3).
To keep axenic cultures running, medium has to be changed ideally every 1–2 weeks. The cultures should be checked
regularly for bacterial contamination (e.g. by transferring an
aliquot of the supernatant to bacterial broth), as should the
amoebae themselves for endocytobionts. To reduce the risk
of contamination, antibiotics (e.g. 200 IU penicillin and
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
200 lg/mL streptomycin) and/or antimycotics (e.g. amphotericin B) can be added to the culture medium.
Liquid culture is not suited to initial clinical samples, as
bacteria and fungi would overgrow the cultures (a clinical
sample from the eye surface is never sterile).
2.5.3. Cell culture pathogenicity
Trophozoites are harvested from axenic cultures by centrifugation (700 g/7 min.) and transferred onto a monolayer of
human (e.g. HeLA, HEp-2 or keratinocytes) or animal cells
(e.g. VERO) in an amoeba/cell ratio of 1:10. The amoebae
are designated as highly cytopathic, when the monolayer is
completely lysed within 24 h.
2.5.4. Mouse inoculation
Trophozoites are harvested from axenic cultures by centrifugation (700 g/7 min.), re-suspended in sterile PBS and inoculated into mice intra-nasally or intra-cerebrally. Young mice
are generally more susceptible to an Acanthamoeba infection.
Pathogenic strains lead to death within a few days up to
4 weeks. Importantly, amoebae can lose their pathogenicity
during long-term axenic laboratory culture.
7
(50 -GACTACGACGGTATCTGATC-30 ) [113], which amplifies
a shorter (~300 bp) fragment of the 18S rRNA gene.
In the past years, several protocols for real-time PCR have
also been published [43, 61, 81, 84]. A highly sensitive and
specific assay is the multiplex real-time PCR established by
Qvarnstrom et al. [81], which for AK diagnostics can also be
run as a singleplex.
2.8. Genotyping
Sequences of the PCR amplicons can be obtained by direct
sequencing or by cloning. Generally, it is recommended to
obtain sequences from both strands and assemble them to give
a consensus sequence. For genotyping, obtained sequences are
compared to sequences of Acanthamoeba reference strains by
multiple alignments with all available genotypes at that time
(currently 19) with the model assumption of a <5% sequence
dissimilarity within one genotype as established by Gast
et al. [33] and Stothard et al. [97]. Worldwide, the vast majority
of AK cases are caused by Acanthamoeba genotype T4, but
genotypes T3 and T11 are also commonly associated with
AK, and in fact most genotypes known to date have at least
once been involved in an AK case [11, 116].
3. Pathogenesis of Acanthamoeba keratitis
2.6. DNA isolation
6
For genotyping, actively growing amoebae (~10 cells) are
harvested from culture plates and resuspended in 100 lL of
sterile 0.9% NaCl for DNA isolation. Whole-cell DNA can
be isolated from the amoebal suspensions using a commercial
DNA isolation kit following the manufacturer’s protocol for the
respective type of material. When larger tissue samples are
received, we recommend homogenization of the material prior
to DNA isolation.
2.7. PCR/real-time PCR
The most frequently used PCR for Acanthamoeba diagnostics is probably the one established by Schroeder et al. [90, 97]
amplifying a fragment of the 18S rRNA gene using the
JDP1 (50 -GGCCCAGATCGTTTACCGTGAA-30 ) and JDP2
(50 -TCTCACAAGCTGCTAGGGAGTCA-30 ) primers. In this
PCR, the length of the amplicon varies between 423 and
551 bp depending on the genotype, and DNA sequencing of
the amplicon allows for genotyping in most cases. Generally,
whichever diagnostic PCR is used, it should be run with at least
two different dilutions from each sample (as the proportion
amoebal DNA: human DNA can vary greatly) and a genotype
T4 reference strain should be used as a positive and DNA-free
water as a negative control. Amplicons are visualized by agarose-gel electrophoresis and, if genotyping is required, the
respective bands are extracted from the gel, purified and
subjected to DNA sequencing.
For samples that had been fixed in formaldehyde,
we employ a modified PCR using the JDP1 primer
from the PCR described above and the P2r primer
The devastating nature of Acanthamoeba keratitis and the
problems associated with its diagnosis and successful therapy
suggest a need for complete understanding of the pathogenesis
and pathophysiology to find alternative therapeutic interventions. Another major concern during the course of therapy is
the ability of Acanthamoeba to transform into dormant cyst
forms, which may resist recommended levels of antimicrobial
chemotherapy. The ability of Acanthamoeba to produce infection requires specific adhesins, production of toxins, and its
ability to resist immune/environmental factors and chemotherapeutic agents, which likely enable this pathogen to produce
infection. For simplicity, the information is divided into factors
contributing directly and indirectly to Acanthamoeba pathogenicity (Fig. 6).
3.1. Factors contributing directly to the
pathogenicity of Acanthamoeba
3.1.1. Adhesion
Adhesion is an important step in the pathogenic cascades
of Acanthamoeba keratitis leading to secondary events and
amoebae crossing biological barriers (Fig. 7). Several adhesins
have been identified in Acanthamoeba, including a mannosebinding protein [30], a laminin-binding protein with a
predicted molecular mass of 28.2 kDa [40] and a 55 kDa
laminin-binding protein [87]. Notably, oral immunization with
recombinant mannose-binding protein ameliorates Acanthamoeba keratitis in the Chinese hamster model [30, 31], and
has shown that the mannose-binding protein gene in Acanthamoeba contains six exons and five introns that span 3.6 kbp.
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
Figure 6. Factors contributing to the pathogenicity of Acanthamoeba.
Figure 7. Acanthamoeba-mediated corneal epithelial cell death.
The 2.5 kbp cDNA codes for an 833 amino acids precursor
protein with a signal sequence (residues 1–21 aa), an
N-terminal extracellular domain (residues 22–733 aa) with five
N- and three O-glycosylation sites, a transmembrane domain
(residues 734–755 aa), and a C-terminal intracellular domain
(residues 756–833 aa).
On the host side, parasite binding to specific host cell
receptor(s) remains incompletely understood. However
Toll-like receptor-4 (TLR-4) is shown to provide a docking site
for Acanthamoeba [82, 83]. Complete identification of adhesins involved in binding to various cell types, tissues and surfaces together with specific receptor(s) is a largely unexplored
area, offering tremendous research opportunities. The binding
of Acanthamoeba to host cells interferes with the host intracellular signalling pathways. For example, TLR activation leads to
TLR4-Myeloid differentiation primary response gene 88
(MyD88)-Nuclear Factor-Kappa B (NF-kappaB) and TLR4Extracellular signal-regulated kinases1/2 (ERK1/2) pathways
[82, 83]. This was confirmed using anti-TLR antibodies or
specific inhibitors pyrrolidinedithiocarbamate (PDTC) (for
the NF-kappa B pathway) and U0126 (for the ERK pathway).
Using cell cycle microarrays, it has been shown that adhesion
of Acanthamoeba to host cells regulates the expression of a
number of genes important for the cell cycle such as
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
9
Figure 8. Acanthamoeba-mediated corneal epithelial cell death.
GADD45A and p130 Rb, associated with cell cycle arrest, as
well as inhibiting the expression of other genes, such as those
for cyclins F, G1 and cyclin dependent kinase-6 that encode
proteins important for cell cycle progression [95]. Acanthamoeba inhibited pRb phosphorylation (a master regulator of
cell cycle) in human corneal epithelial cells, indicating that
Acanthamoeba induces cell cycle arrest in the host cells. Acanthamoeba-mediated host cell death is dependent on the activation of phosphatidylinositol 3-kinase [96]. This was shown
using
LY294002,
a
specific
phosphatidylinositol
3-kinase inhibitor, which blocked Acanthamoeba-mediated
host cell death. These findings were further confirmed using
host cells expressing dominant negative p85, i.e. a regulatory
subunit of phosphatidylinositol 3-kinase. The host cells
expressing dominant negative phosphatidylinositol 3-kinase
were significantly less susceptible to Acanthamoeba-mediated
damage (Fig. 8). Chusattayanond et al. [16] have shown that
host cell apoptosis induced by Acanthamoeba is caspasedependent, mediated by over-expression of pro-apoptotic
proteins in the mitochondrial pathway, while Tripathi et al.
[102] demonstrated the role of the cytosolic phospholipase
A2a (cPLA2a) pathway in host cell apoptosis.
3.1.2. Phagocytosis
Adhesion of Acanthamoeba leads to secondary processes
such as phagocytosis or secretion of toxins. The primary role
of Acanthamoeba phagocytosis is to take up food particles.
However, the ability of Acanthamoeba to form food cups or
amoebastomes during incubations with the host cells suggests
it has a role in the pathogenesis of Acanthamoeba [26, 47, 76].
Within 40 s, bound particles are surrounded by pseudopods,
brought into the cytoplasm, and released as phagosome into
the cytoplasmic stream. The oxidative metabolism in Acanthamoeba has some remarkable similarities to the respiratory burst
oxidase of neutrophils [13]. Cytochalasin D, an inhibitor of
actin polymerization blocked Acanthamoeba-mediated host
cell death, confirming that actin-mediated cytoskeletal rearrangements play an important role in the pathogenesis of
Acanthamoeba [70, 100]. Genistein (a protein tyrosine kinase
inhibitor) inhibited, while sodium orthovanadate (protein
tyrosine phosphatase inhibitor), stimulated Acanthamoeba
phagocytosis, indicating that tyrosine kinase-induced actin
polymerization is important in Acanthamoeba phagocytosis
[3]. Rho kinase inhibitor, Y27632, partially blocked Acanthamoeba phagocytosis. Y27632 is known to block stress fibre
formation by inhibiting myosin light chain phosphorylation
and cofilin phosphorylations but independent of the profilin
pathway. LY294002, a specific inhibitor of phosphatidylinositol
3-kinase, inhibited Acanthamoeba phagocytosis. Inhibition
of Src kinase using a specific inhibitor, PP2 (4-amino-5(4 chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d] pyrimidine) but
not its inactive analog, PP3 (4-amino-7-phenylpyrazolo
[3,4-d] pyrimidine), hampered the phagocytic ability of
A. castellanii [93]. The precise elucidation of molecular
mechanisms associated with Acanthamoeba phagocytic
pathways will be of value in the development of therapeutic
interventions.
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3.1.3. Ecto-ATPases
3.1.6. Acanthamoeba-induced plasminogen activation
Ecto-ATPases are glycoproteins expressed in the plasma
membranes with their active sites facing the external medium.
Ecto-ATPases hydrolyze extracellular ATP and other nucleoside triphosphates. The resultant ADP can have toxic effects
on the host cells. For example, it has been shown that ADP
released by Acanthamoeba bind to P2y2 purinergic receptors
on the host cells, causing an increase in intracellular calcium,
inducing caspase-3 activation and finally resulting in apoptosis
[59]. A P2 receptor antagonist, suramin, inhibited Acanthamoeba-mediated host cell death [59], suggesting that ectoATPases play an important role in Acanthamoeba pathogenesis
in a contact-independent mechanism. The clinical isolates of
Acanthamoeba exhibited higher ecto-ATPase activities
compared with weak pathogenic isolates [94]. Several ectoATPases of approximate molecular weights of 62, 100, 218,
272 and more than 300 kDa have been described in Acanthamoeba. However, further research is needed to elucidate their
function in Acanthamoeba biology, and investigate their
precise role in Acanthamoeba pathogenesis.
Acanthamoeba displayed plasminogen activator activity by
catalyzing the cleavage of host plasminogen to form plasmin,
which can activate host proteolytic enzymes, such as
pro-matrix metalloproteases. Once activated, the matrix
metalloproteases degrade the basement membranes and the
components of the extracellular matrix such as type I and type
II collagens, fibronectins and laminin. Thus, the matrix
metalloproteinases are involved in tissue remodelling.
The pathogenic Acanthamoeba showed positive chemotactic
response to the endothelial extracts [109].
3.1.4. Neuraminidase activity
Acanthamoeba exhibited neuraminidase activity. The
enzyme activity is optimal at pH 5 and at temperatures of
25–30 C. The live amoebae released sialic acid from the
human cells. Therefore, the neuraminidase of Acanthamoeba
could be relevant in the colonization of amoebae, and important in producing damage of the sialic acid-rich corneal epithelium. Interestingly, neuraminidases of Trypanosoma cruzi and
Acanthamoeba are immunologically related, as demonstrated
by antibodies against neuraminidase of Trypanosoma cruzi,
which reacted with Acanthamoeba in immunofluorescence,
immunoblotting and enzyme-linked immunosorbent assays
[73, 74].
3.1.5. Superoxide dismutase
The enzyme superoxide dismutase catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. It is
an important antioxidant defence exposed to oxygen. Superoxide is one of the main reactive oxygen species in the cell and as
such, superoxide dismutase plays an important antioxidant
role. Two superoxide dismutases have been identified in Acanthamoeba: an iron superoxide dismutase (approximate molecular weight of 50 kDa) and a copper-zinc superoxide
dismutase (approximate molecular weight of 38 kDa). These
enzymes occur as cytoplasmic and detergent-extractable fractions. They may be potential virulence factors of Acanthamoeba by acting both as anti-oxidants and anti-inflammatory
agents. They may also provide additional targets for chemotherapy and immuno-diagnosis of Acanthamoeba infections
[15]. A. castellanii iron superoxide dismutase may play essential roles in the survival of amoebae not only by protecting
themselves from endogenous oxidative stress, but also by
detoxifying oxidative killing of amoebae by host immune
effector cells [50].
3.1.7. Elastase
Acanthamoeba is known to produce elastase with broad
specificity. Moreover, elastases are known to degrade a range
of connective tissue proteins such as elastin, an elastic fibre,
fibrinogen, collagen, and proteoglycans, which together determined the mechanical properties of the connective tissue.
Tissues altered by prior elastase treatment are more susceptible
to oxygen radical attack, suggesting their involvement in the
pathogenesis and pathophysiology of Acanthamoeba infections
[18, 46, 47, 54]. The elastases were in the region of
70–130 kDa and serine peptidases were found to be possible
elastase candidates [28].
3.1.8. Proteases
Proteases are degradative enzymes that catalyze the total
hydrolysis of proteins. Acanthamoeba is shown to exhibits proteolytic activities. The primary role of Acanthamoeba proteases is to degrade food substances for feeding purposes.
Pathogenic Acanthamoeba exhibit increased extracellular protease activities. The link between pathogenicity and the
increased levels of extracellular proteases suggests that pathogenic Acanthamoeba utilize proteases to facilitate invasion of
the host. Acanthamoeba is known to produce serine, cysteine
and metalloproteases. Several serine proteases have been identified ranging in molecular weights from >20 kDa to
>200 kDa. They are shown to possess collagen degradation
activity, plasminogen activator, and degradation of fibronectin,
fibrinogen, IgG, IgA, albumin, haemoglobin, protease inhibitors, interleukin-1, chemokines and cytokines [46, 54, 55, 60].
A 133 kDa serine protease, called MIP133 has been identified
as a crucial component of the pathogenic cascade of Acanthamoeba pathogenesis. The MIP133 serine protease is shown to
induce the degradation of keratocytes, iris ciliary body cells,
retinal pigment epithelial cells, corneal epithelial cells and corneal endothelial cells, and induce apoptosis in macrophage-like
cells. The properties of serine proteases facilitate Acanthamoeba invasion of the corneal stroma, leading to secondary
reactions such as oedema, necrosis and inflammatory
responses. A direct functional role of serine proteases in
Acanthamoeba infections is indicated by the observations that
intrastromal injections of Acanthamoeba conditioned medium
produced corneal lesions in vivo, similar to those observed in
Acanthamoeba keratitis patients, and this effect is inhibited
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
by phenylmethylsulfonyl fluoride, a serine protease inhibitor
[37, 67, 68]. In addition, the chemically synthesized siRNA
against the catalytic domain of the extracellular serine proteases of Acanthamoeba reduced protease activity and Acanthamoeba-mediated host cell cytotoxicity. These results support
the idea that extracellular serine proteases are directly involved
in the pathogenesis and virulence of Acanthamoeba [55]. In
addition, several cysteine proteases have been identified in
Acanthamoeba, including 43, 65, 70 and 130 kDa cysteine
proteases [46, 47]. In addition to serine and cysteine proteases,
there is evidence for metalloprotease activity in Acanthamoeba. An 80 kDa metalloprotease was identified in cocultures of Acanthamoeba and host cells, but its origin (whether
Acanthamoeba or the host cells) was not established. Later studies identified a 150 kDa extracellular metalloprotease from
Acanthamoeba isolate of the T1 genotype. This metalloprotease
exhibited properties of extracellular matrix degradation, as
demonstrated by its activity against collagen I and III (major
components of the collagenous extracellular matrix), elastin
(elastic fibrils of the extracellular matrix), plasminogen
(involved in proteolytic degradation of the extracellular matrix),
as well as degradation of casein, gelatine and haemoglobin.
Recently, the complete sequence of a type-1 metacaspase
from Acanthamoeba was reported, comprising 478 amino acids
[102]. Later studies revealed that A. castellanii metacaspases
associate with the contractile vacuole and have an essential role
in cell osmoregulation suggesting its attractiveness as a possible target for treatment therapies against A. castellanii infection [89]. These studies showed that Acanthamoeba exhibited
diverse proteases and elastases, which could play important
roles in Acanthamoeba infections. The precise mechanisms
of protease mode of action at the molecular level are only
beginning to emerge. Proteases have been shown to be ‘‘druggable’’ targets, as evidenced by the widespread use of protease
inhibitors as effective therapy for hypertension and AIDS, and
the current clinical development of protease inhibitors for diabetes, cancer, thrombosis, and osteoporosis. As long as issues
such as the difficulty of achieving selectivity can be addressed
through targeting allosteric sites, protease-based drug therapy
has tremendous potential in the treatment of many infectious
diseases. Future studies will further determine the role of proteases as vaccine targets, search for novel inhibitors by screening of chemical libraries, or rational development of drugs
based on structural studies should enhance our ability to target
these important pathogens.
3.1.9. Phospholipases
During phagocytosis, there is a large turnover of the plasma
membrane in Acanthamoeba, indicating that there is controlled
local degradation of phospholipids leading to instability of the
membrane phospholipid bilayer, which would then reform after
the acylation of the lysophospholipid. In support of this, all of
the enzymes that are needed for such a cycle are present in the
plasma membrane of Acanthamoeba, including phospholipase
A2, acyl CoA:lysolecithin acyltransferase, and acyl CoA synthetase. Phospholipase A1 and lysophospholipase are also present in the plasma membrane of Acanthamoeba. The plasma
membrane lysophospholipase may also serve to protect the cell
11
from the lytic effect of lysophospholipids either of exogenous
or endogenous origin. The plasma membranes have the enzymatic capability of modulating the fatty acyl composition of
phospholipids by de-acylation and acylation. Our knowledge
of phospholipases in the virulence of Acanthamoeba is fragmented, however several studies have shown that pathogenic
Acanthamoeba that exhibit cytopathic effects on mammalian
cells in vitro liberate more phospholipase, suggesting their possible involvement in Acanthamoeba infections. Because phospholipases cleave phospholipids, it is reasonable to suspect that they
play a role in membrane disruptions, penetration of host cells, and
cell lysis. However this remains to be determined. Other actions
of phospholipases may involve interference with intracellular
signalling pathways. Phospholipases generate lipids and
lipid-derived products that act as second messengers. A. castellanii lysates and their conditioned medium exhibited phospholipase activities [66]. Sphinganine, a PLA2 inhibitor showed
robust amoebistatic properties but had no effect on the viability of
A. castellanii. These studies suggest that Acanthamoeba
phospholipases and/or lysophospholipases may play a role in
producing host cell damage or affect other cellular functions such
as induction of inflammatory responses, thus facilitating
Acanthamoeba virulence. More studies are needed to identify
and characterize Acanthamoeba phospholipases and to determine their potential role in the development of therapeutic
intervention. This is not a novel concept: earlier studies have
shown that phospholipase C from Clostridium perfringens
induced protection against C. perfringens-mediated gas gangrene. In addition, targeting of phospholipases using synthetic
inhibitor compounds has been shown to prevent Candida
infections. Antibodies produced against Acanthamoeba
phospholipases may also be of potential value in the development
of sensitive and specific diagnostic assays as well as of
therapeutic value [42].
3.1.10. Glycosidases (also called glycoside hydrolases)
Glycoside hydrolases catalyze the hydrolysis of glycosidic
linkage to generate smaller sugars. Glycoside hydrolases are
ubiquitous in nature and involved in the degradation of biomass such as cellulose and in a variety of cellular functions.
Together with glycosyltransferases, glycosidases form the
major catalytic machinery for the synthesis and breakage of
glycosidic bonds. Acanthamoeba exhibits glycosidase activities
including beta-glycosidase, alpha-glucosidase, beta-galactosidase, beta-N-acetyl-glucosaminidase, beta-N-acetyl-galactosaminidase and alpha-mannosidase [38, 39]. Acanthamoeba
extracts mediate enzymatic lysis of cell walls from several
species of bacteria including Micrococcus lysodeikticus,
Micrococcus roseus, Streptococcus faecalis, Bacillus megaterium, Sarcina lutea, Micrococcus radiodurans and limited
activity against Bacillus subtilis, Bacillus cereus, but has no
effects on Acanthamoeba cyst walls or chitin. Exhaustive
digestion of Micrococcus lysodeikticus cell walls released free
N-acetyl-glucosamine, N-acetyl-muramic acid, glycine, alanine, glutamic acid and lysine, suggesting that Acanthamoeba
possesses both endo- and exo-hexosaminidases and betaN-acetyl-hexosaminidases. Since Acanthamoeba is known to
utilize maltose, cellobiose, sucrose or lactose, some of the
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
glycosidases indicated above may suggest the utilization of
these disaccharides [42].
of a given isolate [46, 47, 54]. The precise mechanisms by
which pathogenic Acanthamoeba adapt to higher temperatures
and maintain their metabolic activities require further studies.
3.1.11. Acanthaporin
Recently, acanthaporin, the first pore-forming toxin was
described from Acanthamoeba [64]. Acanthaporin was isolated
from extracts of virulent A. culbertsoni by tracking its poreforming activity, molecularly cloning the gene of its precursor,
and recombinant expression of the mature protein in bacteria.
Acanthaporin was cytotoxic for human neuronal cells and
exerted antimicrobial activity against a variety of bacterial
strains by permeabilizing their membranes [64]. The tertiary
structures of acanthaporin’s active monomeric form and inactive dimeric form, both solved by NMR spectroscopy, revealed
a currently unknown protein fold and a pH-dependent trigger
mechanism of activation.
3.2. Factors contributing indirectly to
Acanthamoeba pathogenicity
The ability of Acanthamoeba to produce human diseases is
a multifactorial process and is, amongst other factors, dependent on its ability to survive outside its host and under diverse
conditions (high osmolarity, varying temperatures, food deprivation and resistance to chemotherapeutic drugs) [24, 48, 106,
107, 114]. The ability of Acanthamoeba to overcome such conditions can be considered as contributory factors towards disease and are indicated below.
3.2.1. Morphological features
The infective forms of Acanthamoeba or trophozoites do
not have a distinct morphology. However, they do possess
spine-like structures known as acanthopodia on their surface,
which allow them to modulate binding of Acanthamoeba to
biological and inert surfaces. In addition, their amoeboid
motion resembles that of macrophages/neutrophils and it is
likely that Acanthamoeba employ similar strategies to traverse
biological barriers and invade tissues using the paracellular
route.
3.2.2. Temperature tolerance, osmotolerance and growth
at different pH
Being a free-living amoeba, Acanthamoeba is exposed to
various temperatures, osmolarity and pH. Similarly contact
with tear film exposes Acanthamoeba to high osmolarity
(due to salinity in tears), high temperatures as well as alterations in pH. For successful transmission, Acanthamoeba must
withstand such stress and exhibit biological activity. Pathogenic Acanthamoeba showed high levels of heat shock proteins
(i.e. HSP60 and HSP70) compared with weak pathogens [75,
77]. The higher levels of heat shock proteins in Acanthamoeba
may indicate their involvement in (i) tolerance to hosts’ stressors and/or (ii) in species’ virulence [75]. The ability of Acanthamoeba to grow at high temperatures and high osmolarity
correlates with the pathogenicity of Acanthamoeba isolates,
and may provide a good indicator of the pathogenic potential
3.2.3. Cellular differentiation
Cellular differentiation is the ability of Acanthamoeba to
differentiate into a morphologically distinct dormant cyst form
or a vegetative trophozoite form. This is a reversible change,
dependent on environmental conditions. Cysts are resistant to
various antimicrobial agents and adverse conditions such as
extremes in temperatures, pH, osmolarity, desiccation and cysts
can be airborne: all of which presents a major problem in chemotherapy because their persistence may lead to recurrence of
the disease. Furthermore, Acanthamoeba cysts can survive several years while maintaining their pathogenicity [62]. These
characteristics suggested that the primary functions of cysts
lie in withstanding adverse conditions and in the spread of
amoebae throughout the environment. In addition, this may
represent the ability of Acanthamoeba to alternate expression
of surface proteins/glycoproteins, in response to changing environments and/or immune surveillance. Cellular differentiation
represents a major factor in the transmission of Acanthamoeba
and recurrence of its infection. However, the underlying
molecular mechanisms in these processes remain incompletely
understood and warrant further investigation.
3.2.4. Chemotaxis
Chemotaxis directs amoeba movement according to certain
chemicals in their environment. This is important as
Acanthamoeba moves towards the highest concentration of
food molecules, or to flee from poisons. Acanthamoeba
exhibits chemosensory responses as observed by their response
to a variety of bacterial products or potential bacterial
products by moving actively towards the attractant.
Acanthamoeba responded to the chemotactic peptides formylmethionyl-leucyl-phenylalanine,
formyl-methionyl-leucylphenylalaninebenzylamide, lipopolysaccharide and lipid A.
In addition, significant responses to cyclic AMP, lipoteichoic
acid and N-acetyl-glucosamine were also found. Interestingly,
chemotactic peptide antagonists, mannose, mannosylated
bovine serum albumin and N-acetyl-muramic acid all yielded
non-significant responses. Pretreatment of Acanthamoeba with
chemotactic peptides, bacterial products and bacteria reduced
the directional response to attractants. Acanthamoeba grown
in the presence of bacteria appeared more responsive to chemotactic peptides. Treatment of Acanthamoeba with trypsin
reduced the response of cells to chemotactic peptides, though
sensitivity was restored within a couple of hours [7, 92]. These
findings suggest that Acanthamoeba membranes possess receptors sensitive to these bacterial substances, which are different
from the mannose-binding protein involved in binding to the
host cells to produce cytotoxicity or involved in binding to bacteria during phagocytosis. The rate of movement is relatively
constant (ca. 0.40 lm per sec), indicating that the locomotor
response to these signals is a taxis, or possibly a klinokinesis,
but not an orthokinesis [92].
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
3.2.5. Ubiquity
Acanthamoeba has been found in diverse environments,
from drinking water to distilled water wash bottles, deep ocean
bottom and Antarctica. It is therefore not surprising that human
beings encounter and interact regularly with these organisms,
as is evidenced by findings that in some regions, up to 100%
of the population tested possess Acanthamoeba antibodies,
suggesting that these are one of the most ubiquitous protists
and often come into contact with human beings, and given
the opportunity (e.g. contact lens wear), can cause serious
infections.
3.2.6. Biofilms
Biofilms are known to play an important role in the
pathogenesis of Acanthamoeba keratitis. Biofilms are microbially-derived sessile communities, which can be formed in aqueous environments as well as on any materials and medical
devices including intravenous catheters, contact lenses, scleral
buckles, suture material, and intraocular lenses. In the instance
of contact lenses, biofilms are formed through contamination of
the storage case. Once established, biofilms provide attractive
niches for Acanthamoeba by fulfilling their nutritional requirements as well as providing resistance to disinfectants. In addition, this allows higher binding of Acanthamoeba to contact
lenses. For example, Acanthamoeba exhibits significantly
higher binding to used and Pseudomonas biofilm-coated hydrogel lenses compared to unworn contact lenses. The abundant
nutrient provided by the biofilm encourages transformation of
Acanthamoeba into the vegetative, infective trophozoite form,
and it is important to remember that binding of Acanthamoeba
to the human corneal epithelial cells most likely occurs
during the trophozoite stage as cysts exhibit minimal binding.
These findings suggest that biofilms play an important role in
Acanthamoeba keratitis in wearers of contact lenses and
preventing biofilm formation is perhaps an important preventative strategy [9, 118].
3.2.7. Host factors
The factors that enable Acanthamoeba to produce disease
are not limited solely to the pathogen, but most likely involve
host determinants [17, 115]. Evidence for this comes from
recent studies in the UK, Japan and New Zealand, which suggested that the storage cases of contact lenses of 400–800 per
10,000 asymptomatic wearers are contaminated with Acanthamoeba. This number is remarkably high compared with the
incidence rate of Acanthamoeba keratitis in wearers of contact
lenses, which is around 0.01–1.49 per 10,000 [46]. These findings suggest that factors such as host susceptibility, tissue specificity, tear factors, sIgA, corneal trauma, as well as
environmental factors such as osmolarity may be important
in initiating Acanthamoeba infections. However, the extent to
which such host factors contribute to the outcome of Acanthamoeba keratitis is unclear because host factors are more complex and difficult to study than those of the pathogen. Overall,
it can be concluded that Acanthamoeba traversal of biological
barriers and to produce disease is a complex process that
involves both pathogen as well as host factors.
13
4. Acanthamoeba keratitis treatment: a
problem with no simple solution?
The treatment of Acanthamoeba keratitis has evolved
since the first medical cure was reported in 1985 [46, 54, 86,
103–106, 111, 112]. Early diagnosis and aggressive medical
therapy has improved the management of this difficult infection. Other reported factors that may facilitate effective medical therapy and an improved outcome include early epithelial
debridement (to remove the majority of organisms) and penetrating keratoplasty in medically-resistant cases.
So far, no chemotherapeutic agent has been described as a
single effective treatment against AK, regardless of the isolate
or genotype that causes it. This is because there are many factors, including the wide range of virulence traits that different
isolates possess, which makes it almost impossible to establish
a correlation between in vitro and in vivo efficacies. Nevertheless, to establish the most effective treatment regimen is not
easy for several reasons such as the relatively small number
of reported cases of AK, variable pathogenicity of different
strains, and the intrinsic fluctuating nature of the disease
process.
4.1. Is there a single effective treatment against
AK?
There are currently no methods or a single drug that can
eliminate both cystic and trophozoite forms, while the trophozoite form is much more readily eliminated [46, 47, 54, 56].
The available reported treatment regimens in the literature
have varied widely depending on the manifestation of the disease, the general health status of the infected cornea, and the
experience of the clinician. For example, in the original AK
case reported by Naginton et al. [reviewed in 46, 54], numerous topical antimicrobial preparations were tried in conjunction
with steroids, but both eyes eventually required grafting.
Recent years have brought us knowledge of more specific antimicrobials, although unfortunately, surgical grafting of the cornea remains the last solution in case of severe infection.
4.2. Current therapeutic approaches
Current AK treatment consists of topical antimicrobial
agents, which can achieve high concentrations at the site of
infection. Moreover, due to the existence of a cyst form in
Acanthamoeba that is highly resistant to therapy, a combination
of agents is generally used.
Most of the currently used topical agents are effective
against trophozoites and cysts of Acanthamoeba such as biguanides, (i) PHMB [52, 54, 58], which is effective at low concentrations (0.02%), but is unfortunately toxic to human
corneal cells [52, 54], and (ii) chlorhexidine, which is effective
against both amoebic forms, and at minimal concentrations is
not toxic to corneal epithelial cells [52, 54, 58, 86]. Chlorhexidine 0.02% is often used in combination with aromatic diamidines such as 0.1% propamidine isethionate Brolene (Sanofi,
UK), 0.15% dibromopropamidine, hexamidine 0.1% Desomedine (Chauvin, France) and neomycin, showing good results
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
if the treatment is applied early in the development of the
infection [54, 86]. Unfortunately, propamidine and hexamidine
are not available in all countries.
These topical antimicrobials are administered every hour
immediately after corneal debridement or for the first several
days of therapy. These agents are then continued hourly for
3 days (at least nine times/day is recommended) depending
on clinical response. The frequency is then reduced to every
3 h. Two weeks may be required before a response is observed,
and the total duration of therapy is a minimum of 3–4 weeks.
Some authors also recommend treatment for 6–12 months.
Moreover, when therapy is discontinued, close observation of
the patient is suggested in order to avoid recurrent infection.
Some patients have been successfully treated using an antiseptic as monotherapy; if this is attempted, it should be reserved
for patients with early disease.
4.3. Biguanides as first line treatment against AK
PHMB and chlorhexidine have been reported to be the
most effective drugs for treatment of infection and in combination they have been reported to be effective against both cysts
and trophozoites [23, 52, 54, 79, 108]. Regarding these two
drugs, it is important to mention that they are active against
a wide spectrum of pathogens by increasing cytoplasmic membrane permeability. Chlorhexidine and PHMB both contain
highly charged positive molecules capable of binding to the
mucopolysaccharide plug of the ostiole resulting in penetration
of the amoeba. The drug then binds to the phospholipid bilayer
of the cell membrane which is negatively charged resulting in
damage, cell lysis and death [52].
Among the observed side effects, toxic keratopathy may
develop at any time, necessitating significant alteration in this
treatment plan [46, 54, 98]. Elevated intraocular pressure as
well as increased inflammation often requires the use of
antiglaucoma medication and cycloplegics. The role of topical
corticosteroids as well as surgical intervention with therapeutic
penetrating keratoplasty in the management of this infection
remains controversial and is discussed later.
Brolene may be accompanied by drug toxicity and resistance and topical treatment with miconazole can lead to epithelial toxicity [29, 54, 56]. According to Turner et al. [107],
resistance is mainly due to the exocyst and endocyst, which
forms a double-walled protective barrier to biocides. Cysts
may also be resistant to biocides because they show little or
no metabolic activity and because of selection pressure due
to continuous drug exposure [46, 106, 107]. If resistance to
drugs occurs, keratoplasty may be used [29, 46, 51, 54]. Ueki
et al. [108] stated that recommended treatment for AK includes
corneal scrape with antifungal drugs and antibiotic treatment.
However, antifungal, antibacterial, antiviral and even corticosteroids used can complicate matters because they cause initial
improvement then worsening of the disease [46, 54].
4.4. Steroids controversy in AK treatment
No clear consensus exists about use of steroids. Most
authorities recommend that steroid use is probably best
avoided. Patients receiving steroids should continue antiamoebic therapy for several weeks after the steroids are stopped.
In the case of a persistent infection with inflammation, corticosteroids may be used. However, their use is controversial
because they cause suppression of the immunological response
of the patient. Moreover, corticosteroids produce inhibition of
the processes of encystation and excystation of Acanthamoeba,
which could be a cause of the appearance of resistance problems [103]. Recent studies have highlighted an association of
topical corticosteroids and a diagnostic delay of AK. Moreover,
there is some evidence that suggests that steroid use may result
in increased pathogenicity of the amoebae [85]. McClellan
et al. [63] demonstrated in an in vivo model that the addition
of topical corticosteroids, even at low doses, promotes an
increase in the number of trophozoites, produced by excystment in the infected corneal stroma. This exposes patients to
the risk of significantly greater corneal destruction through
an increase in amoebic load, which may be greater than the
increased chemotherapy effect on trophozoites compared with
the more resistant cysts [63, 85].
Furthermore, corneal transplantation (keratoplasty) is
another therapeutic option when topical treatment has failed.
This intervention is recommended if in the acute phase of
infection, the cornea becomes too thin or has been damaged,
and vision is limited [51, 69]. Nevertheless, there is a risk of
not eliminating all the trophozoites or cysts that could colonize
the new cornea [98]. A variation of keratoplasty called DALK
(Deep Anterior Lamellar Keratoplasty) has been proposed to
be more effective in increasing the survival of transplanted corneal cells and to prevent entry of pathogenic organisms at the
time of surgery [72].
4.5. In vitro drug sensitivity testing and
personalised treatments in AK patients
In vitro drug sensitivity testing, although rarely used, may
be helpful in refractory cases. However, such testing has its
limitations and may not be practical for the clinician. Not only
may drug sensitivities be variable between strains, but a
strain may also become resistant to formerly effective drugs.
In addition, testing results may differ between laboratories
and in some cases may not correlate with the clinical course.
Despite these problems, drug sensitivity testing may offer the
clinician a small edge in improving chances of therapeutic success and should be employed when possible. Recently, a patient
suffering from severe AK was healed after a personalized treatment with voriconazole, when sensitivity to this drug was
assayed after isolation of the amoebae from the patient’s
cornea [5].
4.6. Surgical management of AK
Therapeutic penetrating keratoplasty was the mainstay of
treatment for AK before the development of early diagnosis
and aggressive medical therapy [10, 21, 36, 72]. The role
and timing of penetrating keratoplasty in AK still remains
poorly defined. Certainly pending or frank corneal perforation
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
is a clear indication for surgical intervention. However, other
indications for surgery are not well defined.
Therapeutic penetrating keratoplasty should be considered
when the infectious process spreads to the paracentral corneal
stroma despite maximum antiamoebic therapy [21]. Performing this procedure on a more localized infection may allow
total removal of the organisms by excising the clinically
involved tissue as well as a rim of clear surrounding cornea.
This procedure allows the donor tissue to be placed into a relatively undamaged and hence non-immunocompromised recipient bed. After surgery, medical therapy should be
continued for at least several months to help ensure elimination
of any residual Acanthamoeba organisms in the recipient stromal tissue. Once the infection has spread into the peripheral
cornea, however, the likelihood of achieving a surgical cure
is markedly diminished. Intensive medical management is
required for several months to eradicate the organism prior
to keratoplasty. Unfortunately, the prognosis in these cases is
poor and reinforces the rationale for earlier rather than later
surgical treatment.
Recently, bipedicle conjunctival flap (CF) and cryopreserved amniotic membrane graft (AMG) have been reported
to be effective in AK. They restore ocular surface integrity
and provide metabolic and mechanical support for corneal
healing. Nevertheless, in the case of large corneal perforation,
penetrating keratoplasty to restore ocular integrity remains as
the only effective surgical option [1].
4.7. Novel therapeutic approaches
Recently, the widespread use of photorefractive surgery has
inspired its use in the setting of AK. Kandori et al. [44]
reported four cases in 2010, where early stage AK was treated
with standard topical therapy, but developed large corneal
abscesses in the upper third thickness of the stroma. These
were removed using laser phototherapeutic keratectomy
(PTK); all eyes experienced no disease recurrence and final
acuities ranged from 20/16 to 20/25.
Cross-linking is another relatively new treatment
option that has been applied to AK. While in vitro studies by
Kashiwabuchi et al. [45] and Del Buey et al. [25] have shown
no amoebicidal effect of riboflavin combined with UVA exposure, clinical case reports have shown a much more promising
picture. Garduño-Vieyra et al. [32] administered collagen
cross-linking to a patient in Mexico instead of topical medical
therapies, which were not commercially available. Significant
improvement was observed after 24 h, with symptoms resolving within 3 months, and 20/20 vision was obtained after
5 months. Khan et al. [49] have since reported three similar
cases which responded equally well to cross-linking, with all
ulcers closing within 7 weeks. In subsequent PK surgery for
scarring, no organisms were detected in excised tissue. It is
possible that the collagen stabilizing effect prevents further
tissue damage and isolates and prevents reproduction of the
amoebae. Although individual case report results seem promising, there are no formal clinical trials thus far to recommend
incorporation into standard practice.
15
4.8. In the search of novel drugs against AK
A new path may be the application of alkylphosphocholines. These are phosphocholines esterified to aliphatic alcohols. They exhibit in vitro and in vivo antineoplastic activity
and have been shown to be cytotoxic against Leishmania
spp., Trypanosoma cruzi and Entamoeba histolytica. A recent
study has demonstrated that particularly hexadecylphosphocholine (miltefosine) is highly effective also against various
strains of Acanthamoeba. Moreover, it has recently been
applied in combination with PHMB in AK in Austria with
successful outcomes [8, 78, 79].
Recently, the creation of a ‘‘pharmaceutical phylogeny’’
has been started for Acanthamoeba in order to elucidate and
select new therapeutic targets [54, 58, 86]. The phylogeny of
Acanthamoeba is closer evolutionarily to human beings than
many other eukaryotic pathogens [69]. Therefore, part of the
hypothesis is that many biochemical processes as well as therapeutic targets are evolutionarily conserved and are similar in
related organisms. This implies that there will be a large number of processes, some still unknown, in Acanthamoeba that
are similar to those of human beings. Therefore, treatments
that affect the host could also affect the parasite, for example,
phospholipid analogues as mentioned above which have been
demonstrated to be effective against Acanthamoeba [2, 86].
However, even though many biological processes are similar
in Acanthamoeba to other eukaryotic cells, some proteins such
as tubulins are not sensitive to inhibitors that are normally used
against them [86]. Therefore, it is also interesting to find those
targets that are specific to Acanthamoeba in order to attack the
parasite without affecting the host. These targets may be of a
different nature: specific gene products, biological processes
themselves, transcription or translation mechanisms or physical attributes such as cell membranes. In addition, many antibiotics active against Acanthamoeba have a mechanism of
action which is still unknown. In recent years, the possibility
to design and synthesize specific small interfering RNAs
(siRNAs) for gene silencing have made RNAi techniques a
powerful tool for the study and understanding of new cellular
pathways of proteins whose functions are still unknown, as
well as for their use as a therapy against various diseases
[14, 88]. In the case of Acanthamoeba, siRNAs have been used
successfully to identify potential therapeutic targets and even
recently to establish a target and propose statins as a future
therapy against Acanthamoeba strains [58].
Other drug targets that have been validated using the same
approach, but without the elucidation of an available chemical
alternative (active principle/drug) include glycogen phosphorylase and other cellulose synthesis related enzymes, serine proteases and myosin IC [4, 27, 53, 55, 57, 65]. The recently
published Acanthamoeba castellani genome data will further
assist in the development of novel therapeutics in the near
future [19].
5. Concluding remarks
The number of reported cases of Acanthamoeba keratitis is
increasing worldwide every year, due to increasing contact lens
16
J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
use for vision correction and cosmetic purposes. Increased
awareness combined with early diagnosis of the disease is currently a good pathway towards better outcomes. However, knowledge about the pathogenesis and cellular differentiation
processes in Acanthamoeba are still not fully known and urgently
require further investigation. They hold the key to improved diagnosis and to development of effective therapeutic approaches.
Acknowledgements. JLM was supported by the Ramón y Cajal Subprogramme from the Spanish Ministry of Economy and Competitivity RYC-2011-08863 and by the grants RICET (Project No. RD12/
0018/0012 of the programme of Redes Temáticas de Investigación
Cooperativa, FIS), Spanish Ministry of Health, Madrid, Spain and
the Project FIS PI13/00490 ‘‘Protozoosis Emergentes por Amebas
de Vida Libre: Aislamiento, Caracterización, Nuevas Aproximaciones Terapéuticas y Traslación Clínica de los Resultados’’ from
the Instituto de Salud Carlos III and Project ref. AGUA3 ‘‘Amebas
de Vida Libre como Marcadores de Calidad del Agua’’ from CajaCanarias Fundación. JLM is grateful to the laboratory members of
the Free-Living Amoebae Laboratory at the University Institute of
Tropical Diseases and Public Health of the Canary Islands, University of La Laguna, Spain.
NAK was supported by the Higher Education Commission, and Aga
Khan University, Pakistan, the British Council for the Prevention of
Blindness, UK, and the Royal Society, UK.
JW was supported by the Medical University of Vienna, Austria and
would like to thank all members of the Molecular Parasitology
laboratory at the Institute of Specific Prophylaxis and Tropical
Medicine, Vienna, Austria.
This review was invited by the Editor at the occasion of ICOPA XIII
(Mexico, 2014). Its publication is sponsored by the publisher of
Parasite, EDP Sciences.
Images from Figure 1 were kindly provided by Dr Francisco ArnalichMontiel, Cornea Unit, Hospital Ramón y Cajal, Madrid, Spain.
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J. Lorenzo-Morales et al.: Parasite 2015, 22, 10
Cite this article as: Lorenzo-Morales J, Khan NA & Walochnik J: An update on Acanthamoeba keratitis: diagnosis, pathogenesis and
treatment. Parasite, 2015, 22, 10.
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