6. ➢Obligate aerobes are organisms that grow only in the presence of oxygen.
They obtain their energy through aerobic respiration.
➢Obligate anaerobes are organisms that grow only in the absence of
oxygen and, in fact, are often inhibited or killed by its presence. They
obtain their energy through anaerobic respiration or fermentation.
➢Facultative anaerobes are organisms that grow with or without oxygen,
but generally better with oxygen. They obtain their energy through aerobic
respiration if oxygen is present, but use fermentation or anaerobic
respiration if it is absent. Most pathogenic bacteria are facultative
anaerobes.
➢Aerotolerant anaerobes do not use oxygen to transform energy but can
tolerate and grow in its presence. They obtain energy only by fermentation
and are also known as obligate fermenters.
➢Microaerophiles are organisms that require a low concentration of oxygen
(2% to 10%) for growth, but higher concentrations are inhibitory. They
obtain their energy through aerobic respiration. .
7. OXYGEN REACTIVITY
➢ Oxygen is a very reactive molecule and is a potent
cellular poison unless a cell has a mechanisms to
enzymatically inactivate it.
➢ Various reactive oxygen radicals (ROS) are invariably
generated by cells in the presence of O2.
➢ These ROS are H2O2 (peroxide), superoxide (O2
-), singlet
oxygen (O-), peroxide anions and hydroxyl radicals (OH-).
8. OXYGEN DETOXIFICATION
➢All organisms which can live in the presence of O2
(whether or not they utilize it in their metabolism) contain
enzyme superoxide dismutase, which destroys superoxide
(O2
-) ion.
➢Nearly all organisms also contain the enzyme catalase,
which decomposes H2O2.
➢Some organisms decompose H2O2 by means of peroxidase
enzymes which derive electrons from NADH2 to reduce
peroxide to H2O.
9. The action of superoxide dismutase, catalase and
peroxidase enzymes on toxic oxygen radicals
10. Distribution of superoxide dismutase, catalase and
peroxidase in prokaryotes with different O2 tolerances.
Group Superoxide
dismutase
Catalase Peroxidase
Obligate
aerobes and
most facultative
anaerobes (e.g.
Enterics)
+ + -
Most
aerotolerant
anaerobes (e.g.
Streptococci)
+ - +
Obligate
anaerobes (e.g.
Clostridia)
- - -
11. Terms used to describe O2 Relations of Microorganisms
Group Aerobic Anaerobic O2 Effect
Obligate aerobe Growth No growth Required
(utilized for
aerobic
respiration)
Obligate
anaerobe
No growth Growth Toxic
Facultative
anaerobe
(Facultative
aerobe)
Growth Growth Preferentially
utilized when
available
12. Microaerophile Growth if level
not too high
No growth Required but at
levels below 0.2
atm
Aerotolerant
anaerobe
Growth Growth Not required
and not utilized
14. Capnophiles
▪ A small amount of CO2 is required by all organisms for
growth, which is provided by metabolism.
▪ Some organisms requires relatively high concentrations
of carbon dioxide (1-5%) for their optimum growth,
particularly for primary isolation; these are known as
capnophiles, e.g., Brucella abortus
16. Bacteria have a minimum, optimum, and maximum temperature for
growth and can be divided into following groups based on their
optimum growth temperature (temperature at which an organism
grows best):
➢ Psychrophiles are cold-loving bacteria. Their optimum growth
temperature is between -5°and 15°C with maximum up to 20°C. They
are usually found in the Arctic and Antarctic regions and in streams
fed by glaciers.
➢ Mesophiles are bacteria that grow best at moderate temperatures.
Their optimum growth temperature is between 25° and 40°C with
maximum below 45°C. Most bacteria are mesophilic and include
common soil bacteria and PATHOGENS.
➢ Thermophiles are heat-loving bacteria. Their optimum growth
temperature is between 45° and 70°C and are commonly found in
hot springs and in compost heaps, e.g. Bacillus stearothermophilus
17. ➢ Hyperthermophiles - grow at very high temperatures. Their
optimum growth temperature is between 70° and 110°C. They are
usually members of the Archae and are found growing near
hydrothermal vents at great depths in the ocean, e.g. Sulfolobus
acidocaldarium
▪ Obligate psychrophiles – are unable to grow above 20ºC.
▪ Facultative psychrophiles - are able to grow above 20ºC, but not in
mesophilic range.
▪ Psychrotroph - A variant of a psychrophile which can grow at
temperature in the range of mesophiles
▪ Thermoduric - Mesophilic organisms that can survive brief
exposures to relatively high temperatures are termed thermoduric.
18. ➢ Psychrophilic bacteria are adapted to their cool environment due
to high concentration of unsaturated fatty acids in their plasma
membranes.
➢ Thermophilic bacteria adapted to their hot environment due to high
concentration of highly saturated fatty acids in their plasma
membranes.
➢ Often thermophiles have a high G + C content in their DNA
➢ The membranes of hyperthermophiles are composed of repeating
subunits of the C5 compound, phytane, which help them to live in
superheated environments.
TEMPERATURE TOLERANCE
19. Terms used to describe microorganisms in relation to
temperature requirements for growth
Group Minimum Optimum Maximum Comments
Psychrophile Below 0 10-15 Below 20 Grow best at
relatively low T
Psychrotroph 0 15-20 Above 25 Able to grow at
low T but
prefer
moderate T
Mesophile 10-15 25-40 Below 45 Most bacteria
especially those
living in
association with
warm-blooded
animals
20. Thermoduric 10-15 30-40 45-60 Mesophiles that
can survive brief
exposures to
relatively high
temperatures
Thermophile 45 50-70 Above
100 ºC
(boiling)
Shows wide
variation in
optimum and
maximum T
Hyperthermophile 70 80-110 Above
121
occurring in hot
springs or deep sea
vents in mid- ocean
ridges
22. Microorganisms can be placed in one of the following groups
based on their optimum pH requirements:
Acidophiles - Organisms whose optimum pH is moderately to
highly acidic, usually 5.5. Several genera of Archaea, including
Sulfolobus and Thermoplasma, are obligate acidophiles.
Among eukaryotes, many fungi are acidophiles.
Neutrophiles - Organisms whose optimum pH is 7± 1.5 with a
range of 5 to 8, e.g. Staphylococcus aureus (7.0-7.5), E. coli
(6.0-7.0)
Alkaliphiles - Organisms whose optimum pH is usually above
8.0, e.g. Nitrobacter
23. WATER AVAILABILITY
➢ Water is the solvent in which the molecules of life are dissolved,
and the availability of water is, therefore, a critical factor that
affects the growth of all cells.
➢ The availability of water for a cell depends upon its presence in the
atmosphere (relative humidity) or its presence in solution (water
activity).
➢ Microorganisms live over a range of Aw from 1.0 to 0.7.
➢ The concept of lowering water activity in order to prevent bacterial
growth is the basis for preservation of foods by drying (in sunlight
or by evaporation) or by addition of high concentrations of salt or
sugar.
➢ Xerophiles - Organisms which live in dry environments (made dry by
lack of water).
24. OSMOTIC PRESSURE
Most bacteria require an isotonic environment or a
hypotonic environment for optimum growth.
Osmophiles - organisms that are able to live in
hypertonic environments high in salt or sugar.
Osmotolerant - organisms that can tolerate salt
concentration up to approximately 10%.
25. Depending on the requirement of salt concentrations
microorganisms are classified as follows:
▪ Non-halophiles require less than 1% salt (E. coli or
Pseudomonas)
▪ mild halophiles require 1-6% salt
▪ moderate halophiles require 6-15% salt (S. aureus)
▪ Extreme halophiles require 15-30% NaCl for growth
(archaea such as Halococcus).
▪ Halotolerant - bacteria that are able to grow at moderate
salt concentrations, even though they grow best in the
absence of NaCl.
27. NUTRITIONAL REQUIREMENTS
All living organisms are grouped into different nutritional
groups according to their energy source, carbon source and
hydrogen / electrons source.
Energy source
Phototrophs use light as the primary source of energy
Chemotrophs use chemical compounds as the primary
source of energy
28. Carbon source
Autotrophs require only carbon dioxide as a carbon source. An
autotroph can synthesize organic molecules from inorganic nutrients.
Heterotrophs require organic forms of carbon. A heterotroph cannot
synthesize organic molecules from inorganic nutrients.
Hydrogen/Electron source
Lithotroph use inorganic substrates such as ammonium, nitrite,
hydrogen sulfide or elementary hydrogen as a source of hydrogen/
electron donor
Organotroph use reduced organic compounds such as glucose and
fatty acids as a source of hydrogen/ electron donor
29. Major nutritional types of prokaryotes
Nutritional Type Energy
Source
Carbon
Source
Examples
Photoautotrophs Light CO2 Cyanobacteria,
some Purple and
Green Bacteria
Photoheterotrophs Light Organic
compounds
Some Purple and
Green Bacteria
Chemoautotrophs
or Lithotrophs
(Lithoautotrophs)
Inorganic
compounds,
e.g. H2, NH3,
NO2, H2S
CO2 A few Bacteria and
many Archaea
Chemoheterotrophs
or Heterotrophs
Organic
compounds
Organic
compounds
Most Bacteria,
some Archaea
32. Major elements, their sources and functions in bacterial cells
Element % of
dry
weight
Source Function
Carbon 50 organic
compounds or
CO2
Main constituent of
cellular material
Oxygen 20 H2O, organic
compounds, CO2,
and O2
Constituent of cell
material and cell water;
O2 is electron acceptor in
aerobic respiration
Nitrogen 14 NH3, NO3, organic
compounds, N2
Constituent of amino
acids, nucleic acids
nucleotides, and
coenzymes
33. Hydrogen 8 H2O, organic
compounds, H2
Main constituent of organic
compounds and cell water
Phosphorus 3 inorganic
phosphates (PO4)
Constituent of nucleic acids,
nucleotides, phospholipids, LPS,
teichoic acids
Sulfur 1 SO4, H2S, So,
organic sulfur
compounds
Constituent of cysteine,
methionine, glutathione, several
coenzymes
Potassium 1 Potassium salts Main cellular inorganic cation,
cofactor for enzymes
Magnesium 0.5 Magnesium salts Inorganic cellular cation, cofactor
for enzymes
Calcium 0.5 Calcium salts Inorganic cellular cation, cofactor
for certain enzymes, a component
of endospores.
Iron 0.2 Iron salts Component of cytochromes and
certain nonheme iron-proteins,
cofactor for some enzymes
34. Growth factors
➢ Growth factors are organic compounds such as amino
acids, purines, pyrimidines, and vitamins that a cell must
have for growth but cannot synthesize itself.
➢ Organisms having complex nutritional requirements and
needing many growth factors are said to be fastidious.