Burkholderia cenocepacia AU1054 is an opportunistic pathogen isolated from the blood of a person ... more Burkholderia cenocepacia AU1054 is an opportunistic pathogen isolated from the blood of a person with cystic fibrosis. AU1054 is a multihost pathogen causing rapid pathogenicity to Caenorhabditis elegans nematodes. Within 24 h, AU1054 causes greater than 50% mortality, reduced growth, emaciated body, distended intestinal lumen, rectal swelling, and prolific infection of the nematode intestine. To determine virulence mechanisms, 3,000 transposon mutants were screened for attenuated virulence in nematodes. Fourteen virulence-attenuated mutants were isolated, and the mutant genes were identified. These genes included paaA , previously identified as being required for full virulence of B. cenocepacia K56-2. Six mutants were restored in virulence by complementation with their respective wild-type gene. One of these contained an insertion in gspJ , predicted to encode a pseudopilin component of the type 2 secretion system (T2SS). Nematodes infected with AU1054 gspJ had fewer bacteria pres...
Microbial populations stochastically generate variants with strikingly different properties, such... more Microbial populations stochastically generate variants with strikingly different properties, such as virulence or avirulence and antibiotic tolerance or sensitivity. Photorhabdus luminescens bacteria have a variable life history in which they alternate between pathogens to a wide variety of insects and mutualists to their specific host nematodes. Here, we show that the P. luminescens pathogenic variant (P form) switches to a smaller-cell variant (M form) to initiate mutualism in host nematode intestines. A stochastic promoter inversion causes the switch between the two distinct forms. M-form cells are much smaller (one-seventh the volume), slower growing, and less bioluminescent than P-form cells; they are also avirulent and produce fewer secondary metabolites. Observations of form switching by individual cells in nematodes revealed that the M form persisted in maternal nematode intestines, were the first cells to colonize infective juvenile (IJ) offspring, and then switched to P fo...
The nematode Heterorhabditis bacteriophora is the vector for transmitting the entomopathogenic ba... more The nematode Heterorhabditis bacteriophora is the vector for transmitting the entomopathogenic bacterium Photorhabdus luminescens between insect larvae. The dauer juvenile (DJ) stage nematode selectively retains P. luminescens in its intestine until it releases the bacteria into the hemocoel of an insect host. We report the results of studying the transmission of the bacteria by its nematode vector. Cells of P. luminescens labeled with green fluorescent protein preferentially colonized a region of the DJ intestine immediately behind the basal bulb, extending for various distances toward the anus. Incubation of DJ nematodes in vitro in insect hemolymph induced regurgitation of the bacteria. Following a 30-min lag, the bacteria migrated in a gradual and staggered movement toward and ultimately exited the mouth. This regurgitation reaction was induced by a low-molecular-weight, heat- and protease-stable, anionic component present in arthropod hemolymph and in supernatants from insect cell cultures. Nematodes anesthetized with levamisole or treated with the antihelmenthic agent ivermectin did not release their bacteria into hemolymph. The ability to visualize P. luminescens in the DJ nematode intestine provides the first clues to the mechanism of release of the bacteria during infection of insect larvae. This and the partial characterization of a component of hemolymph triggering release of the bacteria render this fascinating example of both a mutualistic symbiosis and disease transmission amenable to future genetic and molecular study.
Photorhabdus is a member of the family Enterobacteriaceae that lives in a mutualistic association... more Photorhabdus is a member of the family Enterobacteriaceae that lives in a mutualistic association with a Heterorhabditis nematode worm. The nematode worm burrows into insect prey and regurgitates Photorhabdus, which goes on to kill the insect. The nematode feeds off the growing bacteria until the insect tissues are exhausted, whereupon they reassociate and leave the cadaver in search of new prey. This highly efficient partnership has been used for many years as a biological crop protection agent. The dual nature of Photorhabdus as a pathogen and mutualist makes it a superb model for understanding these apparently exclusive activities. Furthermore, recently identified clinical isolates of Photorhabdus are helping us to understand how human pathogens can emerge from the enormous reservoir of invertebrate pathogens in the environment. As Photorhabdus has never been found outside a host animal, its niche represents an entirely biotic landscape. In this review we discuss what molecular adaptations allow this bacterium to complete this fascinating and complex life cycle.
Burkholderia cenocepacia AU1054 is an opportunistic pathogen isolated from the blood of a person ... more Burkholderia cenocepacia AU1054 is an opportunistic pathogen isolated from the blood of a person with cystic fibrosis. AU1054 is a multihost pathogen causing rapid pathogenicity to Caenorhabditis elegans nematodes. Within 24 h, AU1054 causes greater than 50% mortality, reduced growth, emaciated body, distended intestinal lumen, rectal swelling, and prolific infection of the nematode intestine. To determine virulence mechanisms, 3,000 transposon mutants were screened for attenuated virulence in nematodes. Fourteen virulence-attenuated mutants were isolated, and the mutant genes were identified. These genes included paaA , previously identified as being required for full virulence of B. cenocepacia K56-2. Six mutants were restored in virulence by complementation with their respective wild-type gene. One of these contained an insertion in gspJ , predicted to encode a pseudopilin component of the type 2 secretion system (T2SS). Nematodes infected with AU1054 gspJ had fewer bacteria pres...
Microbial populations stochastically generate variants with strikingly different properties, such... more Microbial populations stochastically generate variants with strikingly different properties, such as virulence or avirulence and antibiotic tolerance or sensitivity. Photorhabdus luminescens bacteria have a variable life history in which they alternate between pathogens to a wide variety of insects and mutualists to their specific host nematodes. Here, we show that the P. luminescens pathogenic variant (P form) switches to a smaller-cell variant (M form) to initiate mutualism in host nematode intestines. A stochastic promoter inversion causes the switch between the two distinct forms. M-form cells are much smaller (one-seventh the volume), slower growing, and less bioluminescent than P-form cells; they are also avirulent and produce fewer secondary metabolites. Observations of form switching by individual cells in nematodes revealed that the M form persisted in maternal nematode intestines, were the first cells to colonize infective juvenile (IJ) offspring, and then switched to P fo...
The nematode Heterorhabditis bacteriophora is the vector for transmitting the entomopathogenic ba... more The nematode Heterorhabditis bacteriophora is the vector for transmitting the entomopathogenic bacterium Photorhabdus luminescens between insect larvae. The dauer juvenile (DJ) stage nematode selectively retains P. luminescens in its intestine until it releases the bacteria into the hemocoel of an insect host. We report the results of studying the transmission of the bacteria by its nematode vector. Cells of P. luminescens labeled with green fluorescent protein preferentially colonized a region of the DJ intestine immediately behind the basal bulb, extending for various distances toward the anus. Incubation of DJ nematodes in vitro in insect hemolymph induced regurgitation of the bacteria. Following a 30-min lag, the bacteria migrated in a gradual and staggered movement toward and ultimately exited the mouth. This regurgitation reaction was induced by a low-molecular-weight, heat- and protease-stable, anionic component present in arthropod hemolymph and in supernatants from insect cell cultures. Nematodes anesthetized with levamisole or treated with the antihelmenthic agent ivermectin did not release their bacteria into hemolymph. The ability to visualize P. luminescens in the DJ nematode intestine provides the first clues to the mechanism of release of the bacteria during infection of insect larvae. This and the partial characterization of a component of hemolymph triggering release of the bacteria render this fascinating example of both a mutualistic symbiosis and disease transmission amenable to future genetic and molecular study.
Photorhabdus is a member of the family Enterobacteriaceae that lives in a mutualistic association... more Photorhabdus is a member of the family Enterobacteriaceae that lives in a mutualistic association with a Heterorhabditis nematode worm. The nematode worm burrows into insect prey and regurgitates Photorhabdus, which goes on to kill the insect. The nematode feeds off the growing bacteria until the insect tissues are exhausted, whereupon they reassociate and leave the cadaver in search of new prey. This highly efficient partnership has been used for many years as a biological crop protection agent. The dual nature of Photorhabdus as a pathogen and mutualist makes it a superb model for understanding these apparently exclusive activities. Furthermore, recently identified clinical isolates of Photorhabdus are helping us to understand how human pathogens can emerge from the enormous reservoir of invertebrate pathogens in the environment. As Photorhabdus has never been found outside a host animal, its niche represents an entirely biotic landscape. In this review we discuss what molecular adaptations allow this bacterium to complete this fascinating and complex life cycle.
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