Rank: Species
Lineage: Bacteria;Proteobacteria;Gammaproteobacteria;Enterobacterales;Morganellaceae;Photorhabdus;Photorhabdus asymbiotica
Description: Photorhabdus. Photorhabdus is currently subdivided into three species, luminescens, temperate and asymbiotica all of which have been isolated as symbionts of heterorhabditid nematodes. This organism is unusual in that it is symbiotic within one insect, and pathogenic in another, the only organism that is known to exhibit this dual phenotype. This bacterium lives within the gut of an entomopathogenic nematode that attacks the insect larvae of another species. Once the nematode enters the target insect bloodstream bacteria are released, rapidly killing (24-48 hrs) the target via secreted toxins. Enzymes are then released by the bacteria that result in rapid degradation of the insect body, allowing both bacteria and nematode to feed and reproduce. During this period Photorhabdus releases bacteriocidal products, including antibiotics and bacteriocins, that prevent infection of the larva by competitive microbes. The result is promotion of Photorhabdus-nematode interactions that result in continuation of the symbiotic relationship. In order to engage in a symbiotic relationship with the nematode and a pathogenic one with the insect larva, the bacterium encodes specific factors that encourage both. Pathogenic functions are encoded within a number of pathogenicity islands (PAIs) contained on the bacterial chromsome. These include a large number of genes that code for secreted toxins and enzymes, as well as genes that encode products for the production of antibiotics and bacteriocins. Secretion of these products occurs by an array of systems including type I, type II, and type III secretion systems. The type III system is closely related to the Yersinia plasmid-encoded type III system. Genes that promote symbiotic relationships are also encoded on genomic islands on the chromosome including some that affect nematode development. Virulence genes appear to be active during exponential growth. Symbiotic genes appear to function during stationary phase (post-exponential) growth. The switch from one state to another is controlled.Photorhabdus is capable of giving off light, a complex process that requires the products of the lux operon. The lux system has been used extensively as a molecular marker in many experiments. The exact biological function of bioluminescence is not known, although it may have a role in protection against reactive oxygen species (ROS).Photorhabdus asymbiota. Photorhabdus asymbiota, formerly Xenorhabdus luminescens, has been isolated from human wound and blood infections often in association with spider bites. This species can also be isolated from the entomopathogenic nematode Heterorhabditis indica. Photorhabdus asymbiota is divided into two subspecies, subsp. australis which contains the Australian clinical isolates and subsp. asymbiota which contains the North American isolates.
Reference Material:
https://www.ncbi.nlm.nih.gov/genome/?term=Photorhabdus%20asymbiotica
