Bacteria Genomes - PHOTORHABDUS LUMINESCENS

Photorhabdus luminescens is a Gram-negative gamma proteobacterium belonging to the family Enterobacteriaceae. It is symbiotic with soil entomopathogenic nematodes of the family Heterorhabditidae and pathogenic to a wide range of insects.

P. luminescens promotes its own transmission among susceptible insect populations using its nematode host. Upon invasion of the host the bacteria are released directly into the open blood system of the insect. Its life cycle comprises a symbiotic stage in the nematode's gut and a virulent stage in the insect larvae, which it kills through toxemia and septicemia.

After the nematode attacks a prey insect and P. luminescens is released, the bacterium produces a wide variety of virulence factors ensuring rapid insect killing. Bioconversion of the insect cadaver by exoenzymes produced by the bacteria allow the it to multiply and the nematode to reproduce. During this process P. luminescens produces antibiotics to prevent invasion of the insect cadaver by bacterial or fungal competitors and the carcass becomes visibly luminescent due to the bioluminescence of P. luminescens.

Once the infection is established, the insect cadaver is bioconverted into a source of nutrients for both the bacteria and the nematodes. Infective juvenile nematodes subsequently re-acquire the bacteria and leave the insect to infect new hosts.

Iron acquisition seems to be of particular importance for the life cycle of P. luminescens, as this bacterium has the largest known set of iron, heme, hemin and siderophore transporters. Within an insect, P. luminescens needs to get access to the available nutrients and has to deal with low-iron conditions.

P. luminescens has also been found as an opportunistic pathogen in human wounds. It was suspected of causing wounds of some Civil War combatants to glow, thus providing a curative effect against infections caused by other microbes.

Now that the complete genome has been sequenced, the next challenge is to identify genes involved in symbiosis that could be used to increase the production of the worms for the biological control of insects. In tests Photorhabdus luminescens reduced Colorado potato beetles by 100 percent in lab conditions. The potato beetle is notorious for developing resistance to insecticides, so scientists are seeking non-chemical controls as possible natural insecticides.

Strain TT01 possesses a single circular chromosome of 5,688,987 bp with an average GC content of 42.8% and has the largest genome of its class, including E. coli.

References:

Nat. Biotechnol. , 21 (11):1307-13 (2003)
http://www.genomenewsnetwork.org/articles/10_03/toxic_glow.shtml
http://jade.ccccd.edu/starnes/bacterium.htm
http://www.ars.usda.gov/is/pr/2001/010529.htm
http://www.seedquest.com/News/releases/usa/Usda/n3915.htm