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The Microbe Blog (at http://www.smallthingsconsidered.us)

Elio Schaechter and Merry Youle, fellow writer and editor

The Two Faces of Photorhabdus
http://schaechter.asmblog.org/schaechter/2009/03/the-two-faces-of-photorhabdus.html

 

Nematodes-those small, nondescript roundworms-are among the most numerous animals, found in virtually every ecological niche. With an estimated 400,000 to possibly 10,000,000 species, their diversity might match that of the insects, but no one knows for sure. Two families of entomopathogenic nematodes have garnered much attention due to the company they keep. (More about this below.) Some 350 million years ago, each family, it is thought, independently began a mutualistic association with a lineage of enteric bacteria-one with the lineage leading to the genus Photorhabdus, the other with the Xenorhabdus lineage. These nematode-bacterial teams are deadly parasitoids that prey on diverse insects. Both teams have evolved obligate mutualisms with similar life cycles, but the underlying molecular mechanisms are distinctly different. We'll focus on Photorhabdus luminescens, the most "illuminating" example. Although these are soil-dwelling nematodes, only one larval stage, called the infective juvenile (IJ), is found free in the soil. The name is appropriate, as the IJ is arrested at this larval stage and its sole task is to find and enter an insect host. The rest of the life cycle, including reproduction, takes place within a host.

Each IJ carries quiescent Photorhabdus in its gut. Once inside a new host, the IJ migrates to the hemolymph and regurgitates the Photorhabdus. The bacteria quickly launch into a phase pathogenic to the insect, secreting toxins, enzymes, and antibiotics. Toxins effectively disable the insect's innate immune defenses; enzymes convert host tissues into a nutrient soup; antibiotics preserve the cadaver. Experimental injection of fewer than five bacteria into an insect's hemolymph kills within 48-72 hours. In contrast, the insect's innate immune response can handle similar infection by >106 Escherichia coli.

The Photorhabdus grow exponentially for the first 48 hours, reaching populations of 109 per host. By then, all of the host tissue has been converted into bacterial biomass-essentially a monoculture of Photorhabdus. Meanwhile, the IJ exits from diapause, begins to feed, and continues development into an adult hermaphrodite. Photorhabdus then stops growing, and its pathogenic phase gives way to the mutualistic phase. The accumulated bacterial biomass literally nourishes the nematodes, providing most, if not all, of their nutritional requirements.

The nematodes reproduce for several generations inside the cadaver. Then, a final generation of eggs develops into IJs within the body of their mother nematode. They receive a vital bacterial inoculum by maternal transmission, not haphazardly from the population free in the cadaver. Thus armed, the IJs emerge from the cadaver and go forth in search of a new host, to repeat the cycle yet again. Under optimal conditions in the lab, the entire cycle takes 10-20 days and 100,000 IJs can be produced from infection of one insect by a single IJ. Definitely a well-run operation.

Photorhabdus luminescens, as the name suggests, carries out bioluminescence. It also infects wounds. This is the stuff of legends, with stories going back to the American Civil War. It seemed that soldiers with wounds that glowed in the dark were more likely to recover. Their improved prognosis has been attributed to the antibiotics produced by these bacteria. Photorhabdus is the only terrestrial bacteria known to exhibit bioluminescence. They luminesce only during their mutualistic phase, inside the insect cadaver. What use is this? Makes you wonder if Photorhabdus luminescens really needs to be luminescens.

Clarke, D. J. 2008. Photorhabdus: a model for the analysis of pathogenicity and mutualism. Cell. Microbiol. 10:2159-2167.


Talmudic Question of the Month* by Daniel Portnoy

Why don't mammals make antibiotics of the type so readily made by bacteria and fungi?

Answers? Add a comment online to Talmudic Question #46, March 19, 2009.
http://schaechter.asmblog.org/schaechter/2009/03/talmudic-question-46.html

*We use this term to denote questions whose answers cannot be found by a Google search.