Eating in, eating out

For a Kafkaesque tale of murder and mayhem, of matricide and insecticide, of pathogenesis and mutualism, and yes, of friendship and cooperation, we now turn our attention to Photorhabdus luminescens. But, be warned: this is not a tale for the weak-hearted! P. luminescens, a close relative of the Yersiniae, are gram-negative bacteria that reside peacefully within the gut of the nematode, Heterorhabditis spp.¹ The free-living form of this nematode is found in the soil as a non-feeding and long-lived ‘infective juvenile’ (IJ). The IJ actively seeks out any of a number of insect larvae within which it will complete the rest of its life cycle. Once a victim is found, the nematode penetrates into the blood stream of the unfortunate larva and regurgitates its bacterial partner. L-proline, the most abundant amino acid in insect blood, signals the bacteria to turn on an array of virulence genes and also serves as its preferred energy source.² These genes are involved in adherence, colonization and subversion of the insect innate immune system. P. luminescens also encodes the greatest number of putative toxins compared to any other sequenced bacterial genome.

The insect larva does not have a chance against this onslaught. Within two days, the insect tissue is replaced by a nutrient broth supporting the growth of about 10⁹ bacteria. As the bacteria reach post-exponential phase of growth, the larva dies and all that remains is an intact cuticle. Twenty-two loci on the P. luminescens chromosome, accounting for nearly 6% of the bacteria's genome, are predicted to encode polyketide synthases and non-ribosomal peptide synthases. These genes contribute to the production of a number of secondary metabolites and antibiotics that prevent other bacteria and fungi from taking advantage of the rich nutrient biomass.

In this idyllic environment, the IJ awakens and feeds on the exponentially growing monoculture of P. luminescens. The Heterorhabditis worms are fastidious eaters and will only feed on P. luminescens, although the basis for this specificity is currently unclear (other worms have a similar one-to-one relationship with their chosen bacteria). The worm completes its development into an adult hermaphrodite even as the insect larva is in the last throes of its unfortunate existence. The 200–300 eggs produced by the hermaphrodite hatch, feed on the bacteria and develop through the four larval stages; reproduction occurs for another 1–2 generations. A few bacteria that avoid becoming worm food enter the gut intact and migrate to the rectal gland.

In the final generation, the eggs hatch inside the adult hermaphrodite and consume the mother for food, a process colorfully named matricida endotokia. This generation responds to an unknown signal and enters the alternative IJ stage. At the same time, rupture of the parental rectal gland releases P. luminescens into the body cavity. The developing juveniles inherit a single bacterium from this population. The bacterium colonizes the pre-intestinal valve of the IJ, replicating to yield about 100 bacteria. A single IJ infecting an insect can result in the production of 10⁵ worms in the 10–20 days it takes to complete the cycle (Fig. 1). Not exactly Phoenix rising from the ashes, and perhaps less colorful than a butterfly emerging from a cocoon, but definitely awe-inspiring.

Figure 1.

Murder most foul: An insect cadaver infected with a single IJ and dissected after seven days. The intact insect, cuticle, the large numbers of nematodes and the bacterial biomass (arrow) can be clearly seen. Image kindly provided by Dr. David Clarke, University College, Cork, Ireland.

The alternate lifestyles of P. luminescens help address fundamental questions about mutualism and pathogenesis.³ A recent study identified P. luminescens mutants unable to colonize the nematode; several, but not all, of the mutants were also unable to infect insects suggesting both common and divergent mechanisms for the two modes of interaction. The mutations were primarily in genes involved in assembly and maintenance of lipopolysaccharide and other cell surface molecules. P. luminescens is the only known terrestrial bacterium that is bioluminescent. There is emerging evidence to suggest that luminescence may facilitate the mutualistic relationship between bacteria and their hosts. In a fascinating study of the association between the marine squid Euprymna scolopes and its bioluminescent tenant Vibrio fischerii, it was demonstrated that luminescence (or perhaps the consequent lowering of oxygen concentrations due to the luminescence reaction) induces the expression of genes that facilitate colonization of the squid's light organ.⁴

In keeping with its Jekyll-and-Hyde personality, Photorhabdus spp. can be either a friend or foe to humans. The P. luminescens-nematode combination is such a powerful insect killing machine that it has been marketed commercially for use on crops. Photorhabdus spp. and other related bacteria produce a vast array of secondary metabolites that are currently being explored for potential pharmacological and other uses. It has been speculated that the increased survival rates of civil war soldiers with ‘glowing wounds’ could be due to antibiotics produced by P. luminescens that might have colonized the wounds. But then again, there is Photorhabdus asymbiotica, an emerging pathogen of humans capable of causing soft tissue infections and disseminated bacteremia in immunocompromised humans. P. asymbiotica virulence in humans may have resulted from the acquisition of a plasmid related to pMT1, the virulence plasmid in the plague bacillus, Yersinia pestis.⁵ Photorhabuds certainly has our attention now, and it promises to keep it that way for some time to come!