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 spp1. 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 source2. 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.