• In the non-infestant-stage nematode living in the soil,
Xenorhabdus spp. are carried in a specialized region of the
intestine, termed the receptacle. • At the third-stage of development, the infective juvenile (IJs) invade the
hemocoel of susceptible insect hosts. • The bacteria are released in the insect hemocoel, where they overcome the insect's defense systems and produce numerous virulence factors such as hemolysin and cytotoxin. They participate in suppressing insect immunity and killing the host. • The bacteria proliferate to high levels in the insect cadaver and produce diverse antimicrobial compounds that suppress the growth of antagonistic microorganisms.
Xenorhabdus spp. also secrete an array of exoenzymes that stimulate macromolecular degradation, the products of which, together with the bacteria themselves, are thought to provide a nutrient base for
nematode growth and reproduction. • When nematode numbers become high and nutrients become limiting in the insect cadaver, nematode progeny re-associate with bacteria and differentiate into colonized, non-feeding IJs that emerge into the soil to forage for new hosts.
Xenorhabdus, like
Photorhabdus bacteria, has a striking feature of phase variation. Phase I variants are involved in the symbiotic relationship with entomopathogenic nematodes and are isolated from the nonfeeding infective stage nematodes and the body cavities of insects killed by these nematodes. No role in symbiosis has yet been determined for phase II, which is associated only with entomopathogenic nematodes under laboratory conditions. ==Phylogeny==