Xenorhabdus

Xenorhabdus is a genus of motile, gram-negative bacteria from the family of the Morganellaceae. All the species of the genus are only known to live in symbiosis with soil entomopathogenic nematodes from the genus "Steinernema".
Although no free-living forms of Xenorhabdus have ever been isolated outside of the nematode host, the benefits for the bacteria are still unknown. 
However, it has been demonstrated that the nematode can't establish within his insect host without the bacteria.
The tripartite Xenorhabdus-nematode-insect interaction represents a model system in which both mutualistic and pathogenic processes can be studied in a single bacterial species.
In laboratory, some species are virulent directly injected within the insect host, whereas others species need the nematode to penetrate into the insect.
Lifecycle.
"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. 
Biological pest control.
The mutualistic association between "Xenorhabdus" and "Steinernema" represent an insectidical complex, active against a large range of insect pests. 
Indeed, the complex is used in biological pest control, and is very efficient against insects such as "Spodoptera exigua" (Lepidoptera), "Cydia pomonella" (Lepidoptera), "Leptinotarsa decemlineata" (Coleoptera), family Tipulidae (Diptera). These bacteria inhabit the gut of the Asian corn borer, a moth pest of maize in East Asia, and kills it within 48 hours.
"Xenorhabdus nematophila" is the most widely used species in biological control, in association with "Steinernema carpocapsae" and "S. feltiae".
The pathogenicity of the complex is "species-specific", which means that the complex can only be active against a specific range of insects.
The Steinernema-Xenorhabdus association is currently sold as a biocontrol agent by private companies, like Biobest, SUMI AGRO, and Biosafe.
Perspectives.
A study carried out by Furgani G. & Al
suggests that the antibiotic compounds produced by Xenorhabdus to preserve the insect cadaver from others bacteria may be used in the aim of controlling mastitis caused by bacteria. Indeed, "X. budapestensis", "X. szentirmaii" and "X. nematophila" appear to be efficient against pathogens such as "Staphylococcus aureus" and "Escherichia coli".