Entomopathogenic nematodes and their bacterial symbionts: The inside out of a mutualistic association

Entomopathogenic nematodes Steinernema and Heterorhabditis spp. (Nematoda: Steinernematidae, Heterorhabditidae) and their bacterial symbiont bacteria Xenorhabdus and Photorhabdus spp (Gram-negative Enterobacteriaceae) represent an emerging model of terrestrial animal-microbe symbiotic relationships. Xenorhabdus and Photorhabdus spp. are harbored as symbionts in the intestine of the only free-living stage of the nematodes, also known as the infective juvenile or 3rd stage infective juvenile. The bacterium-nematode pair is pathogenic for a wide range of insects and has successfully been implemented in biological control and integrated pest management programs worldwide. Moreover, realization of the practical use of these nematodes has spurred developments across a far broader scientific front. Recent years have seen an intensive worldwide search for fresh genetic materials resulting in an exponential growth of described new species and the discovery of thousands of new isolates worldwide. These nematodes and their bacterial symbionts are now considered a tractable model system that is amenable to study physiological, chemical, structural and developmental aspects of beneficial symbiotic associations. We herein provide an overview of the research done in relation to the study of the symbiotic interactions between Steinernema and Heterorhabditis nematodes and their bacterial symbionts.