Early Colonization Events in the Mutualistic Association between Steinernema carpocapsae Nematodes and Xenorhabdus nematophila Bacteria

Abstract

The bacterium Xenorhabdus nematophila is a mutualist of the entomopathogenic nematode Steinernema carpocapsae. During its life cycle, the bacterium exists both separately from the nematode and as an intestinal resident of a nonfeeding nematode form, the infective juvenile (IJ). The progression of X. nematophila from an ex vivo existence to a specific and persistent colonization of IJs is a model to understand the mechanisms mediating the initiation and maintenance of benign host-microbe interactions. To help characterize this process, we constructed an X. nematophila strain that constitutively expresses green fluorescent protein, which allowed its presence to be monitored within IJs. Using this strain, we showed that few bacterial cells initiate colonization of an individual IJ and that these grow inside the lumen of the IJ intestine in a reproducible polyphasic pattern during colonization. In accordance with these two observations, we demonstrated that the final population of bacteria in a nematode is of predominantly monoclonal origin, suggesting that only one or two bacterial clones initiate or persist during colonization of an individual nematode. These data suggest that X. nematophila initiates IJ colonization by competing for limited colonization sites or resources within the nematode intestine. This report represents the first description of the biological interactions occurring between X. nematophila and S. carpocapsae during the early stages of the colonization process, provides insights into the physiology of X. nematophila in its host niche, and will facilitate interpretation of future data regarding the molecular events mediating this process.The mutualistic, monospecific partnership between the gram-negative enterobacterium Xenorhabdus nematophila and its entomopathogenic nematode host, Steinernema carpocapsae, is one of several emerging models to study benign hostmicrobe interactions (9, 13, 27). The natural life cycle of S. carpocapsae nematodes includes reproductive stages that occur exclusively within larval-stage insects and that are not colonized by X. nematophila and also includes a nonreproductive, nonfeeding, soil-dwelling stage known as the infective juvenile (IJ), which is colonized, at a discrete intestinal location termed the vesicle, by a monoculture of X. nematophila bacteria (9).Colonized IJs exist in the soil until they invade a susceptible insect host, seeking the blood system or hemolymph. Once there, the IJs release their X. nematophila symbionts by defecation (19, 31; E. C. Martens and H. Goodrich-Blair, unpublished data). At this point in their life cycles, bacteria and nematodes exist separately although in close proximity to one another. The released X. nematophila bacteria contribute to the killing of the insect host and grow to high density in the resulting cadaver. These specific bacteria are essential for nematode growth and development presumably both by serving as a direct food source and by supplying nutrients through degradation of the insect carcass (2, 22).When nema...

Interference competition in entomopathogenic nematodes: male Steinernema kill members of their own and other species

“…Steinernema can have several generations within the host. As the host resources are depleted IJs form, are colonised by the symbiont (Martens et al, 2003) and disperse in search of fresh hosts. More than 100,000 IJs can be produced from a single host cadaver (Dutky et al, 1964;Lindegren et al, 1993;Shapiro-Ilan et al, 2002).…”

Olfaction shapes host–parasite interactions in parasitic nematodes

“…IJs then infect the host either by entering through natural orifices or by penetrating through the insect cuticle (20). Following infection, IJs release a bacterial endosymbiont into the insect host and resume development (21)(22)(23). The bacteria proliferate inside the insect, producing an arsenal of secondary metabolites that lead to rapid insect death and digestion of insect tissues.…”

Ready or Not: Microbial Adaptive Responses in Dynamic Symbiosis Environments

“…Once a few bacterial cells colonize the IJ receptacle, they grow slowly into a population that persists in the host for periods lasting up to months until the nematode enters a new insect host (30,52). The IJ receptacle has some spatial and temporal nutrient complexity that could help trigger adaptive gene expression as IJs age.…”