Résumé
Background: The infective juveniles (free form) of the nematodes Steinernema are in closed symbiotic association with the intestinal bacteria Xenorhabdus (Enterobacteriaceae). The couple Steinernema – Xenorhabdus represents an effective insecticidal complex against a wide range of insect pests. The Steinernema carpocapsae – Xenorhabdus nematophila couple was the most studied. The bacterial symbiont X. nematophila is an entomopathogenic bacterium since it kills several lepidopteran insects after injection without its nematode host. By contrast, other Xenorhabdus species belonging to other Steinernema – Xenorhabdus couples have recently been identified as non-virulent against lepidopteran insects.Objectives: The existence of non-virulent Xenorhabdus symbiont led us to search for an auxiliary microbiota that could participate to the Steinernema – Xenorhabdus infectivity in the insect. Methods: The auxiliary bacterial microbiota of five Steinernema – Xenorhabdus couples was isolated by classical culture techniques after crushing of infective juveniles. This microbiota was compared with the one isolated in lepidopteran model insects infested by the same Steinernema – Xenorhabdus couples. Biochemical (API tests) and molecular (sequencing of the 16S rRNA gene) characterization were undergone to identify the OTUs. Antimicrobial activities against Xenorhabdus strains and pathology assays by direct injection into larvae of Spodoptera littoralis were conducted for identified taxon.Conclusion: We identified up to seven cultivable colonial morphotypes associated with each of the five Steinernema – Xenorhabdus couples. The matching OTUs belong to the genera Delftia, Providencia, Acinetobacter, Serratia, Stenotrophomonas, Ochrobactrum and Pseudomonas depending on the Steinernema – Xenorhabdus couples. Some of them were also identified in insect cadavers after Steinernema – Xenorhabdus infestation and displayed antimicrobial and virulence properties. Altogether, these data suggest that other bacterial taxons than Xenorhabdus could participate to the infectious process in insect.