Bacterial symbionts of entomopathogenic nematodes of the genus Steinernema from South Italy: phenotypic features and evolutionary implications

Three strains of Xenorhabdus kozodoii and seven of X.bovienii, obtained from different species and isolates of the genus Steinernema (S. feltiae, S. vulcanicum, S. kraussei, S. apuliae and S. ichnusae), were analysed by genotypic and phenotypic assays in order to investigate in vitro biofilm production, antibiotics susceptibility and growth properties. For identification purposes we used a genotypic analysis, such as a 16S rDNA partial sequence and ERIC PCR profile. Phenotypic profiles were obtained by testing, among others: growth on selective media, haemolytic activity, ß-galactosidase. Moreover, bacterial toxin was isolated and tested in vivo on insect larvae upon direct injection into the haemolymph. The results indicate that X. bovienii isolates present a broad phenotypic spectrum, particularly for biofilm production and cephalosporin susceptibility, suggesting that this species underwent adaptive changes, presumably following the evolutionary divergence of Steinernema species or isolates. X. kozodoii diverges from X. bovienii for lower levels of biofilm production and for lactose utilisation. However, both species synthesized a highly active toxin, which killed larvae in a manner resembling direct infection by EPN.