Mechanisms of specificity of association between the nematode Steinernema scapterisci and its symbiotic bacterium.

We suggest a new mechanism for the maintenance of specificity of the association between the entomopathogenic nematode Steinernema scapterisci and its symbiotic bacteria. We evaluated the development and reproduction of infective and non-infective juvenile S. scapterisci in monoxenic combinations with its symbiotic bacteria, Xenorhabdus sp. 'S' and with the bacterial symbiont of Steinernema carpocapsae and Steinernema riobravis. Although development of non-infective stages occurred on all Xenorhabdus spp., the development of infective juveniles to the 4th stage ('dauer' recovery) was significantly delayed and reduced with X. nematophilus and Xenorhabdus sp. 'R', the bacterial symbionts of S. carpocapsae and S. riobravis, respectively. 'Dauer' recovery improved significantly when the cultures of X. nematophilus and Xenorhabdus sp. 'R' were supplemented with cell-free filtrates from Xenorhabdus sp. 'S'. The infective juvenile S. scapterisci produced in all 3 cultures were virulent to Galleria mellonella larvae, confirming successful retention of Xenorhabdus from other steinernematids in their intestine. In fact, S. scapterisci infective juveniles containing X. nematophilus or Xenorhabdus sp. 'R' were more virulent to G. mellonella than those containing their natural symbiont, Xenorhabdus sp. 'S'. We believe that this is the first demonstration of the symbiont-specific exit of infective juveniles from the 'dauer' phase which represents the finest level of specificity of bacteria-nematode association. This is also the first report of successful isolation of the natural symbiont of S. scapterisci.