Morphological and Functional Dimorphism in Xenorhabdus spp., Bacteria Symbiotically Associated with the Insect Pathogenic Nematodes Neoaplectana and Heterorhabditis

Xenorhabdus spp., entomopathogenic bacteria symbiotically associated with the nematodes Neoaplectana and Heterorhabditis, occur in two forms. In general, only one form, designated the primary form, is transmitted into new hosts by the infective stage of the nematode. The significance of the relationship between the two forms has been examined with X. nematophilus, the symbiont of N. feltiae. The forms of X. nematophilus can be differentiated by their colony characteristics but by only two biochemical tests. The two forms of X.nematophilus are equally pathogenic when injected into the haemocoel of Galleria larvae. However, the primary form when injected into Galleria larvae with axenic nematodes provides better conditions for reproduction of the nematodes than the secondary form, for which a role has not been determined. Although the primary form readily converts to the secondary form in vitro and occasionally in vivo, the secondary form is usually stable. Possible causes of the instability have been investigated.