Bacterial symbionts of insect pathogenic nematodes of the families Steinernematidae and Heterorhabditidae

Insect pathogenic nematodes of the families Steinermatidae and Heterorhabditidae from 42 populations (four, genera and over nine species) from Australasia, Europe and North America were found to be symbiotically associated with bacteria of the genus Xenorhabdus. A taxonomic study of Xenorhabdus showed that the bacterial symbionts of the Heterorhabditidae were all X. Zuminescens and those of the Steinernematidae, with one possible exception, were all X. nematophilus. Numerical analysis of the taxonomic data indicated that the genus might be heterogeneous. Although the symbionts of most of the Steinernematidae (including Steinernema kraussei which had previously been reported to be associated with a Flavobacterium sp.) were classified within one species, the differences between the bacteria associated with the various steinernematid species were great enough to warrant the erection of four subspecies. Results obtained with some tests used in the taxonomic study differed from those previously reported. Amended descriptions of the genus Xenorhabdus and its two species are proposed. Each Xenorhabdus species was found to produce two forms of colony. One form, designated the primary form, promoted significantly greater nematode fecundity than did the secondary; it produced antimicrobial substances whereas the secondary did not; and it was the form usually found in the infective stage nematodes. The primary form was unstable under many conditions, resulting in production of the secondary form which in two subspecies was also unstable reverting to the primary form. Attempts were made to elucidate the mechanism determining the change of form.