Plasmids and phase variation in Xenorhabdus spp

Three strains of Xenorhabdus nematophilus (A24, F1, NC116) and strain Dan of Xenorhabdus bovienii were tested to evaluate whether the phase variation observed in these bacteria was in any way connected with plasmids. The plasmid patterns of both phases of A24 and F1 strains were the same, whereas the two NC116 phases had only one band each. No difference was observed between the undigested or digested plasmid patterns of the two phases from the three strains. No plasmid was detected in either phase of strain Dan. The plasmid probes were prepared from the six bands of A24 phase 1. By hybridization studies, three plasmids in two forms (open circular and supercoiled) were detected in the strain A24. Two were estimated at 12 kb, and the smallest was about 4 kb. Attempts to hybridize plasmid probes with either undigested or digested chromosomal DNA of the two phases of strain A24 were unsuccessful. The results suggest that neither a difference in plasmid content nor a plasmid recombination with the chromosome is involved in phase variation. The hybridizations revealed homologous DNA sequences among the three plasmids of strain A24 and among the plasmids of strains such as A24 and NC116, which were isolated from geographically distant countries, suggesting that plasmids may encode similar proteins.