Homology among nearly all plasmids infecting three Bacillus species

We have surveyed naturally occurring plasmids in strains of Bacillus subtilis and the closely related species B. mojavensis and B. licheniformis. Previous studies have failed to find host-benefitting functions for plasmids of these species, suggesting that these plasmids are nonmutualistic. Only one type of plasmid was found in each plasmid-bearing strain, suggesting that most of the plasmids infecting these Bacillus species are in the same incompatibility group. A sample of 18 plasmids from these species ranged in size from 6.9 to 16 kb, with all but 6 plasmids falling into three size groups. These groups differed in the sizes of their host ranges and geographical ranges. All but 1 of the 18 plasmids from these three host species are homologous with one another. The cryptic plasmids from these three species are far less diverse than are plasmids (from other species) that are known to benefit their bacterial hosts. The low-level diversity among these cryptic plasmids is consistent with the hypothesis that host-benefitting adaptations play an important role in fostering the coexistence of plasmid populations, but other explanations for the low-level plasmid diversity are possible. Comparison of the phylogenies of the plasmids with those of their hosts suggests that Bacillus plasmids are horizontally transferred in nature at a low rate similar to that found for the colicin plasmids of Escherichia coli.