Closely related plasmids from Staphylococcus aureus and soil bacilli

Antibiotic resistance plasmids from staphylococci and soil bacilli have been isolated and compared. A tetracycline resistance (Tc^r) plasmid, indistinguishable from pT181, which is typical of Tc^r plasmids that are widely dispersed among human clinical isolates of S. aureus, has been found also in bovine mastitis isolates. This plasmid, however, shows no detectable homology to a family of related Tc^r plasmids, typified by pBC16, that is widely dispersed among aerobic spore-forming bacilli. However, and rather unexpectedly, pBC16 is highly homologous to and incompatible with pUB110, an S. aureus plasmid specifying kanamycin resistance. The two plasmids are homologous except for the region occupied by their resistance determinants, which has the appearance of a heterologous substitution. These results suggest the occurrence of natural plasmid transfer between staphylococci and soil bacilli.