A total of 342 staphylococci representing 13 different recognized species were screened for plasmid composition using agarose gel electrophoresis techniques. Plasmids of molecular weight (MW) 30 × 106 or larger in size were uncommon in staphylococci. Moderately large plasmids of MW 15 × 106 to 29 × 106 were not or only occasionally found in the species Staphylococcus sciuri, S. intermedius, S. hyicus, or S. simulans, but were common in S. aureus (55%) and members of the S. epidermidis (79%) and S. saprophyticus (86%) species groups. Small plasmids were common in most of the species. They produced more complex profiles in simian subspecies or biotypes than in human subspecies or biotypes, e.g., in the species S. warneri, S. haemolyticus, S. cohnii, S. xylosus, and S. aureus. Although some of the species appeared to have rather unique plasmid patterns, we would currently hesitate to use this feature as a taxonomic character, considering the occurrence of exceptional strains and possible plasmid transfer between species.
Identification and Characterization of Large Plasmids in Rhizobium meliloti using Agarose Gel Electrophoresis
