Transfer of resistance plasmids from Staphylococcus epidermidis to Staphylococcus aureus: evidence for conjugative exchange of resistance

Abstract

The ability of S. epidermidis to transfer antimicrobial resistance to S. aureus was tested by mixed culture on filter membranes. Two of 6 clinical isolates examined were able to transfer resistance to S. aureus strains 879R4RF, RN450RF and UM1385RF. Subsequent S. aureus transconjugants resulting from matings with S. epidermidis donors were able to serve as donors to other S. aureus strains at similar frequencies. Cell-free and mitomycin C-induced filtates of donors and transconjugants showed no plaque-forming ability. Addition of DNAse I, citrate, EDTA, calcium chloride and human sera to mating mixes and agar showed no effect on transfer. Nonviable donor cells were unable to transfer resistance and transfer did not occur at 4.degree. C. Cell-to-cell contact was required since transfer did not occur in broth or when filters of donor and recipient, respectively, were placed back-to-back so cells were not in direct contact. Analysis of DNA from S. epidermidis isolate UM899, its subsequent S. aureus transconjugants and cured derivatives demonstrated that all resistance markers which transferred resided on plasmids. Mating experiments suggested a central role for the gentamicin plasmid pAM899-1 in the transfer process. The results are consistent with a conjugative transfer of resistance from S. epidermidis to S. aureus analogous to plasmid transfer demonstrated in streptococcal species for plasmids such as pAM.beta.1. This represents a novel mechanism for gene exchange among staphylococci.