We have previously reported the transfer of gentamicin resistance (Gmr) plasmids in a mixed culture inter-and intraspecifically between strains of Staphylococcus aureus and Staphylococcus epidermidis isolated at Michael Reese Hospital (Jaffe et al., Antimicrob. Agents Chemother. 21:773-779, 1982). We have now shown that representatives of these plasmids were transferred between apparently nonlysogenic strains of S. aureus either in mixed culture in broth or by filter-mating on agar medium. The mechanism of transfer appeared to be conjugation. A transferable Gmr plasmid (pSH8) mobilized or cotransferred a tetracycline R-plasmid and a chloramphenicol R-plasmid that were not independently transferable. The transfer of Gmr plasmids was accompanied by a high incidence of deletion mutations with varied loss of plasmid resistance determinants and, with some mutants, loss of the ability to effect self-transfer. Restriction endonuclease digestion of pSH8 and its deletion mutants made it possible to assign the property of self-transfer to a specific segment of the pSH8 genome and provided the basis for a physical and genetic map of that plasmid. Similar Gmr plasmids from S. aureus strains isolated in locations remote from Michael Reese Hospital had resistance determinants and transfer properties comparable to those of pSH8. Our observations provide evidence for the conjugal transfer of some staphylococcal plasmids, apparently independent of the presence of phage. This mechanism may be of significance in the intra-and interspecific dissemination of resistance to aminoglycosides and other antibiotics in Staphylococcus spp.We have reported the recovery of five classes of closely related gentamicin resistance (Gm) plasmids from Staphylococcus aureus and Staphylococcus epidermidis (13,14). Each plasmid class was defined by its spectrum of antibiotic resistance markers, molecular mass, and restriction endonuclease digestion pattern. Apparently identical Gmr plasmids of each class were present in epidemiologically related isolates of S. aureus and S. epidermidis (35). Similar results have recently been reported by others (2). Evidence of identity between tetracycline resistance plasmids in isolates of S. aureus and S. epidermidis has also been published recently (10,12,34). These findings support the hypothesis that the transfer of plasmids between S. aureus and S. epidermidis occurs in nature.Transduction has been favored as the principal mechanism of genetic exchange between S.

The hypothesis that emergence of gentamicin-resistant strains of Staphylococcus aureus and Staphylococcus epidermidis in a neonatal special care nursery was the result of transfer of a single plasmid between these two species was examined. In experiments with mixtures of staphylococci, either in mixed cultures or on human skin, isolates of S. aureus and S. epidermidis transferred their gentamicin-resistance plasmids both intra- and interspecifically. By electron microscopy, the molecular masses of the plasmids from S. aureus and S. epidermidis were the same, 12.2 +/- 0.36 (standard deviation) and 12.3 +/- 0.56 megadaltons, respectively. Restriction endonuclease analysis of the plasmids from five isolates of S. aureus and two isolates of S. epidermidis, with use of the enzymes HaeIII, EcoRI, XbaI, and HindIII, showed no differences in the digestion patterns of the seven gentamicin-resistance plasmids. The results supported the hypothesis that plasmid transfer between S. aureus and S. epidermidis occurs in nature.

We previously described a neonatal nursery epidemic of infections caused by a single strain of Staphylococcus aureus bearing a gentamicin resistance plasmid (Vogel et al., Antimicrob. Agents Chemother. 13:466-472, 1978

Resistance to methicillin was transduced by phage 80 or 53 from two naturally occurring methicillin-resistant strains of Staphylococcus aureus to methicillinsusceptible recipient strains at frequencies of 10-v to 10-9. Ultraviolet irradiation of transducing phage and posttransductional incubation at 30 C were essential for useful frequencies of transduction. Effectiveness as a recipient for this transduction was highly specific. Strain NCTC 8325 (PS47) in its native state was an ineffective recipient but became effective after it had received by transduction one of several penicillinase plasmids. This acquired effectiveness was retained in most cases after elimination of the plasmid by ethidium bromide treatment. Like the donor strain, the progeny were heterogeneous in the degree of their resistance to methicillin, which was expressed by a higher proportion of cells as the temperature of incubation was lowered from 37 to 30 C. Separate transductants varied widely in the degree of resistance acquired by transduction. Methicillin resistance was stable in the donor and transductant strains. We favored the interpretation that methicillin resistance in our strains was determined by a single chromosomal gene, although the possibility that it was determined by two or more closely linked genes could not be excluded.