Frequencies of subpopulations of aminoglycoside- and vancomycin-resistant variants in Staphylococcus aureus and Staphylococcus epidermidis

Selection and regrowth of resistant variants, which are present in low frequencies in the initial inoculum, were seen when large inocula of five strains of Staphylococcus aureus and four strains of Staphylococcus epidermidis were incubated in broth with amikacin, gentamicin, netilmicin and tobramycin. Statistical analysis showed no significant difference between the aminoglycosides in the selective growth of resistant variants (P > 0.5). Vancomycin differed significantly from the aminoglycosides in both the frequency of, and selection of resistant variants (P < 0.001). No bacteria resistant to > 1 x MIC was seen in the vancomycin-exposed cultures of S. aureus and S. epidermidis, while in most aminoglycoside-exposed cultures, bacteria resistant to 4-16 x MIC were seen.