Selection of subpopulations resistnat to amikacin and netilmicin of gentamicin-resistant clinical strains of Staphylococcus aureus and Staphylococcus epidermidis.

Recently we have found several strains of Staphylococcus aureus and Staphylococcus epidermidis, which in spite of containing aminoglycoside-modifying enzymes (AMEs) remained susceptible to antibiotics such as netilmicin (NET) and amikacin (AN). Assuming an interest in this agent from a clinical point of view, the aim of this study was to determine if these strains became resistant after prolonged contact with such antibiotics. We found that minimal inhibitory concentrations (MICs) of the bacterial strains not only increased when using these two agents, but also when using other aminoglycosides such as gentamicin (GM), tobramycin (TM), amikacin (AN) and isepamicin (ISE). In order to see the effect of prolonged use of NET on enzyme production, three strains containing AMEs were selected and we could observe an increase in the enzyme levels after successive passages through media containing NET.