Studies on multi-antibiotic resistant strains of Staphylococcus aureus.

Twenty-seven isolates of Staphylococcus aureus were found to be resistant to methicillin, nafcillin, tobramycin, gentamicin, amikacin, cefoxitin, clindamycin, erythromycin and chloramphenicol by disc diffusion testing and tube dilution studies; they were sensitive to cefamandole, cephalothin and vancomycin by disc testing. A discrepancy between minimum inhibitory and bactericidal concentrations was noted for the cephalosporins which was not appreciated on disc testing. All isolates were very sensitive to rifampin (minimum bactericidal concentration 0.3 mg/l). All isolates were susceptible to phage type 83A. A subpopulation of variant small-colony forms of Staphylococcus aureus was recovered when superinhibitory amounts of aminoglycosides and methicillin were used. The variant Staphylococcus aureus strains retained the same antibiotic susceptibility patterns and phage type as their parent strain of Staphylococcus aureus. Further studies are in progress concerning these isolates and their impact on the hospital flora at our institution.