VANCOMYCIN-GENTAMICIN SYNERGISM REVISITED - EFFECT OF GENTAMICIN SUSCEPTIBILITY OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

Vancomycin monotherapy of deep-seated staphylococcal infection may be associated with poor bacteriological response, We evaluated 24 unique patient isolates of methicillin-resistant Staphylococcus aureus (MRSA) for vancomycin-gentamicin synergism by determining time-kill curves f or vancomycin at 10 mu g/ml and gentamicin at 1 mu g/ml. Nine MRSA str ains showed high-level gentamicin resistance (HLGR) (MIC, >500 mu g/ml ), acid 15 did not, Vancomycin-gentamicin demonstrated synergism again st none of tile HLGR strains. For the non-HLGR strains, gentamicin aga r dilution MICs ranged from 0.5 to >128 mu g/ml. Vancomycin-gentamicin demonstrated synergism against six of these strains and indifference against nine of them. There was no relationship between the agar dilut ion MIC of gentamicin and the occurrence of synergism against non-HLGR strains. We conclude that a gentamicin MIC of >500 mu g/ml predicts a lack of vancomycin-gentamicin synergism for strains of MRSA. For non- HLGR strains, synergism is not predictable from the gentamicin MIG.