Synergistic activity between vancomycin or teicoplanin and gentamicin or tobramycin against pathogenic diphtheroids

The in vitro activities of vancomycin and teicoplanin alone and in combination with gentamicin or tobramycin were studied by time-kill techniques with 11 strains of pathogenic diphtheroids (Corynebacterium group JK). The activities of vancomycin and teicoplanin were similar (MIC for 90% of strains tested [MIC90], 1 microgram/ml), as were those of gentamicin and tobramycin (the MIC90 was 1 microgram/ml for five aminoglycoside-susceptible strains, and the MIC90 was greater than 1,024 micrograms/ml for six aminoglycoside-resistant strains). No consistent synergistic killing could be demonstrated by the combination of glycopeptide and aminoglycoside antibiotics at arbitrarily chosen concentrations within the range of clinically achievable levels. However, by careful adjustment of both vancomycin and gentamicin concentrations within a narrow range below the MIC of each antibiotic, synergistic killing could be seen with an aminoglycoside-susceptible strain but not with an aminoglycoside-resistant strain. Synergism between glycopeptide and aminoglycoside antibiotics occurs with some diphtheroid organisms, but it may not be clinically relevant.