Glycopeptide susceptibility profiles of Staphylococcus haemolyticus bloodstream isolates

Twelve clinical strains of Staphylococcus haemolyticus (eight methicillin r esistant and three methicillin susceptible), isolated from blood cultures b etween 1982 and 1997, were investigated for teicoplanin and vancomycin susc eptibility profiles. On the basis of conventional MIC tests and breakpoints , four isolates were susceptible (MICs, 1 to 8 mug/ml) and eight were resis tant (MICs, 32 to 64 mug/ml) to teicoplanin while all were susceptible to v ancomycin (MICs, 1 to 2 mug/ml). All Pour strains for which the conventiona l teicoplanin MICs were within the range of susceptibility expressed hetero geneous resistance to teicoplanin and homogeneous vancomycin susceptibility . Of the eight strains for which the conventional teicoplanin MICs were wit hin the range of resistance, six expressed heterogeneous and two expressed homogeneous teicoplanin resistance while seven showed heterogeneous vancomy cin resistance profiles (with subpopulations growing on 8 mug of the drug p er mi at frequencies of greater than or equal to 10(-6) for six strains and 10(-7) for one) and one demonstrated homogeneous vancomycin susceptibility . Of six bloodstream isolates of other staphylococcal species (S, aureus, S . epidermidis, and S, simulans), for all of which the conventional teicopla nin MICs were greater than or equal to4 mug/ml and the vancomycin MICs were 2 mug/ml, none exhibited heterogeneous susceptibility profiles for teicopl anin while three showed homogeneous and three showed heterogeneous suscepti bility profiles for vancomycin (with subpopulations growing on 8 mug of the drug per mi found for only one strain). The results of this study indicate that a heterogeneous response to glycopeptides is a common feature of S, h aemolyticus isolates and suggest that susceptibility to glycopeptides as de termined by conventional MIC tests may not be predictive of the outcome of glycopeptide therapy.