Selection for Vancomycin Resistance in Clinical Isolates of Staphylococcus haemolyticus

Abstract

Killing curves were used to characterize Staphylococcus haemolyticus isolates previously reported to contain subpopulations showing increased resistance to vancomycin. Results suggested that vancomycin and teicoplanin were ineffective at a concentration of 8 µg/ml and growth was seen between 24 and 48 h. Conversely, the lipopeptide antibiotic daptomycin at the same concentration rapidly killed tested strains by 6 h. Various staphylococcal strains were examined to determine if vancomycin resistance could be selected in all strains of staphylococci, was specie(s) restricted, or was unique to this patient—s clinical isolates. About 1 × 10⁸ colony-forming units were added to melted brain-heart infusion agar plates containing 12 µg/ml of vancomycin. Plates were examined after 48 h for presence of resistant clones. Results indicated that selection for vancomycin resistance was restricted to S. haemolyticus strains. Further, all S. haemolyticus isolates that displayed a double zone of growth around imipenem agar diffusion discs (Imp^dz) contained stably resistant subpopulations. Vancomycin resistance could not be selected in imipenem-sensitive derivative clones. Imp^dz isolates that were recovered from geographically distinct locations displayed nearly identical SDS-PAGE protein profiles. It appears that a characteristic susceptibility pattern displayed byclinical isolates of S. haemolyticus may provide a marker for those strains that contain subpopulations having increased resistance to vancomycin.