Decreased Susceptibilities to Teicoplanin and Vancomycin among Coagulase-Negative Methicillin-Resistant Clinical Isolates of Staphylococci

Of 41 methicillin-resistant coagulase-negative staphylococcal clinical isolates collected during a 5-month period between late 1995 and early 1996, 28 showed tube dilution teicoplanin MICs of 4 to 8 mg/ml which increased to 16 to 32 mg/ml upon prolonged incubation. Cultures of such bacteria were heterogeneous; they contained subpopulations with frequencies of 10 25 to 10 24 that could grow on up to 50 mg of teicoplanin per ml. The same cultures were also heterogeneous with respect to susceptibility to vancomycin; while the MICs for the majority of cells were 2 to 4 mg/ml, subpopulations that could grow on 6 to 12 mg of vancomycin per ml were also present at frequencies of 10 25 to 10 27 . Selective enrichment of such cultures for the resistant subpopulation occurred with relative ease under laboratory conditions. Heterogeneous phenotypes for teicoplanin (but not for vancomycin) susceptibility were also identified in several Staphylococcus epidermidis isolates collected during the preantibiotic era. The addition of half the MIC of teicoplanin inhibited autolysis and caused formation of cellular aggregates which disintegrated to individual bacteria in the stationary phase when the titer of teicoplanin in the medium fell to undetectable levels, indicating removal of the antibiotic from the culture medium by the bacteria. The appearance of resistance mechanisms against glycopeptide antibiotics among clinical isolates of enterococci (for a review, see reference 25) and the laboratory demonstration of the transfer of the vanA gene complex to Staphylococcus aureus (15) have raised concern about the occurrence of such a genetic transfer in clinical isolates of methicillin-resistant staphylococci, against which the most frequently used chemotherapy is vancomycin based. While clinical isolates of staphylococci with the enterococcal glycopeptide resistance mechanism have not been reported so far, clinical failure of teicoplanin and vancomycin treatment in coagulase-negative staphylococci (CNS) (see reference 25) and, recently, in methicillin-resistant S. aureus (MRSA) infections in Japan (10) and the United States (4, 5) have been reported. The MRSA strains involved exhibited low-level vancomycin resistance through a novel and as-yet-undefined mechanism(s) that differed from that of the enterococcal vancomycin resistance. A different mechanism of vancomycin resistance associated with removal of the drug molecules from the growth medium during growth of the bacteria has been described recently in a laboratory mutant of MRSA (22). The vancomycin-resistant MRSA showed several unique properties: bacteria grown in the presence of vancomycin formed large cellular aggregates, produced large amounts of extracellular material resembling (by electron microscopy) cell wall material, and produced cell wall peptidoglycan with altered muropeptide composition, and the vancomycin removed from the growth medium (bound to the cell walls) could be recovered in biologically active form from the bacteria (22). An identical set of observations was made with a recently isolated highly teicoplanin-resistant laboratory mutant of S. aureus which removed teicoplanin from the medium into a cell-bound form (22a). In order to determine whether this type of glycopeptide resistance mechanism also existed among clinical isolates, we examined a number of methicillin-resistant coagulase-negative staphylococci (MRCNS) with low-level teicoplanin resistance which were recovered from a hospital in New York City.