Antimicrobial susceptibility of coagulase-negative staphylococci and characterization of isolates with reduced susceptibility to glycopeptides

The antimicrobial susceptibility of 239 coagulase-negative staphylococci (C NS) isolates consequently collected from blood culture in patients admitted in a 600-bed teaching hospital was evaluated. The isolates were identified to the species level by conventional methods and the MicroScan Positive Co mbo Panel type 6 system, and their susceptibility to vancomycin, teicoplani n, and oxacillin were tested by agar dilution, disk diffusion, and MicroSca n-WalkAway system. The species distribution, was as follows: Staphylococcus epidermidis 120 (50.2%), S. hominis 29 (12.1%), S. haemolyticus 24 (10.0%) , S. cohnii 14 (5.9%), and isolates from other CNS species 52 (27.8%). The percentage of resistance to oxacillin was 74.5% by agar dilution. The highe st percentages of oxacillin resistance were found among S. haemolyticus (95 .8%) and S. epidermidis (80.8%). Teicoplanin resistance (MIC greater than o r equal to 32 mu g/mL) was detected in fine S. haemolyticus isolates, where as intermediate resistance (MIC = 16 mu g/mL) was detected in nine strains. These isolates with reduced susceptibility to teicoplanin were resistant t o oxacillin, but remained susceptible to vancomycin (MIC less than or equal to 4 mu g/mL). Two isolates, one S. haemolyticus and one S. epidermidis, s howed a vancomycin MIC of 8 mu g/mL, and both MicroScan and disk diffusion methods classified these isolates its susceptible. Our results showed that glycopeptide resistance is emerging among CNS isolates in our institution a nd the disk diffusion method may not detect isolated with decreased suscept ibility to these antimicrobial agents. (C) 1999 Elsevier Science Inc.