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

Abstract

The antimicrobial susceptibility of 239 coagulase-negative staphylococci (CNS) 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 Combo Panel type 6 system, and their susceptibility to vancomycin, teicoplanin, and oxacillin were tested by agar dilution, disk diffusion, and MicroScan-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 highest percentages of oxacillin resistance were found among S. haemolyticus (95.8%) and S. epidermidis (80.8%). Teicoplanin resistance (MIC greater than or equal to 32 mu g/mL) was detected in fine S. haemolyticus isolates, whereas intermediate resistance (MIC = 16 mu g/mL) was detected in nine strains. These isolates with reduced susceptibility to teicoplanin were resistant to 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, showed 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 and the disk diffusion method may not detect isolated with decreased susceptibility to these antimicrobial agents. (C) 1999 Elsevier Science Inc.