Incidence of Coagulase-Negative Staphylococci with Reduced Susceptibilities to Glycopeptides and Comparison of Test Methods

Ki, Chang Seok; Lee, Seong Kyu; Lee, Jang Ho; Lee, Nam Yong

Abstract

BACKGROUND: We tried to evaluate the incidence and clinical significance of coagulase-negative staphylococci (CoNS) with reduced susceptibilities to glycopeptides. In addition, the ability of disk diffusion and Vitek system to detect CoNS with reduced susceptibilities to glycopeptides were compared with the standard agar dilution method.
METHODS
One hundred and nineteen clinical isolates of CoNS were recovered at Samsung Medical Center from June to July 1998 and were examined for their susceptibilities to vancomycin and teicoplanin by disk diffusion method (30-microgram disk), Vitek system with GPS-AA card, and agar dilution for the determination of MICs. The records of all patients, from whom CoNS with decreased susceptibility to glycopeptide was isolated, were reviewed.
RESULTS
All CoNS showed uniform susceptibility to vancomycin by all three methods but 11 strains (9.2%) exhibited reduced susceptibilities to teicoplanin (MICs, 16 to 32 microgram/mL). All but suspected colonized strains were nosocomially acquired and were isolated from 7 different wards. None were previously treated with teicoplanin. The concordance rates of disk diffusion method and Vitek system with agar dilution method were 94.1% and 84%, respectively. However, the sensitivity of disk diffusion method and Vitek system were only 50.0% and 62.5%, respectively.
CONCLUSIONS
This study demonstrates that CoNS with reduced susceptibilities to glycopeptides is not uncommon and may cause true infections in clinical settings. However, neither disk diffusion method nor Vitek system could differentiate these strains from more susceptible isolates.