Glycopeptide-intermediate Staphylococcus aureus: evaluation of a novel screening method and results of a survey of selected U.S. hospitals

Abstract

Isolates of Staphylococcus aureus with decreased susceptibilities to glycopeptide antimicrobial agents, such as vancomycin and teicoplanin, have emerged in the United States and elsewhere. Commercially prepared brain heart infusion agar (BHIA) supplemented with 6 microg of vancomycin per ml was shown in a previous study to detect glycopeptide-intermediate S. aureus (GISA) with high sensitivity and specificity; however, this medium, when prepared in-house, occasionally showed growth of vancomycin-susceptible control organisms. This limitation could significantly impact laboratories that prepare media in-house, particularly if they wished to conduct large surveillance studies for GISA. Therefore, a pilot study to detect GISA was performed with vancomycin-containing Mueller-Hinton agar (MHA) prepared in-house in place of commercially prepared BHIA. MHA was selected for this study because this medium is widely available and well standardized. The results of the pilot study showed that supplementation of MHA with 5 microg of vancomycin per ml was both a sensitive and a specific method for screening for GISA isolates. This method was used to screen for GISA among 630 clinical isolates of methicillin-resistant S. aureus collected during 1997 from 33 U.S. hospitals. Although 14 S. aureus isolates grew on the screening agar, all were vancomycin susceptible (MICs were </=1 microg/ml) by broth microdilution testing. Population analyses of five isolates revealed two with a subpopulation for which vancomycin MICs were 8 microg/ml. In summary, the MHA screen plate containing 5 microg of vancomycin per ml prepared in-house provides a sensitive and cost-effective method for large-scale screening for GISA for which vancomycin MICs are 8 microg/ml. However, confirmation of isolates as vancomycin resistant is critical. This study suggests that GISA was not a widespread problem in the United States in 1997.