Detection of vancomycin and teicoplanin resistance in Staphylococcus isolates: Which method is more reliable?

This study evaluated the reliability of different laboratory methods for detecting resistance to glycopeptide antibiotics-vancomycin and teicoplanin-in clinical Staphylococcus aureus and coagulase-negative staphylococci (CoNS) isolates. While automated systems are widely used in clinical microbiology laboratories due to their efficiency and ease of use, they may yield inaccurate results when assessing glycopeptide susceptibility. A total of 87 previously collected clinical isolates (22 S. aureus and 65 CoNS), initially identified as resistant to at least one of the vancomycin or teicoplanin by an automated system, were retrospectively analyzed. All isolates were stored at -80 °C and retested using three methods: the same automated system (following the manufacturer's protocol), the gradient diffusion method, and the reference broth microdilution (BMD) method. Interpretations were made according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. Upon re-evaluation, all isolates were found to be susceptible to vancomycin and teicoplanin using the BMD method. The automated system yielded 100% concordance with BMD for vancomycin and 77% for teicoplanin, while the gradient method produced similar findings. Notably, five S. aureus isolates (23%) remained resistant to teicoplanin according to both the automated system and the gradient method but were susceptible by BMD. These results emphasize that automated systems, although practical, may lead to overestimation of glycopeptide resistance. Therefore, when resistance is suspected, especially to teicoplanin, confirmatory testing with the BMD reference method is essential to ensure accurate interpretation and avoid misclassification.