Lay summary
Methicillin-resistant Staphylococcus aureus (MRSA) are major pathogens of hospital-acquired and community-acquired infections. Most MRSA-infected patients receive glycopeptide antibiotics, vancomycin or teicoplanin, which are "the drugs of last resort" for multi-resistant isolates. The therapeutic efficacy of vancomycin (or teicoplanin) is widely debated today, and there is a growing concern that their intensive use may promote a stepwise increase in glycopeptide resistance levels in hospital-acquired MRSA isolates. Emergence of low-level glycopeptide resistance in S. aureus (designated under the acronym of GISA) is currently viewed as the progressive selection of subpopulations of GISA precursors by long-term glycopeptide therapy, eventually leading to homogeneous GISA populations. The phenotypic detection of resistance in GISA, which frequently display cell wall thickening, is problematic. The molecular mechanisms of resistance in GISA are also not well understood, but seem to be linked with multiple genomic changes altering in an unknown manner transcription of several cell-wall regulation pathways. Using transcription profiling and genetic methods, we identified two novel genes playing a key role in emergence and expression of glycopeptide resistance in S. aureus. In addition, an innovative molecular strategy was developed to identify a very limited number of mutations potentially linked with expression of glycopeptide resistance. In this approach, comparison by whole genomic sequencing of GISA and non-GISA clonally-related isolates was combined with a systematic genetic approach, where all steps leading to glycopeptide resistance were reconstituted. We also demonstrated that a precise mutation in the bacterial cell-membrane sensor VraS could strongly inhibit the emergence of teicoplanin-resistant and vancomycin-resistant mutants, which suggests that pharmacological blockade of the VraS sensor might be used for preventing emergence of glycopeptide resistance in S. aureus. Finally, we recently developed and validated an improved phenotypic assay for detection of GISA isolates, which shows promising applications for predicting increased risk of glycopeptide therapy failure in patients with bacteremic and orthopedic MRSA infections. The development and validation of molecular and phenotypic assays for detection of glycopeptide-resistant MRSA isolates may be essential for optimizing antibiotic treatment protocols and infection control programs against multi-resistant MRSA. Improved understanding of the molecular mechanisms of glycopeptide resistance may help to unravel novel targets for development of new antibiotics.