Vancomycin Activates σ(B) in Vancomycin-Resistant Staphylococcus aureus Resulting in the Enhancement of Cytotoxicity

The alternative transcription factor σ(B) is responsible for transcription in Staphylococcus aureus during the stress response. Many virulence-associated genes are directly or indirectly regulated by σ(B). We hypothesized that treatment with antibiotics may act as an environmental stressor that induces σ(B) activity in antibiotic-resistant strains. Several antibiotics with distinct modes of action, including ampicillin (12 µg/ml), vancomycin (16 or 32 µg/ml), chloramphenicol (15 µg/ml), ciprofloxacin (0.25 µg/ml), and sulfamethoxazole/trimethoprim (SXT, 0.8 µg/ml), were investigated for their ability to activate this transcription factor. We were especially interested in the stress response in vancomycin-resistant S. aureus (VRSA) strains treated with vancomycin. The transcription levels of selected genes associated with virulence were also measured. Real-time quantitative reverse transcription PCR was employed to evaluate gene transcription levels. Contact hemolytic and cytotoxicity assays were used to evaluate cell damage following antibiotic treatment. Antibiotics that target the cell wall (vancomycin and ampicillin) and SXT induced σ(B) activity in VRSA strains. Expression of σ(B)-regulated virulence genes, including hla and fnbA, was associated with the vancomycin-induced σ(B) activity in VRSA strains and the increase in cytotoxicity upon vancomycin treatment. These effects were not observed in the sigB-deficient strain but were observed in the complemented strain. We demonstrate that sub-minimum inhibitory concentration (sub-MIC) levels of antibiotics act as environmental stressors and activate the stress response sigma factor, σ(B). The improper use of antibiotics may alter the expression of virulence factors through the activation of σ(B) in drug-resistant strains of S. aureus and lead to worse clinical outcomes.