The host environment activates multiple stress responses which correlate with antibiotic tolerance in Staphylococcus aureus

Antibiotic tolerance enables bacteria to survive antibiotic therapy, leading to treatment failure and recurrent infections. While cellular stress responses promote tolerance in model organisms, their role in Staphylococcus aureus infections remains unclear. In this study, we investigate whether host environments activate bacterial stress responses that lead to antibiotic tolerance. We constructed fluorescent reporters to monitor five stress responses: SOS, oxidative stress, stringent response (cell wall stress and starvation pathways), and heat shock response. Using confocal microscopy, we quantified stress response activation in planktonic cultures, in biofilms, in laboratory media versus human serum, and during neutrophil phagocytosis. We then correlated stress response activation with antibiotic tolerance in time-kill assays. Individual stress responses were specifically activated with minimal cross-activation. Pre-activation of stringent and heat shock responses significantly reduced ciprofloxacin susceptibility, while oxidative stress activation slightly increased susceptibility. Growth of S. aureus biofilms in 100% serum activated all stress responses, while biofilms grown in laboratory media showed no activation. Brief serum exposure (1h) activated the stringent response and conferred complete ciprofloxacin tolerance. During neutrophil phagocytosis, bacteria activated all stress responses. We conclude that the host environment strongly activates multiple stress responses in S. aureus, with stringent and heat shock responses correlating with antibiotic tolerance. The infectious microenvironment primes bacteria for tolerance through stress response activation, providing potential therapeutic targets. The dramatic difference between laboratory and host conditions underscores the importance of studying antibiotic tolerance in clinically-relevant environments.