Bacteria sense environmental stressors and activate responses to improve their survival in harsh growth conditions. Neutrophils respond to the presence of bacteria by producing oxidative antibacterial species including hypochlorous acid (HOCl). However, the extent that bacteria detect activated neutrophils or HOCl has not been known. Here, we report that the opportunistic bacterial pathogen Pseudomonas aeruginosa responds to activated neutrophils by activating the fro system, which regulates the expression of antioxidative factors. We show that this response is specific to HOCl and that other oxidative factors including H₂O₂, do not trigger a fro response. The fro system has been previously shown to detect flow that is present in host vasculature, such as in animal circulatory systems. Our data thus suggest a model in which fro serves as an early host detection system in P. aeruginosa that improves its survival against neutrophil-mediated defenses, which could promote colonization in human tissue and increase pathogenicity.

P. aeruginosa primes its defenses against neutrophils by sensing flow and hypochlorous acid via the fro system.