Bacteria detect neutrophils via a system that responds to hypochlorous acid and flow

Neutrophils kill bacteria by producing an array of lethal molecules, including hypochlorous acid (HOCl). The extent to which pathogens detect HOCl from neutrophils and defend themselves has not been understood. We report that the opportunistic pathogen Pseudomonas aeruginosa responds to activated neutrophils by upregulating the flow regulated operon (Fro), which was previously shown to be activated by fluid flow. We found that fro is upregulated in a mouse infection model where neutrophil influx occurs. This upregulation is induced in vitro by HOCl and its secondary product taurine chloramine but not by other neutrophil defense factors including LL-37, histones, or H ₂ O ₂ . HOCl induces the FroR-dependent upregulation of methionine sulfoxide reductases that relieve otherwise lethal oxidative stress. Fro expression is regulated by FroR’s anti-σ factor FroI, which contains the highest density of methionines and cysteines of all anti-σ factors. The second-order rate constants of HOCl are highest with these residues, raising the possibility that the activation of fro could involve oxidation of FroI. These findings suggest a model in which flow transports oxidizing molecules that activate the fro operon, establishing an early warning system for P. aeruginosa that improves its survival against host immune defenses and persistence during infection.