Detoxification of host-relevant H2O2 shapes Pseudomonas aeruginosa population gradients in flow

During infection, bacterial populations need to overcome low levels of host-generated H2O2. However, experiments almost always use high H2O2 levels that are rarely found in hosts. Here, we use microfluidics to investigate how host-relevant H2O2 impacts populations of the human pathogen Pseudomonas aeruginosa in flow. Using long-channel microfluidic devices, we establish that cells at the front of a population remove H2O2 and protect their downstream neighbors. Population-level protection is mediated by three OxyR-regulated scavenging systems (KatA, KatB, AhpCF), which are each sufficient to detoxify host-relevant H2O2. Mutants lacking all three systems are sensitive to H2O2 but can be cross-protected by resistant cells when co-cultured. Cross-protection is abolished in higher flow regimes, where H2O2 is delivered faster than cells can remove it. Our results demonstrate how local detoxification provides global protection, which results in spatial gradients across bacterial populations in flow. Together, our findings highlight how biological, chemical, and physical factors collectively determine the fate of bacterial populations in host-relevant environments.