Detoxification of host-relevant H ₂ O ₂ shapes Pseudomonas aeruginosa population gradients in flow

During infection, bacterial populations need to overcome low levels of host-generated H ₂ O ₂ . However, experiments almost always use high H ₂ O ₂ levels that are rarely found in hosts. Here, we use microfluidics to investigate how host-relevant H ₂ O ₂ 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 H ₂ O ₂ 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 H ₂ O ₂ . Mutants lacking all three systems are sensitive to H ₂ O ₂ but can be cross-protected by resistant cells when co-cultured. Cross-protection is abolished in higher flow regimes, where H ₂ O ₂ 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.