Combining multiple stressors unexpectedly blocks bacterial migration and growth

In nature, organisms experience combinations of stressors. However, laboratory studies typically simplify reality and focus on the effects of an individual stressor. Here, we use a microfluidic approach to simultaneously provide a physical stressor (shear flow) and a chemical stressor (H2O2) to the human pathogen Pseudomonas aeruginosa. By treating cells with levels of flow and H2O2 that commonly co-occur in nature, we discover that previous reports significantly overestimate the H2O2 levels required to block bacterial growth. Specifically, we establish that flow increases H2O2 effectiveness 50-fold, explaining why previous studies lacking flow required much higher concentrations. Using natural H2O2 levels, we identify the core H2O2 regulon, characterize OxyR-mediated dynamic regulation, and dissect the redundant roles of multiple H2O2 scavenging systems. By examining single-cell behavior, we serendipitously discover that the combined effects of H2O2 and flow block pilus-driven surface migration. Thus, our results counter previous studies and reveal that natural levels of H2O2 and flow synergize to restrict bacterial colonization and survival. By studying two stressors at once, our research highlights the limitations of oversimplifying nature and demonstrates that physical and chemical stress can combine to yield unpredictable effects.