Abstract
Cells regularly experience fluid flow in natural systems. However, most experimental systems rely on batch cell culture and fail to consider the effect of flow-driven dynamics on cell physiology. Using microfluidics and single-cell imaging, we discover that the interplay of physical shear rate (a measure of fluid flow) and chemical stress trigger a transcriptional response in the human pathogen Pseudomonas aeruginosa. In batch cell culture, cells protect themselves by quickly scavenging the ubiquitous chemical stressor hydrogen peroxide (H(2)O(2)) from the media. In microfluidic conditions, we observe that cell scavenging generates spatial gradients of H(2)O(2). High shear rates replenish H(2)O(2), abolish gradients, and generate a stress response. Combining mathematical simulations and biophysical experiments, we find that flow triggers an effect like "wind-chill" that sensitizes cells to H(2)O(2) concentrations 100 to 1,000 times lower than traditionally studied in batch cell culture. Surprisingly, the shear rate and H(2)O(2) concentration required to generate a transcriptional response closely match their respective values in the human bloodstream. Thus, our results explain a long-standing discrepancy between H(2)O(2) levels in experimental and host environments. Finally, we demonstrate that the shear rate and H(2)O(2) concentration found in the human bloodstream trigger gene expression in the blood-relevant human pathogen Staphylococcus aureus, suggesting that flow sensitizes bacteria to chemical stress in natural environments.