Upstream Racing: What Morphological Features Make Bacteria Effective Swimmers Against the Flow

Bacteria can swim upstream in confined environments such as microchannels and flow networks, contributing to contamination and biofilm formation. While most previous studies have focused on the upstream motility of E. coli, the influence of morphological traits on upstream swimming, both in E. coli and in other bacterial strains with diverse shapes, remains underexplored. In this study, we nanofabricated a microfluidic competition assay in which multiple bacterial strains race against flow to identify the most effective upstream swimmers. Using E. coli as a baseline, we combine experimental observations and theoretical modeling to systematically evaluate individual morphological traits that enhance bacterial motility against flow. We also perform DNA sequencing to identify genetic determinants associated with strong upstream motility. Our results show that specific morphological features can significantly improve upstream motility, while others hinder it. Furthermore, we reveal that bacteria trapped near constrictions can adapt their morphology under flow, enhancing their ability to swim upstream.

These findings provide insight into the physical and genetic strategies bacteria use to resist flow, with potential applications in controlling bacterial contamination in medical and industrial systems and in guiding the design of bioinspired microswimmers. They also deepen our understanding of how flow shapes microbial competition, dispersal, and colonization in spatially structured environments, with broader implications for bacterial ecology in natural and engineered habitats.