Collective behaviors of bacteria in flow: Divergent roles for transport and microcolony morphology*

Fluid flows are dominant features of many bacterial environments, yet interactions between fluid flow, surface association, and colonization factors remain largely underexplored. One specific example is infective endocarditis, in which bacteria preferentially colonize heart valves, areas with the greatest flow rates. The mechanisms underlying this important yet paradoxical behavior are unknown. We know bacteria interact on different length scales, from single cell gene expression to collective biofilm communities and introducing flow will allow us to further explore the emergent behaviors. We developed a microfluidic system to study flow effects on colonization of two endocarditis-inducing pathogens: Staphylococcus aureus MRSA and Enterococcus faecalis. We observe bacteria growth from single cells to microcolonies in two different flow rates: high flow is 10x larger than low flow. Both species demonstrate a counter-intuitive larger colonization in high flow compared to low flow. Our experimental and computational studies elucidate two different strategies leading to the preferential adherence in high flow: S. aureus is driven by signaling molecule transport while E. faecalis chains experience a mechanical response under flow. Our divergent mechanisms introduce an intriguing area of further study for bacteria in flow environments, particularly focusing on differnt colony morphologies (e.g. clusters, chains) and how those morphologies may provide advantages (or disadvantages) in complex environments.