The effect of hydrodynamic mixing in bacterial populations due to bacterial chemotaxis is a well described phenomenon known as bioconvection. The classic scenario for bioconvection entails bacterial swimming upward along an oxygen gradient, creating a dense layer of cells near the air/liquid interface, followed by sedimentation of bacteria (due to gravity). This often leads to stable cells of circular flow and creates convective mixing throughout the culture. Under certain conditions, we observed buoyant plumes that were created independently of bacterial motility. We propose that these buoyant hydrodynamic flows were caused by solute gradients, resulting from bacterial metabolism. We observed E. coli strains, both a wild-type K12 strain and a non-motile mutant strain, along the bottom of flat-bottomed containers. Cultures of metabolically active E. coli consistently formed plumes visible with the naked eye, leading to complete mixing of the bacterial culture, whereas cultures of stationary phase bacteria never formed plumes, presumably due to a lack of rapid metabolism. The phenomenon of solute-induced buoyancy-driven convection has previously been characterized for growing protein crystals and soft tissues, however, both are much larger than individual bacterial cells. We developed a numerical model to study the effect of nutrient consumption and by-product excretion on extracellular fluid density and the formation of buoyant plumes. The numerical results compared well with analytical calculations and experimental results. Furthermore, an analysis with dimensionless numbers was used to characterize the size-dependent nature of observable buoyant plumes, and to investigate the effect of altered levels of gravity, such as the microgravity conditions encountered during space flight.

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