Influence of bacteria, diffusion and shear on micro-scale nutrient patches, and implications for bacterial chemotaxis

Abstract

Micro-scale nutrient patches were observed in mixtures of isolates consisting of a protozoan, its prey, and chemotactic bacteria. The patches were shown by swarms of bacteria and the events leading to patches were associated with cell lysis and predation events. In such a form, nutrients are experienced by bacteria as pulses. Simulations showed that patches can be consumed by the bacterial community before being dispersed. As a result, even non-motile bacteria may only ever encounter diffuse patches within a sphere of 2 mm radius. If patches are generated randomly in time and space, it can be advantageous for a bacterium to swim, if only to break this 2 mm barrier. Simulations suggested that chemotaxis can increase a bacterium's exposure to nutrients within a patch, but that the enhancement depends on the size of the patch, due to limitations in sensitivity. The patch size limit for achieving enhancement is ~0.1 pmol. Swimming speed is the most important motility parameter influencing efficiency of chemotactic foraging on diffuse patches, with an optimum speed as a function of patch size. Simulations illustrated that shear stretches patches, but that moderate shear does not significantly alter patch volumes within time scales of several minutes. A large proportion of the nutrients within a patch can be encountered by bacteria within that time.