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

Micro-scale nutrient patches were observed in mixtures of isolates consisti ng of a protozoan, its prey, and chemotactic bacteria. The patches were sho wn by swarms of bacteria and the events leading to patches were associated with cell lysis and predation events. In such a form, nutrients are experie nced by bacteria as pulses. Simulations showed that patches can be consumed by the bacterial community before being dispersed. As a result, even non-m otile 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. S imulations 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 achi eving enhancement is similar to 0.1 pmol. Swimming speed is the most import ant motility parameter influencing efficiency of chemotactic foraging on di ffuse patches, with an optimum speed as a function of patch size. Simulatio ns illustrated that shear stretches patches, but that moderate shear does n ot significantly alter patch volumes within time scales of several minutes. A large proportion of the nutrients within a patch can be encountered by b acteria within that time.