  Ephemeral aggregations of bacteria are ubiquitous in the environment, where
they serve as hotbeds of metabolic activity, nutrient cycling, and horizontal
gene transfer. In many cases, these regions of high bacterial concentration are
thought to form when motile cells use chemotaxis to navigate to chemical
hotspots. However, what governs the dynamics of bacterial aggregations is
unclear. Here, we use a novel experimental platform to create realistic
sub-millimeter scale nutrient pulses with controlled nutrient concentrations.
By combining experiments, mathematical theory and agent-based simulations, we
show that individual \textit{Vibrio ordalii} bacteria begin chemotaxis toward
hotspots of dissolved organic matter (DOM) when the magnitude of the chemical
gradient rises sufficiently far above the sensory noise that is generated by
stochastic encounters with chemoattractant molecules. Each DOM hotspot is
surrounded by a dynamic ring of chemotaxing cells, which congregate in regions
of high DOM concentration before dispersing as DOM diffuses and gradients
become too noisy for cells to respond to. We demonstrate that \textit{V.
ordalii} operates close to the theoretical limits on chemotactic precision.
Numerical simulations of chemotactic bacteria, in which molecule counting noise
is explicitly taken into account, point at a tradeoff between nutrient
acquisition and the cost of chemotactic precision. More generally, our results
illustrate how limits on sensory precision can be used to understand the
location, spatial extent, and lifespan of bacterial behavioral responses in
ecologically relevant environments.
