Blue Light Is a Universal Signal for Chemoreceptors.

Blue light has been shown to elicit a tumbling response in ， a nonphototrophic bacterium. The exact mechanism of this phototactic response is still unknown. Here， we quantify phototaxis in by analyzing single-cell trajectories in populations of free-swimming bacteria before and after light exposure. Bacterial strains expressing only one type of chemoreceptor reveal that all five receptors (Aer， Tar， Tsr， Tap， and Trg) are capable of mediating responses to light. In particular， light exposure elicits a running response in the Tap-only strain， the opposite of the tumbling responses observed for all other strains. Therefore， light emerges as a universal stimulus for all chemoreceptors. We also show that blue light exposure causes a reversible decrease in swimming velocity， a proxy for proton motive force. This result is consistent with a previously proposed hypothesis that， rather than sensing light directly， chemoreceptors sense light-induced perturbations in proton motive force， although other factors are also likely to contribute. Our findings provide new insights into the mechanism of phototaxis， showing that all five chemoreceptor types respond to light and their interactions play an important role in cell behavior. Our results also open up new avenues for examining and manipulating taxis. Since light is a universal stimulus， it may provide a way to quantify interactions among different types of receptors. Because light is easier to control spatially and temporally than chemicals， it may be used to study swimming behavior in complex environments. Since phototaxis can cause migration of bacteria in light gradients， light may be used to control bacterial density for studying density-dependent processes in bacteria.