Bacteria bioluminesce in response to fluid shear*

Many bacteria display bioluminescence, through which they convert chemical energy into light that illuminates the sea. While it is known that mechanical stress can trigger bioluminescence in eukaryotes such as dinoflagellates, it has often been suggested that this is not true for bacteria. Here, we combine imaging and rheology experiments to show that bacteria indeed emit light in response to shear within seconds of stimulation. The light intensity increases sigmoidally with shear rate and the response to shear ramps exhibits species-dependent hysteresis. This response is consistent with that of shear-activated ion channels in eukaryotic cells leading to calcium signaling, and hysteresis is a common feature of ion channels in other systems.  We capture the full range of observations with a two-state adaptive kinetic model with the time scale for opening and closing of channels of the order of seconds. The time scale for adaptation of the total number of channels to shear is larger and introduces the unsteadiness in the system that leads to hysteresis.