Breaking of Time-Reversal Symmetry in Bacterial Chemotaxis

Abstract

Vibrio alginolyticus is a marine bacterium with a single polar flagellum when growing in low viscous liquid media. Recently, we found that they carry out a novel cyclic 3-step motility pattern. In one cycle they swim forward, backward and at the end of the backward swimming they are able to use their flagellum to flick and actively change their cell body orientation. To understand how this novel motility pattern is regulated by V. alginolyticus, we studied their chemotactic response to a sudden increase of the attractant serine concentration. We made use of caged serine, which can be released by UV light in a fraction of second to achieve a sudden change in the serine concentration. We compared their swimming behavior before and after the UV flash. We found that in short time, they treat forward and backward swimming equally, i.e. they extend their current swimming interval regardless of the swimming direction. However, in a long time but before adaptation, cells become gradually forwardly biased. This chemotactic strategy enables the bacteria to swim up the attractant gradient quickly as well as to localize around the top of the gradient to get a maximum nutrient exposure. This perhaps is important in an ocean environment where nutrient are distributed intermittently in space and time.