Chemotaxis in Marine Bacterium Vibrio alginolyticus

We investigated swimming behavior of marine bacterium \emph{Vibrio alginolyticus} in an uniform chemical environment. The typical bacterial trajectory consists of consecutive run (forward swimming) and reverse (backward swimming) intervals with occasional sudden changes of swimming directions, which we call \textquotedblleft{}flagellar flicks\textquotedblright{}. This mode of chemotaxis is different from the canonical run-and-tumble strategy adopted by \emph{Escherichia coli} and may be selected for in \emph{V. alginolyticus} due to the ocean environment where nutrients are scarce and are subject to rapid turbulent dispersion. We measured the statistical distributions of run $T_{run}$ and revers $T_{rev}$ time intervals, $P(T_{run})$ and $P(T_{rev})$, and found that while the back-swimming time appears to have a well-defined time scale of $0.5\, s$, the forward swimming time is more broadly distributed, suggestive of a Poisson process. Measurements of the time interval $T_{flick}$ between two consecutive directional changes show that $P(T_{flick})$ is also peaked at a finite time, $T_{flick}\sim1\, s$, and the mean directional change is $\Delta\theta\sim70\,^{0}$. Interestingly, this $\Delta\theta$ observed is nearly optimal for efficient randomization of swimming directions. Altogether, our experiments suggest that \emph{V. alginolyticus }employs both run-and-reverse and flicking activities for chemotaxis, and this behavior presumably optimizes their foraging efficiency in a turbulent environment.