Bacterial flagellum as a propeller and a rudder

Vibrio alginolyticus Enteric bacteria such as Escherichia coli swim by rotating a set of flagella that forms a bundle when the flagellar motors turn in the counterclockwise (CCW) direction The bundle falls apart when one or more motors turns in the clockwise (CW) direction, and the bacterium tumbles. A new swimming direction is selected upon resuming the CCW rotation of the flagellar motors. By modulating the CCW and CW intervals according to external chemical cues, the cells are able to migrate toward attractants or away from repellents.

This paper report observations of motility patterns of marine bacterium Vibrio alginolyticus. The authors fround found that the bacteria employ a unique cyclic three-step (forward–reverse–flick) swimming pattern for chemotaxis; they regulate both forward and backward swimming times according to a given chemical profile. By employing the three-step chemotactic strategy, cells of V. alginolyticus are able to focus on a point source of attractant rapidly and form a compact swarm around it. This is apparently a significant niche for V. alginolyticus, which live in ocean where nutrients are scarce and rapidly dispersed by currents.

Bacterial flagellum as a propeller and as a rudder for efficient chemotaxis. PNAS USA 4th January 4 2011 doi: 10.1073/pnas.1011953108
We investigate swimming and chemotactic behaviors of the polarly flagellated marine bacteria Vibrio alginolyticus in an aqueous medium. Our observations show that V. alginolyticus execute a cyclic, three-step (forward, reverse, and flick) swimming pattern that is distinctively different from the run–tumble pattern adopted by Escherichia coli. Specifically, the bacterium backtracks its forward swimming path when the motor reverses. However, upon resuming forward swimming, the flagellum flicks and a new swimming direction is selected at random. In a chemically homogeneous medium (no attractant or repellent), the consecutive forward tf and backward tb swimming times are uncorrelated. Interestingly, although tf and tb are not distributed in a Poissonian fashion, their difference Δt = |tf – tb| is. Near a point source of attractant, on the other hand, tf and tb are found to be strongly correlated, and Δt obeys a bimodal distribution. These observations indicate that V. alginolyticus exploit the time-reversal symmetry of forward and backward swimming by using the time difference to regulate their chemotactic behavior. By adopting the three-step cycle, cells of V. alginolyticus are able to quickly respond to a chemical gradient as well as to localize near a point source of attractant.


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