Marine Bacterial Chemoresponse to a Stepwise Chemoattractant Stimulus

Abstract

We found recently that polar flagellated marine bacterium Vibrio alginolyticus is capable of exhibiting taxis toward a chemical source in both forward and backward swimming directions. How the microorganism coordinates these two swimming intervals, however, is not known. The work presented herein is aimed at determining the response functions of the bacterium by applying a stepwise chemoattractant stimulus while it is swimming forward or backward. The important finding of our experiment is that the bacterium responds to an identical chemical signal similarly during the two swimming intervals. For weak stimuli, the difference is mainly in the amplitudes of the response functions while the reaction and adaptation times remain unchanged. In this linear-response regime, the amplitude in the forward swimming interval is approximately a factor of two greater than in the backward direction. Our observation suggests that the cell processes chemical signals identically in both swimming intervals, but the responses of the flagellar motor to the output of the chemotaxis network, the regulator CheY-P concentration, are different. The biological significance of this asymmetrical response in polar flagellated marine bacteria is discussed.

Chemotaxis strategies of bacteria with multiple run modes

“…Thus, the time that a flagellum rotates in a clockwise (CW) direction (wrapped mode) is affected by the chemoattractant, whereas the time of CCW rotation (push mode) is not. This asymmetry is reminiscent of the different responses of CW and CCW rotation times observed in Vibrio alginolyticus ( 14 , 16 ) in response to a stepwise temporal increase in chemoattractant ( 17 ).…”

An Element of Determinism in a Stochastic Flagellar Motor Switch

“…This extreme CW bias can be explained as a result of elevated [YP] in the cytoplasm of E. coli that “forces” the motor to run exclusively in the CW direction. If exposure to the repellent has the same effect of elevating [YP] in V. alginolyticus , for which there is little reason to believe otherwise, an inescapable conclusion is that the flagellar motor switch of V. alginolyticus reacts to this regulatory protein very differently from E. coli [ 29 , 30 , 32 ]. This important observation motivates a molecular toggle switch model for the polar flagellar motor to be presented in the Theoretical Modeling section.…”

“…Below we propose a minimal model aimed at mimicking P (Δ f ) and P (Δ b ) seen in our experiment. Because the role of CheY-P on the motor is more-or-less symmetric for the two motor states [ 32 ], in the ensuing discussion it suffices to consider only one of these transitions, say from CCW to CW. We assign the conformation of the subunit to be active (inactive) when the motor is in CW (CCW) direction.…”

“…For V. alginolyticus , on the other hand, CheY-P behaves as a switching facilitator; binding of CheY-P increases the exiting rate regardless of its current state. We posit that this type of regulation is well suited for bacteria that are capable of bidirectional swimming and chemotaxis [ 32 ].…”

Chemotactic drift speed for bacterial motility pattern with two alternating turning events

“…alginolyticus bacteria [26]. Although the response for backward and forward motion might differ by a numerical factor of the order of 2 [27], here for simplicity of calculations we assume the same response for both swimming directions. To quantify the effectiveness of chemotaxis we use the so-called chemotactic drift velocity v d in the concentration of the chemoattractant with a small linear gradient pointing along Oz axis:…”