Two Alternative Models for Spontaneous Flagellar Motor Switiching inHalobacterium salinarium

Halobacteria are light-sensitive microorganisms which swim by means of rotating flagella. Under constant environmental conditions, they reverse their swimming direction about every 10–30 s. This is due to spontaneous switchings of the flagellar motors from clockwise to counterclockwise rotation or vice versa. However,it is unknown how the direction of rotation is regulated. In this paper, two stochastic models which could explain the characteristics of sequences of spontaneous switiching events are discussed. The process is described in terms of a “motor switch” cyclically running through a sequence of states and triggering a reversal whenever a cycle is complete. Referring to previously stated hypotheses, modelling is based on the assumption that transitions between the states of the switch either occur with constant probability per unit time or are actively regulated by an endogenous biochemical oscillator. It is shown that both models yield a good description of the experimental data, though they are mutually exclusive.