Cell orientation of swimming bacteria: from theoretical simulation to experimental evaluation

The motion of small bacteria consists of two phases: relatively long runs alternate with intermittent stops, back-ups, or tumbles, depending on the species. In polar monotrichous bacteria, the flagellum is anchored at the cell pole inherited from the parent generation (old pole) and is surrounded by a chemoreceptor cluster. During forward swimming, the leading pole is always the pole recently formed in cell division (new pole). The flagella of the peritrichous bacterium Escherichia coli often form a bundle behind the old pole. Its cell orientation and receptor positioning during runs generally mimic that of monotrichous bacteria. When encountering a solid surface, peritrichous bacteria exhibit a circular motion with the leading pole dipping downward. Some polar monotrichous bacteria also perform circular motion near solid boundaries, but during back-ups. In this case, the leading pole points upward. Very little is known about behavior near milieu-air interfaces. Biophysical simulations have revealed some of the mechanisms underlying these phenomena, but leave many questions unanswered. Combining biophysics with molecular techniques will certainly advance our understanding of bacterial locomotion.