| Summary: | The bacterial flagellar motor (BFM) is the molecular machine responsible for the swimming and chemotaxis of many species of motile bacteria. The BFM is bidirectional, and changes in the rotation direction of the motor are essential for chemotaxis. It has previously been observed that many species of bacteria also demonstrate brief pauses in rotation, though the underlying cause of such events remains poorly understood. We examine the rotation of Escherichia coli under low mechanical load with high spatial and temporal resolution. We observe and characterize transient pauses in rotation in a strain which lacks a functional chemosensory network, showing that such events are a phenomenon separate from a change in rotational direction. Rotating at low load, the BFM of E. coli exhibits about 10 pauses per second, lasting on average 5 ms, during which time the rotor diffuses with net forwards rotation. Replacing the wild type stators with Na+ chimera stators has no substantial effect on the pausing. We discuss possible causes of such events, which are likely a product of a transient change in either the stator complex or the rotor.
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