We had previously found that the bacterial flagellar motor is resistant to ionizing radiation at a dose that is sufficient to inhibit bacterial growth. This implies that some enzymatic activity remains after irradiation to maintain the metabolic network in cells. In the present study, to estimate the persistence of bacterial motility after irradiation, we investigated the swimming ability of gamma-irradiated cells after maintenance in a non-nutrient motility medium for several hours at room temperature or at 4 °C. Hence, the motility of the gamma-irradiated cells, which showed no colony-forming ability, lasted for more than 1 day. Swimming speed and the motile fraction were not significantly different between intact (unirradiated) and irradiated cells, whereas these parameters decreased gradually with incubation time. Cells stored at 4 °C did not swim; however, motility was recovered after bacteria reached room temperature.

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