Bacterial Olympics: swimming speed is independent of body size

For human swimming at Re ~ 106, size typically matters: the winners' podium is typically occupied by taller athletes. Here, we image the motility of Escherichia coli, a flagellated bacterial strain swimming at Re ~ 10-6, using confocal microscopy. In particular, we explore the correlation between bacterial swimming speeds and their body sizes. Counterintuitively, we find that the speed of bacteria is constant, irrespective of their body size in the range of 1 μm up to 3 μm. To understand this surprising observation, we visualize E. coli flagella by fluorescent labeling. The results demonstrate the relation between the bacterial cell body and flagellar conformation, which is not accounted for in the existing theoretical framework of bacterial motility. Thus, our study provides new insights for bacterial motility relevant to a wide range of biological and biomedical processes.