Cohesive swimming and separation of bacterial pairs in dilute suspensions

In this study, we use numerical simulations to investigate the dynamical behavior of a pair of Escherichia coli bacteria that move toward each other. For wild-type bacteria, our results show that there exists a region in the space defined by the impact parameters, within which the experimentally observed phenomenon of cohesive swimming can be reproduced, where the pair swims side by side for extended periods before the eventual separation. The boundary of the cohesive-swimming region with respect to the non-cohesive-swimming region in the parameter space is then identified. While the cohesive-swimming state can be well characterized by two key features: spatial closeness (small surface-to-surface distance) and orientational alignment (small angle between the major axes of the ellipsoidal bacterial bodies), previous studies largely attribute the eventual separation through misalignment caused by angular diffusion. In this study, our results demonstrate that translational motion plays a crucial role, and cohesive-swimming pairs mostly separate due to large spatial separation while orientationally still aligned. More numerical studies are then performed for bacteria with different size characteristics and the results show that our conclusion is quite robust against variations in bacterial dimensions.