Bacterial surface swimming states revealed by TIRF microscopy

Motility near solid surfaces plays a key role in the life cycle of bacteria and is essential for biofilm formation, biofilm dispersal, and virulence. The alignment of the cell body with the surface during surface swimming impacts bacterial surface sensing. Here, we developed a high-throughput method for characterizing the orientation of the cell body relative to the surface using total internal reflection fluorescence (TIRF) microscopy. The angle between the cell body and the surface was determined by maximizing image cross-correlations between the TIRF image of the cell and a reference library. Utilizing this technique, we surprisingly identified six distinct surface swimming states of Pseudomonas aeruginosa according to the body alignment and the flagellar position. Furthermore, we observed that the near-surface swimming speed is greater in the pull state than in the push state, attributed to hydrodynamic effects near the liquid-solid interface. Hydrodynamic force analysis of the swimming states provided rich insights into the mechanics of bacterial surface swimming. Our technique is readily applicable to the study of surface motility across a wide spectrum of bacterial species.