Helical swimming motion driven by coordinated rotation of flagellar apparatus in marine bacterial cells

The magnetotactic bacterium MO-1 possesses a pair of unique flagellar apparatuses. Each apparatus comprises seven flagella and 24 thin fibrils enclosed in a sheath. Using these apparatuses, the cells swim at high speed through solutions, tracing helical trajectories. The mechanism by which MO-1 cells utilize their two flagellar apparatuses to propel the cell body remains unclear. Due to the short length of the flagellar apparatus, direct observation of individual movements under a microscope is technically challenging. In this study, we performed numerical simulations based on the boundary element method to investigate the swimming motility of MO-1 cells. Our cell models successfully reproduced the helical trajectories of active MO-1 cells. The two flagellar apparatuses were positioned at the front and back of the cell body relative to the direction of movement. The cell body was pulled by the anterior flagellar apparatus and pushed by the posterior apparatus. The swimming motion of the cell model exhibited minimal changes near a wall. This unique swimming behavior of MO-1 cells could only be achieved through the rotation of both flagellar apparatuses. Based on the revolution rate of active MO-1 cells, we estimated the rotational speed of the flagellar apparatus to be approximately 1200 s-1. This rapid rotation may result from the tight packing of flagellar filaments within the sheath. Our study elucidates the mechanism by which active MO-1 cells swim in helical trajectories.