Regulation of flagellar motor mechanosensitivity by stator pool size

Bacterial swimming is powered by protein complexes, stators, which produce the torque necessary to spin flagella. Stators are found either freely diffusing in the inner membrane or attached to the flagellar motor in a torque generating configuration. The motor responds to external conditions in a mechanosensitive manner, recruiting a full complement of stators under high load and releasing them under low load. An E. coli cell is estimated to have approximately 100 stators, more than the number needed to power each motor at maximum capacity. Here, we study the effect of the total number of stators on the motility of E. coli through controlled expression of the stators. We develop a model for stator dynamics that includes the number of diffusing stators. We test our model against measurements of entire cell populations as well as single motors. Our results show that the binding process of stators depends on the total number of stators in a cell. The flagellar motor is sensitive to stator pool size at low load, but the effect of the stator pool diminishes under high load conditions. Our findings both support and extend previous studies, revealing that stator expression is likely an evolutionarily optimized feature of the bacterial flagellar motor.