Swimming of a Non-Flagellated Bacterium by a Non-Rotary Molecular Motor

Abstract

Free-swimming bacteria commonly exploit a system of propeller-like appendages called flagella to generate propulsion, typically attached to rotary molecular motors. In harsh contrast, the helically-shaped and minimalistic genus Spiroplasma lacks such a system but is able to swim anyway. This is achieved by a linear motor generating pairs of kinks propagating down the length of the cell body. The motor itself consists of a contractile chain of approx. 600 identical proteins spanning the entire length of the cell. Kinks are generated by a concerted spread of conformational changes of the motor's subunits within approx. 100ms, always starting at the same end of the cell.Here, we use object-adapted optical trapping and shape-tracking to characterize the dynamics of the swimming pattern of single cells under different metabolic and chemical conditions. Further, we demonstrate a force sensitivity of the motor by optically mimicking micro-environments with different elasticity. The results suggest that the motor performs better under high external loads, i.e., in stiff environments. We present a rate model describing the subsequent and highly cooperative conformational switching of the chain's subunits, allosterically triggered by binding of an effector ligand. This is a further step to draw a complete picture of the maybe simplest principle of autonomous locomotion possible on the single cell level which might have an important impact on constructing bio-inspired, autonomous micro-machines.