Mechanical–electrochemical coupling theory of bacterial cells

The environmental adaption, growth, motion, and other dynamic behaviors of bacteria are closely associated with the coupled mechanical–electrochemical properties of their subcellular structures, but the underlying regulatory mechanisms remain unclear. The mechanical responses of bacteria are difficult to elucidate by traditional models without considering the mechanical, chemical, and electric coupling effects. In this paper, a mechanical–electrochemical theory is constructed to investigate the deformation behavior of bacterial cells. A bacterium is treated as a bilayer structure consisting of a negatively charged polysaccharide capsule and an elastic envelope subjected to turgor pressure. This model is used to reveal the regulating roles of the electrostatic double-layer force and osmosis under different electrolyte conditions. Good agreement is found between the theoretical predictions and the experimentally observed three-stage nanoindentation responses of Klebsiella pneumoniae (K. pneumoniae) bacterial cells. Furthermore, we investigate the mechanical–electrochemical coupling mechanisms in the compression resistance of the bacterial capsule. The results reveal that the osmosis induced by ionic imbalance and polysaccharide–solvent aggregates plays a significant role in the compression resistance of the capsule. The present model not only deepens our understanding of the mechanical–electrochemical coupling mechanisms of bacterial cells at the subcellular scale, but also holds promise for applications in characterizing their mechanical properties.