The motility of B. subtilis determines the timescale of the antimicrobial action of chitosan-based hydrogels

We study the motility of B. subtilis confined within antimicrobial Chitosan(CT)-Polyvinylalcohol(PVA) porous hydrogels using confocal microscopy and particle tracking. We show that with increasing CT content the bacterial motility reduces, as indicated by the average mean squared displacements (MSDs), and the mean bacterial velocities. Furthermore, the short-time motility changes qualitatively from the straight runs characteristic of the run-and-tumble motility to diffusion and sub-diffusion. In the anomalous diffusion regime a large spread of individual MSDs indicates the presence of pronounced dynamical heterogeities. At the same time the ergodicity breaking parameter presents a maximum at small CT content. We interpret these findings in terms of a competition between confinement effects and the electrostatic interactions between bacteria and hydrogel, with the former being important at small CT contents, and leading to a broadening of the motility distribution, while the latter dominating at larger CT fractions and inducing slower but more homogeneous dynamics. We show that the effects on motility are the first stage of the antimicrobial action, that occurs on a timescale that depends on CT content and determines the onset of bacterial damage. Our results thus show that CT content tunes both the strength and the timescale of antimicrobial action.