Understanding the external stimuli induced control over bacterial motility and biofilm formation

Abstract

Bacteria is a type of active matter which consumes food as an energy source and then dissipates the energy to expand on the surface efficiently. They live collectively and form a macroscopic colony that expands over the surface. While expanding it shows a wide range of dynamical characteristics and pattern formations that depend on the physical and chemical conditions of the environment. Over the last couple of decades, there has been significant progress in understanding the physical and biological reasons behind the dynamical and morphological features of the bacterial macroscopic colony. In recent years, diffusion of nutrients into the colony, wettability of the surface were revealed as important factors affecting the expansion of bacterial colony over the surface. Moreover, various chemical cues were shown to impact the dynamical characteristics and emergent features of the colony. Here, we experimentally investigated the growth kinetics of the macroscopic colony of Bacillus subtilis IITKSM1 across different length scales by tuning the wettability and the mesoscopic structure of the underlying substrate by varying its agar concentration. The growth rate of the macroscopic colony increased i.e., accelerated with time for all agar concentrations beyond a specific critical time. However, at later times, we found a transition from the accelerating growth phase to a deceleration for substrates made with agar concentrations greater than 0.5 % . We investigated the time at which this transition occurs and find that it decrease with increase in agar concentration. After a given time for the formation of biofilms, we found an intriguing non-monotonic dependence of the production of biomass: For lower agar concentrations the biomass increased, and at higher agar concentrations the biomass decreased with increase in agar concentrations. While the initial increase might be dominated by the spreading characteristics of the substrate, our simulations, based on Fisher population dynamics, revealed that the later decrease is due to the depletion of nutrients at the growth front. Our results reveal the complex interplay of the surface wetting characteristics and the nutrient availability for biofilm growth. The numerical simulation was done for understanding this transition behavior as a prelude for further research in this regard. We also investigated the effect of NaCl on the surface motility of the bacteria. We report that the presence of NaCl increases the surface motility of the bacteria by enhancing the motility of the individual cells. The microscopic studies revealed that bacteria exhibit subdiffusive motion on a long time scale in the absence of NaCl whereas diffusive motion in presence of NaCl. Interestingly, the addition of NaCl decreased the overall dynamic heterogeneity observed at the expanding edge of the colony. Complementary efforts at the biological side allowed us to reveal NaCl-induced differences in the expression/suppression of selected genes, which shed light on the observed microscopic and macroscopic dynamical characteristics