A thin-film extensional flow model for biofilm expansion by sliding motility
Date:
Abstract: Biofilms are sticky communities of microbial cells and fluid that exist on surfaces. Since bacterial and yeast biofilms are highly resistant to antimicrobials, they are a leading cause of hospital-acquired infections. In biofilm formation experiments, yeast cells adhere weakly to the agar substratum, leading experimentalists to hypothesise that yeast biofilms expand by sliding motility. This involves a sheet of cells and fluid spreading as a unit, facilitated by cell proliferation and weak biofilm–substratum adhesion. The objective of our paper was to derive an accurate model for the sliding motility mechanism, and to provide quantitative evidence that it can explain the biofilm expansion observed in experiments.
We modelled the biofilm as a two-phase (living cells and an extracellular matrix) viscous fluid mixture, and incorporated the movement, uptake, and consumption of nutrients. Since the governing equations obtained from conservation of mass and momentum are complicated, we used the thin-film approximation in the extensional flow regime to derive a simpler one-dimensional axisymmetric model. We obtained excellent agreement between the experimental expansion speed and numerical solutions to the model using parameters estimated from experiments. Having established the biological relevance of the model, we then used further numerical solutions to predict the effect of parameters on expansion speed and the biofilm profile. These results confirmed that sliding motility is a possible explanation for yeast biofilm expansion, and identified features that inhibit or enhance biofilm growth.