A multi-phase extensional flow model for sliding motility in yeast biofilms
Date:
Abstract: Most bacteria and fungi exist in biofilm colonies, consisting of adherent cells embedded in a fluid extracellular matrix. Yeast biofilms can form on indwelling medical devices, making them a leading cause of bloodstream infection. As biofilms are highly resistant to anti-fungal treatment, the mortality rate of these infections can approach 50% for patients in intensive care units. Due to this, yeast biofilms have attracted significant research attention. However, although inducing biofilm formation in the laboratory is possible, less is known about the physics governing their expansion.
Based on similar observations in bacteria, one hypothesis is that yeast biofilms expand by sliding motility. This involves a sheet of cells spreading as a unit, enabled by weak adhesion to the substratum. We construct a multi-phase fluid model for a biofilm growing in a nutrient-limited environment. We assume that the biofilm consists of two viscous fluid phases: living cells and an extracellular matrix. As the width of a biofilm significantly exceeds its height, we employ an extensional flow thin-film approximation to simplify the governing equations. This gives rise to a one-dimensional axisymmetric model for the biofilm height, cell volume fraction, nutrient concentration, and fluid velocity.
To test the sliding motility hypothesis, we compare numerical solutions with yeast biofilm formation experiments. We find good agreement in expansion speed between numerical solutions and data from multiple experiments. The model also reproduces the ridge-like structure that is observed in experiments. From these findings, we conclude that sliding motility is a possible mechanism for yeast biofilm formation.