Motile bacteria, active biohybrids and cellular physiology
Le Nagard, Lucas
This thesis investigates bacterial motility from active matter and physiological perspectives using experiments and theoretical modelling. In the first part, I design and characterize a system made of motile Escherichia coli encapsulated in giant lipid vesicles. For slightly deflated vesicles, the bacteria extrude active membrane tubes that can propel the vesicles. I show that the propulsion arises from a physical coupling between the lipid membrane tubes and the flagella of the encapsulated bacteria and develop a simple theoretical model to estimate the propulsive force. In a second part, I present two studies using motility as a tool to gain insight into bacterial physiology. First, I study the motility of dense suspensions of Escherichia coli fermenting glucose. Using new experimental data gathered by others, I develop a semi-empirical model that quantitatively links the swimming speed of the bacteria to the concentration of protonated organic acids in anaerobic conditions. Secondly, I focus on bacterial motility during complete starvation. Combining single-cell and population-level experiments, I show that Escherichia coli maintains a motile phenotype in the early stages of starvation, but that the swimming speed and motile fraction decay over a few tens of hours. I show that the complete decay of motility in these conditions happens on a much faster timescale than cell death. Interestingly, while swimming speed and flagellar motor measurements both show that the motility fully decays in about 24 h in these conditions, they seem to return different temporal dynamics.