Understanding the way motile micro-organisms such as bacteria explore their environment is central to many ecological, medical and biotechnological issues. Here, I will present recent advances on the kinematics of bacteria such as E.coli, undergoing sequences of runs and tumbles which finally lead to a random-walk exploration process. The extreme sensitivity of the flagella motor rotation switch to the presence of a phosphorylated protein in its vicinity, inherently leads to a behavioral variability of the run-times characterized by a non-Markovian memory and a large log-normal distribution [1]. This mechanism prevails in most Newtonian fluids with important consequences on the residence times at surfaces [2] as well as on the large scale dispersion properties in confined environments [3].

However, when the surrounding fluid is a yield-stress fluid, the locally high resistance to penetration takes control of the exploration process. The presence of mechanical heterogeneity coupled to the bending of the flagella bundle, takes control of the bacteria reorientation eventually leading to a reduction of the run persistence times, the emergence of transient trapping and finally, the onset of a motility barrier.Those mechanisms we describe as «medium assisted tumbling» are cruicial to understand the penetration process and trapping of bacteria in many biological situations such as in mucus layers an important line of defence of epithelial cells in the intestinal track.

References:

[1] N. Figueroa-Morales et al Phys.Rev.X, 10, 021004 (2020).

[2] G.Junot et al., Phys.Rev.Lett., 128, 248101  (2022).

[3] N.Figueroa-Morales et al; Science Advances, 6, eaay0155 (2020).