Migration of chemotactic bacteria in soft agar: role of gel concentration

TL;DR

This study investigates how gel concentration affects chemotactic bacterial migration in soft agar, revealing transitions from chemotactic rings to broad bands and front instabilities, supported by an extended Keller-Segel model.

Contribution

It introduces a novel extended Keller-Segel model with new transport parameters that explain the impact of agar collisions on bacterial chemotaxis.

Findings

Chemotactic rings form at low agar concentrations (<0.35%).

At 0.35% agar, bacteria migrate as broad growth/diffusion waves.

Migration front speed decreases sharply above 0.25% agar.

Abstract

We study the migration of chemotactic wild-type Escherichia coli populations in semisolid (soft) agar in the concentration range C = 0.15-0.5% (w/v). For C < 0.35%, expanding bacterial colonies display characteristic chemotactic rings. At C = 0.35%, however, bacteria migrate as broad circular bands rather than sharp rings. These are growth/diffusion waves arising because of suppression of chemotaxis by the agar and have not been previously reported experimentally to our knowledge. For C = 0.4-0.5%, expanding colonies do not span the depth of the agar and develop pronounced front instabilities. The migration front speed is weakly dependent on agar concentration at C < 0.25%, but decreases sharply above this value. We discuss these observations in terms of an extended Keller-Segel model for which we derived novel transport parameter expressions accounting for perturbations of the chemotactic response by collisions with the agar. The model makes it possible to fit the observed front speed decay in the range C = 0.15-0.35%, and its solutions qualitatively reproduce the observed transition from chemotactic to growth/diffusion bands. We discuss the implications of our results for the study of bacteria in porous media and for the design of improved bacteriological chemotaxis assays.