Lost in the thicket: how agar can suppress bacterial chemotaxis

Bacteria often dwell in porous habitats (e.g. soil, food, ...) where successful colonisation and survival often depend crucially on chemotactic** motility. Agar, a polysaccharide gel, is seldom thought of as porous when studying bacteria. High agar concentrations (10-20 g/litre) are mixed with nutrients to make ''hard'' gels on which cells are cultured. When studying bacterial chemotactic motility, however, lower (''soft'') concentrations (1-4 g/litre) are employed to make gels which are penetrable by bacteria. The chemotactic motion of bacteria in liquid environments has been extensively studied, and is well understood. The study of bacteria in gels, however, is found wanting. The understanding of bacteria in similarly porous media is more advanced, but also incomplete. One would expect bacterial chemotactic motion in a gel (porous medium) to be perturbed by the latter's microstructure. Indeed, amongst other results, in this seminar I will present evidence of the drastic changes a gelatinous environment can affect on bacterial motion: increasing agar concentration (within the ''soft'' range) can ''switch off'' chemotaxis in a population of Escherichia coli cells genetically capable of chemotactic response.

** The ability of organisms to direct their motion towards chemically favourable environments and away from toxic ones is known as chemotaxis.