Competition on Surfaces

Competition on surfaces is quite different from competition in well-mixed liquid environments. Selection becomes decoupled from colony growth rates. Theory predicts, and experiments confirm, that selection in the basal layer of a biofilm effectively ceases for bacteria with tiny detachment rates. Theory predicts, and experiments with colicinogenic Escherichia coli demonstrate, that while toxic interference of another competitor produces a priority effect in a well-mixed environment, it is favored on surfaces where benefits are sequestered locally. Patches of colicin-producing (C), -sensitive (S) and -resistant (R) strains chase one another across agar surfaces (C beats S beats R beats C). Mixing produces a three-way competition, in which case the resistant strain wins. Sectoring in colonies, founded by dropping a mixture of fluorescently marked strains onto agar plates, forms a visual record of the history of random genetic drift and selection. A slow-growing strain may yet win the competition by blocking access of the fast-growing strain to nutriment diffusing in from the surroundings. Chemotaxis, commonly assumed to be a search for fresh resources, is a way of staking out territory prior to consuming those resources at leisure. Costly to maintain, slow-growing chemotactic strains can form a balanced polymorphism with fast-growing nonmotile strains in mixed colonies.