Rivalry in Bacillus subtilis Colonies: Enemy or Family?

A bacterial colony constitutes a complex microcosm where bacteria interact with each other and the external environment to ensure the survival of the colony. Two adjacent Bacillus subtilis colonies growing on a nutrient rich agar plate have been found to exhibit two distinct interactions: they either coalesce as they grow or form an interface at the colony fronts without complete coalescence. The nature of interaction has been found to be dependent on the agar concentration (C) in the growth medium and the initial separation between the colonies (d). Interactions of a colony with solid structures and liquid drops have indicated that biochemical rather than physical factors are responsible for the interface formation. A simple mathematical model has been formulated to numerically simulate the two interaction patterns. The model predicts the nature of interaction for the given combination of C and d. The model prediction agrees well with experimental findings and generates a C-d phase diagram containing coalescence and interface regimes. The study provides valuable insights into the dynamics of bacterial colonies, especially when multiple colonies interact with each other.