Periodic Colony Formation by Bacterial Species Bacillus subtilis

We have investigated the periodic colony growth of bacterial species Bacillus subtilis. A colony grows cyclically with the interface repeating an advance (migration phase) and a rest (consolidation phase) alternately on a surface of semi-solid agar plate under appropriate environmental conditions, resulting in a concentric ring-like colony. It was found from macroscopic observations that the characteristic quantities for the periodic growth such as the migration time, the consolidation time and the terrace spacing do not depend so much on nutrient concentration Cn, but do on agar concentration Ca. The consolidation time was a weakly increasing function of Ca, while the migration time and the terrace spacing were, respectively, weakly and strongly decreasing function of Ca. Overall, the cycle (migration-plus-consolidation) time seems to be constant, and does not depend so much on both Cn and Ca. Microscopically, bacterial cells inside the growing front of a colony keep increasing their population during both migration and consolidation phases. It was also confirmed that their secreting surfactant called surfactin does not affect their periodic growth qualitatively, i.e., mutant cells which cannot secrete surfactin produce a concentric ring-like colony. All these suggest that the of the nutrient and the surfactin are irrelevant to their periodic growth.