Oscillatory expansion of Bacillus subtilis biofilms on MSgg agar medium.

We investigated the oscillatory expansion of Bacillus subtilis biofilms on minimal salts glycerol glutamate (MSgg) agar by modulating the concentration of the nutrient glutamate and the rigidity of the surface via agar concentration. The concentration of glutamate and agar influenced the parameters of biofilm oscillation. Specifically, elevated glutamate levels boosted the initial growth rate (within the first 15 h) and prolonged the oscillation period. Conversely, increasing the agar concentration physically restrained colony expansion and dampened the amplitude of growth rate variations, leading to a more uniform, yet oscillatory, progression. Beyond altering the physicochemical environment, we probed the biofilm under competitive condition; the oscillatory period lengthened and resulted in asymmetric colony morphology. Furthermore, during self-healing following mechanical disruption, the oscillatory pattern was intact; the biofilm regenerated, with rapid recovery at the colony edges than at the center. To validate these experimental observations, we developed a spatial-temporal model of biofilm metabolism. This computational model successfully simulated the oscillatory growth on solid surfaces. It confirmed that the interplay between localized nutrient availability and the development of physical heterogeneity in biofilm thickness is a fundamental driver of the oscillatory patterns observed in B. subtilis biofilms, providing a unified theoretical basis for our empirical findings.