Fractal Growth of<i>Bacillus subtilis</i>on Agar Plates

TLDR

Bacteria can exhibit diverse morphologies on agar plates depending on growth conditions. A Bacillus subtilis strain was inoculated on agar and incubated at 35 °C. The colonies formed two‑dimensional, randomly branched patterns that were self‑similar with a fractal dimension of 1.716±0.008, matching the diffusion‑limited aggregation model; interior branches ceased growth due to screening, and on moist plates the colonies grew radially with regular branches, indicating a dense‑radial morphology.

Abstract

Bacteria have been shown to grow with various morphologies under different conditions on agar plates. A Bacillus subtilis strain was inoculated on the plate surface and incubated at 35°C. Colonies grew two-dimensionally with random branches, similar to clusters of the diffusion-limited aggregation (DLA) model. The colony patterns were analyzed and found to be self-similar with a fractal dimension of 1.716±0.008, in excellent agreement with the expected value of the DLA model. Interior branches were observed to stop growing in spite of their open neighborhood during the incubation period, implying the existence of a screening effect. These results clearly suggest that the colony pattern of the organism was formed through the DLA process. Moreover, the colonies were found to grow radially with almost regular branches on agar plates with moist surfaces, reminiscent of "dense radial" morphology.