Fractal morphogenesis by a bacterial cell population

Abstract

Many species of bacteria have been found to form fractal colonies. Environmental (physicochemical) and biological factors for this fractal morphogenesis have been examined for their roles in the genesis of fractal and pattern diversity. Morphology of a bacterial colony on a solid agar medium depends on the nutrient diffusion field (two-dimensional). When concentrations of nutrients are low, point-inoculated bacteria (e.g., Bacillus subtilis) exert diffusion-limited growth. A self-similar fractal colony formed slowly under such a condition has the same morphology as one made by the diffusion-limited aggregation (DLA) model. The value of fractal dimensions (ca. 1.72) and the appearance of specific phenomena (screening and repulsion effects) are consistent with computer simulations of the DLA fractal model. On the other hand, a round colony recognized on an agar-rich medium was considered to be the product of reaction-limited growth and was simulated by the Eden model. When motile bacteria are point inoculated onto semi-solid agar media, bacterial spreading behavior also is morphogenic. Branching patterns with various morphologies (e.g., dense-branching morphology) have been recognized and examined for factors responsible for pattern changes. By microscopic inspection of the extending branch, multicellular behavior of bacteria has been observed in the structured cell distribution. Besides cell division and translocation activities, wetting agents produced by some species of bacteria (e.g., serrawettins produced by Serratia marcescens) are considered to be important microbial factors for efficient space occupation and specific cell transpositions in various surface environments.