Abstract
The emergence of diffusion-limited growth (DLG) within a microbial colony on a solid substrate is studied using a combination of mathematical modelling and experiments. Using an agent-based model of the interaction between microbial cells and a diffusing nutrient, it is shown that growth directed towards a nutrient source may be used as an indicator that DLG is influencing the colony morphology. A continuous reaction–diffusion model for microbial growth is employed to identify the parameter regime in which DLG is expected to arise. Comparisons between the model and experimental data are used to argue that the bacterium Bacillus subtilis can undergo DLG, while the yeast Saccharomyces cerevisiae cannot, and thus the non-uniform growth exhibited by this yeast must be caused by the pseudohyphal growth mode rather than limited nutrient availability. Experiments testing directly for DLG features in yeast colonies are used to confirm this hypothesis.
Highlights
When placed on a solid substrate, many types of unicellular microbes, such as bacteria and fungi, grow into colonies consisting of numerous individual cells
The model could be used to quantify the behaviour of each phenomenon, showing that biased growth provides a good indicator of diffusion-limited growth (DLG)
By matching the model results to experimental data, it was found that the typical growth of the bacterium B. subtilis corresponds to a region in which directed growth due to DLG may occur, while that of the yeast S. cerevisiae does not
Summary
When placed on a solid substrate, many types of unicellular microbes, such as bacteria and fungi, grow into colonies consisting of numerous individual cells. The colony may undergo diffusion-limited growth (DLG), which typically manifests as a non-uniform colony shape While such a change can occur without any change in the morphology of individual cells, as exhibited by the bacterium Bacillus subtilis, certain microbes may respond actively to low nutrient levels. The cell performing the random walk may instead be interpreted as a nutrient particle that is absorbed by an existing cell upon contact, allowing the existing cell to reproduce and place a new cell in an adjacent site Under this interpretation, the DLA model represents colony growth at very low nutrient concentrations and represents a form of DLG. Simulations using this model suggested that outer branches ‘screen’ inner sites and grow faster
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