Characterizing Bacillus subtilis colony morphology in response to environmental acidification
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Abstract
Bacterial community dynamics in the human gut microbiome can have profound effects on human health and disease, yet experimental limitations make it extremely difficult to study this environment with sufficient ecological validity. To work toward physiological relevance, this study ostensibly aimed to assess the morphological changes in Bacillus subtilis colonies in response to pH transience. However, before observing colony responses at pH transition points, static hard substrate experiments in acidic conditions revealed a novel colony growth phenotype. The bulk of the work that follows this observation aims to characterize and explain the novel ‘microcolony’ phenotype; this phenotype is characterized by an asymmetrical formation of cell clusters within the inoculation drop initially, which then converge to form a dense, cohesive colony with minimal radial expansion. Experiments revealed a lack of swarming motility and increased chaining in acidic conditions – with no evidence to support a pH sensation and chemotaxis element to the phenotype. Our results suggest that apparent increased cell-cell adhesion under acidic conditions is a result of both pH-driven matrix protein aggregation and a pH-driven defect in cell division machinery. While both theories are strongly supported by our findings and the existing literature, we recommend further experiments to assess the contributions of each of these factors in determining colony morphology.