Supplementary material from "Deciphering the Influence of NaCl on Social Behaviours of Bacillus subtilis"

Various environmental signals, such as temperature, pH, nutrient levels, salt content, and the presence of other microorganisms, can influence the biofilm's development and dynamics. However, the innate mechanisms that govern at the molecular and cellular levels remain elusive. Here, we report the impact of physiologically relevant concentrations of NaCl on the biofilm formation and the associated differences in an undomesticated natural isolate of Bacillus subtilis. NaCl exposure and its uptake by bacterial cells induced substantial changes in the architecture of pellicle biofilm and an upsurge in the expansion of biofilm colonies on agar surfaces. We have observed the upregulation of genes involved in motility and the downregulation of genes involved in the biosynthesis of extracellular matrix components through the transcription factor sigD, suggesting the possible underlying mechanisms. To further support these observations, we have used ΔsigD and ΔsrfAC null mutants, which showed compromised NaCl-induced effects. Our results indicate that NaCl induces a lifestyle shift in B. subtilis from a sessile biofilm state to an independent unicellular motile state. Overall, we present evidence that NaCl can reprogram gene expression and alter cellular morphology and the state of cells to adapt to motility, which facilitates the expansion of bacterial colonies.