Physiological and cell morphology adaptation of B acillus subtilis at near‐zero specific growth rates: a transcriptome analysis

Nutrient scarcity is a common condition in nature, but the resulting extremely low growth rates (below 0.025 h −1 ) are an unexplored research area in B acillus subtilis . To understand microbial life in natural environments, studying the adaptation of B . subtilis to near‐zero growth conditions is relevant. To this end, a chemostat modified for culturing an asporogenous B . subtilis sigF mutant strain at extremely low growth rates (also named a retentostat) was set up, and biomass accumulation, culture viability, metabolite production and cell morphology were analysed. During retentostat culturing, the specific growth rate decreased to a minimum of 0.00006 h −1 , corresponding to a doubling time of 470 days. The energy distribution between growth and maintenance‐related processes showed that a state of near‐zero growth was reached. Remarkably, a filamentous cell morphology emerged, suggesting that cell separation is impaired under near‐zero growth conditions. To evaluate the corresponding molecular adaptations to extremely low specific growth, transcriptome changes were analysed. These revealed that cellular responses to near‐zero growth conditions share several similarities with those of cells during the stationary phase of batch growth. However, fundamental differences between these two non‐growing states are apparent by their high viability and absence of stationary phase mutagenesis under near‐zero growth conditions.