Different Resource Allocation in a Bacillus subtilis Population Displaying Bimodal Motility

To cope with sudden changes in their environment, bacteria can use a bet-hedging strategy by dividing the population into cells with different properties. This so-called bimodal or bistable cellular differentiation is generally controlled by positive feedback regulation of transcriptional activators. Due to the continuous increase in cell volume, it is difficult for these activators to reach an activation threshold concentration when cells are growing exponentially. This is one reason why bimodal differentiation is primarily observed from the onset of the stationary phase, when exponential growth ceases. An exception is the bimodal induction of motility in Bacillus subtilis, which occurs early during exponential growth. Several mechanisms have been put forward to explain this, including double-negative feedback regulation and the stability of the mRNA molecules involved. In this study, we used fluorescence-assisted cell sorting (FACS) to compare the transcriptomes of motile and nonmotile cells and noted that expression of ribosomal genes is lower in motile cells. This was confirmed using an unstable green fluorescent protein (GFP) reporter fused to the strong ribosomal rpsD promoter. We propose that the reduction in ribosomal gene expression in motile cells is the result of a diversion of cellular resources to the synthesis of the chemotaxis and motility systems. In agreement with this, single-cell microscopic analysis showed that motile cells are slightly shorter than nonmotile cells, an indication of slower growth. We speculate that this growth rate reduction can contribute to the bimodal induction of motility during exponential growth.