Different Resource Allocation in a Bacillus subtilis Population Displaying Bimodal Motility

Bacteria often use bet-hedging strategies to adapt to unpredictable environments. These strategies involve splitting a population into cells with distinct traits, such as motile and nonmotile states. Typically, this bimodal differentiation occurs when growth slows, such as during the stationary phase. However, Bacillus subtilis shows bimodal motility early in exponential growth, which is unusual. Prior studies have proposed mechanisms like double-negative feedback and mRNA stability to explain this phenomenon. Still, the exact link between gene expression and motility remains unclear. This gap motivated the current study to investigate the transcriptomic differences between motile and nonmotile cells. The researchers aimed to determine how resource allocation might influence the bimodal expression of motility in B. subtilis. By comparing gene expression profiles, they sought to uncover the underlying regulatory mechanisms. This work builds on existing knowledge of bacterial differentiation but introduces new insights into resource allocation during exponential growth. The findings could help clarify how cells prioritize different functions under varying growth conditions.

The study aimed to explore the gene expression differences between motile and nonmotile cells in B. subtilis during exponential growth. The researchers focused on understanding how resource allocation affects motility and growth. They hypothesized that motile cells might divert energy from ribosome synthesis to motility-related processes. To test this, they used fluorescence-assisted cell sorting (FACS) to isolate and compare transcriptomes of motile and nonmotile cells. The goal was to identify genes whose expression levels differ significantly between the two cell types. They also wanted to confirm their findings using a reporter system based on a ribosomal gene promoter. The study sought to determine whether reduced ribosome expression in motile cells correlates with slower growth. This approach allowed the researchers to link gene expression patterns to observable cellular traits like cell length and growth rate.

The researchers used fluorescence-assisted cell sorting (FACS) to separate motile and nonmotile cells from a B. subtilis culture. They then compared the transcriptomes of these two cell types to identify differentially expressed genes. To validate their findings, they constructed a reporter system using an unstable green fluorescent protein (GFP) fused to the promoter of the ribosomal gene rpsD. This reporter allowed them to measure ribosomal gene expression at the single-cell level. They also performed single-cell microscopic analysis to assess cell length and growth rates. By comparing the fluorescence intensity of the reporter in motile versus nonmotile cells, they could quantify differences in ribosomal gene expression. The FACS method enabled precise isolation of cells with distinct motility states. The use of a GFP reporter provided a direct readout of ribosomal gene activity. The microscopic analysis confirmed that motile cells were shorter than nonmotile cells, suggesting a growth rate difference.

The transcriptome comparison revealed that motile cells expressed fewer ribosomal genes compared to nonmotile cells. This finding was confirmed using the rpsD-GFP reporter system, which showed lower fluorescence in motile cells. The reduced ribosomal gene expression suggested a reallocation of cellular resources toward motility-related functions. Single-cell microscopic analysis indicated that motile cells were slightly shorter than nonmotile cells, which is consistent with slower growth. This growth difference may explain the bimodal induction of motility during exponential growth. The data suggest that motile cells prioritize the synthesis of motility systems over ribosome production. This reallocation of resources leads to a temporary reduction in growth rate. The study provides evidence that resource allocation influences the expression of motility traits in B. subtilis.

The study found that motile cells in B. subtilis express fewer ribosomal genes than nonmotile cells. This suggests a shift in resource allocation toward motility systems rather than ribosome synthesis. The reduced ribosome expression correlates with slower growth in motile cells, as observed through microscopic analysis. The researchers propose that this reallocation of resources contributes to the bimodal induction of motility during exponential growth. The findings support the idea that growth rate reduction can stimulate bimodal differentiation. The use of FACS and a reporter system provided strong evidence for the proposed mechanism. The results align with the authors' hypothesis that resource allocation plays a key role in motility regulation. These conclusions are based directly on the observed gene expression and growth patterns in the study.