Temporal ^B stress-response profiles impact Bacillus subtilis fitness

The Gram-positive model organism Bacillus subtilis responds to environmental stressors by activating the alternative sigma factor sigma(B). The sensing apparatus upstream of sigma(B) activation is thought to consist of cytoplasmic stressosomes-megadalton-sized protein complexes that include five paralogous proteins known as RsbRs. The RsbRs are presumed to be involved in stress sensing and the subsequent response. Perturbations to the RsbR complement in stressosomes by engineering cells that produce only one of the RsbR paralogs ("single-RsbR strains") lead to altered sigma(B) response dynamics with respect to timing and magnitude. Here, we asked whether such changes to sigma(B) response dynamics impact the relative fitness of a strain. We competed strain pairs with different RsbR complements under ethanol and sodium chloride stress and found not only differences in relative fitness among wild-type and single-RsbR strains but also different relative fitness values in the two different stressors. We found that the presence of RsbRA, which dominates the wild-type sigma(B) response, enhances fitness in ethanol but is detrimental to fitness in NaCl. Meanwhile, RsbRD-only cells were among the most fit in NaCl. Strains producing hybrid RsbR fusion proteins displayed different fitness values that depended on the RsbR proteins from which they were derived. Our results here suggest that sigma(B) response dynamics can impact fitness, highlighting the physiological importance of the unusual stressosome-based general stress response system of B. subtilis.IMPORTANCE The model bacterium Bacillus subtilis uses cytoplasmic multiprotein complexes, termed stressosomes, to activate the alternative sigma factor sigma(B) when facing environmental stresses. We have previously shown that genetically manipulating the complement of putative sensor proteins in stressosomes can alter the dynamics of the sigma(B) response in terms of its magnitude and timing. However, it is unknown whether these response dynamics impact the fitness of cells challenged by environmental stressors. Here, we examine the fitness of strains with different sigma(B) responses by competing strain pairs in exponential-phase co-cultures under environmental stress. We find that strains with different response dynamics show different competitive indices that differ by stressor. These results suggest that the dynamics of the sigma(B) response can affect the fitness of cells facing environmental stress, highlighting the relevance of different sigma(B) dynamics.