Modulation of B. subtilis spore inner membrane fluidity by fatty acid composition and its impact on resistance and germination.

Bacterial spore inner membrane (IM) properties are believed to play a central role in spore resistance and germination, making the identification of factors governing IM organization highly relevant for improving the control of spore-forming bacteria in food preservation. In vegetative cells, changes in fatty acid (FA) composition constitute the main adaptive mechanism regulating cytoplasmic membrane fluidity in response to temperature fluctuations. However, the contribution of FA-mediated changes in the IM physical state to spore phenotypes remains poorly understood. In this study, we investigated the relationship between IM FA composition, membrane fluidity, and resistance and germination properties of Bacillus subtilis spores. First, we confirmed that targeted modifications of IM FA composition, through genetic manipulation of the desaturase system or supplementation of the sporulation medium with precursors of branched-chain FA, could alter membrane fluidity, as assessed by DPH-labelled fluorescence anisotropy. However, the FA profiles alone were insufficient to predict membrane anisotropy. Increased IM rigidity was associated with enhanced resistance to IM-targeting dodecylamine and heat, along with impaired nutrient-induced germination efficiency. However, the changes in IM fluidity induced by sporulation temperature did not correlate with the expected spore phenotypes. Overall, our results demonstrate that although FA-driven changes in IM fluidity can presumably influence spore behavior, it is not the dominant determinant of the resistance and germination properties of B. subtilis spores formed under different sporulation conditions.