CotG controls spore surface formation in response to the temperature of growth in Bacillus subtilis

Impact of sporulation temperature on germination of Bacillus subtilis spores under optimal and adverse environmental conditions

Bacillus subtilis spores are important food spoilage agents and are occasionally involved in food poisoning. In foods that are not processed with intense heat, such bacterial spores are controlled by a combination of different hurdles, such as refrigeration, acidification, and low water activity (aw), which inhibit or delay germination and/or growth. Sporulation temperature has long been regarded as a relevant factor for the assessment of germination in chemically defined media, but little is known about its impact on food preservation environments. In this study, we compared germination dynamics of B. subtilis spores produced at optimal temperature (37 °C) with others incubated at suboptimal (20 °C) and supraoptimal (43 °C) temperatures in a variety of nutrients (rich-growth medium, L-alanine, L-valine, and AGFK) under optimal conditions as well as under food-related stresses (low aw, pH, and temperature). Spores produced at 20 °C had a lower germination rate and efficiency than those incubated at 37 °C in all the nutrients, while those sporulated at 43 °C displayed a higher germination rate and/or efficiency in response to rich-growth medium and mostly to L-alanine and AGFK under optimal environmental conditions. However, differences in germination induced by changes in sporulation temperature decreased when spores were activated by heat, mainly due to the greater benefit of heat for spores produced at 20 °C and 37 °C than at 43 °C, especially in AGFK. Non-heat-activated spores produced at 43 °C still displayed superior germination fitness under certain stresses that had considerably impaired the germination of the other two populations, such as reduced temperature and aw. Moreover, they presented lower temperature and pH boundaries for the inhibition of germination in rich-growth medium, while requiring a higher NaCl concentration threshold compared to spores obtained at optimal and suboptimal temperature. Sporulation temperature is therefore a relevant source of variability in spore germination that should be taken into account for the accurate prediction of spore behaviour under variable food preservation conditions with the aim of improving food safety and stability.

Giving a signal: how protein phosphorylation helps Bacillus navigate through different life stages

Protein phosphorylation is a universal mechanism regulating a wide range of cellular responses across all domains of life. The antagonistic activities of kinases and phosphatases can orchestrate the life cycle of an organism. The availability of bacterial genome sequences, particularly Bacillus species, followed by proteomics and functional studies have aided in the identification of putative protein kinases and protein phosphatases, and their downstream substrates. Several studies have established the role of phosphorylation in different physiological states of Bacillus species as they pass through various life stages such as sporulation, germination, and biofilm formation. The most common phosphorylation sites in Bacillus proteins are histidine, aspartate, tyrosine, serine, threonine, and arginine residues. Protein phosphorylation can alter protein activity, structural conformation, and protein-protein interactions, ultimately affecting the downstream pathways. In this review, we summarize the knowledge available in the field of Bacillus signaling, with a focus on the role of protein phosphorylation in its physiological processes.

CotG Mediates Spore Surface Permeability in Bacillus subtilis

Proteins and glycoproteins that form the surface layers of the Bacillus spore assemble into semipermeable arrays that surround and protect the spore cytoplasm. Such layers, acting like molecular sieves, exclude large molecules but allow small nutrients (germinants) to penetrate. We report that CotG, a modular and abundant component of the Bacillus subtilis spore coat, controls spore permeability through its central region, formed by positively charged tandem repeats. These repeats act as spacers between the N and C termini of the protein, which are responsible for the interaction of CotG with at least one other coat protein. The deletion but not the replacement of the central repeats with differently charged repeats affects the spore resistance to lysozyme and the efficiency of germination-probably by reducing the coat permeability to external molecules. The presence of central repeats is a common feature of the CotG-like proteins present in most Bacillus species, and such a wide distribution of this protein family is suggestive of a relevant role for the structure and function of the Bacillus spore. IMPORTANCE Bacterial spores are quiescent cells extremely resistant to a variety of unphysiological conditions, including the presence of lytic enzymes. Such resistance is also due to the limited permeability of the spore surface, which does not allow lytic enzymes to reach the spore interior. This article proposes that the spore permeability in B. subtilis is mediated by CotG, a modular protein formed by a central region of repeats of positively charged amino acid acting as a "spacer" between the N and C termini. These, in turn, interact with other coat proteins, generating a protein layer whose permeability to external molecules is controlled by the distance between the N and C termini of CotG. This working model is most likely expandable to most sporeformers of the Bacillus genus, since they all have CotG-like proteins, not homologous to CotG of B. subtilis but similarly characterized by central repeats.