Bacterial Endospores: Stress Resistance and Germination

Dear Colleagues,

Bacterial spores are the ultimate survival capsule occurring in nature. They have been around for eons and have evolved an unprecedented survival ability amongst all Life on Earth, even in the face of many adverse environmental conditions, such as: i) extreme acid and alkaline conditions; ii) the environmental niche of the gastrointestinal tract of hosts; iii) desiccation; iv) extreme temperatures, including those used in contemporary food manufacturing. The resistance conferred by the spores of both anaerobes, such as Clostridia and aerobic organisms, from the genus Bacillus has been long enigmatic, as the structures involved in spore resistance were, by nature, refractive to detailed biochemical analysis methods. In recent decades, genetic analyses and, more recently, genomic and proteomic analysis have contributed to unravelling some of the secrets of bacterial spore stress resistance, as well as spore germination mechanisms. Nevertheless, the mechanism(s) of the extreme high heat resistance of many spore formers remains unresolved, as do questions about spore germination triggering and the stress resistance of spores of many recently identified gut microbes.

Interestingly, imaging and molecular techniques applied to spore biology have shown tremendous phenotypical heterogeneity in genetically homogeneous populations, demonstrating this at the actual germination level by assessing the kinetics of spore water uptake and calcium dipicolinic acid secretion,, as well as at the level of individual germination proteins. It has been shown that these latter spore proteins are organized differently, at least in spores of the human pathogen Clostridiades diffcile, compared to the most extensively studied Bacillus subtilis. In the latter, germinant receptor (GR) proteins are organized in what has been termed germinosome complexes in the spore inner membrane (IM), and GR activation triggers germination, whereas in the former, such IM GRs are not observed. Instead, C. difficile spores have pseudoproteases and proteases in their cortex, which interact with (co-)germinants and activate a cortex lytic enzyme zymogen, thus starting germination. Finally, both mechanisms converge upon the need to open a SpoVA channel, allowing secretion of the spore core’s huge depot of calcium dipicolinc acid and, in a yet-to-be-defined manner, the uptake of water. This exemplifies the open major research questions in the field, such as: (1) where do the germinants actually bind on germinant receptor proteins or their functional equivalents; (2) what structural modification does such binding induce to allow for a functional interaction with the SpoVA channel proteins and what exactly is this “interaction”; (3) what drives the heterogeneity in the molecular composition of the germination apparatus of spores within genetically homogeneous populations; (4) which structures confer extreme heat resistance upon spores and how does wet heat kill spores; and (5) how widespread are the spore germination molecular physiological characteristics identified in Bacillus subtilis?

Prof. Dr. Stanley Brul
Prof. Dr. Peter Setlow
Guest Editors

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• spores
• microbiome
• microbiota
• food chain
• stress resistance
• heat stress
• germination heterogeneity
• germinosome
• SpoVA channel
• calcium dipicolinc acid