| Summary: | Bacterial spore germination is triggered by specific nutrients detected by germinant receptors (GRs) located in the inner membrane. While GR diversity and function are well-characterized in some Bacillus and Clostridium spp., they remain poorly understood in (Para)Geobacillus spp., despite the relevance of their spores in food spoilage and biotechnology. This study analyzed 105 genomes from (Para)Geobacillus strains to identify GR-encoding operons and evaluated the germination of 22 representative strains. All strains carried between two and five GRs, mostly orthologous to GRs in B. subtilis, Priestia megaterium, B. cereus, and B. anthracis, with wide variation among species and in some cases, such as G. stearothermophilus, among strains. Among GRs, the most commonly conserved GR across all strains was an ortholog of GerK from B. subtilis 168 and Priestia megaterium QM B1551. All strains germinated in rich nutrient medium (TSBYE), but none responded to common amino acids or nucleosides, and only a few (G. stearothermophilus ATCC 12980T and ATCC 10149, G. thermodenitrificans DSM 465T, and several P. thermoglucosidasius strains) germinated in the presence of various sugars. Notably, differences in germination responses did not align with GR diversity. Additionally, all strains displayed poor outgrowth on rich nutrient agar, with colony formation efficiencies ranging from 3 % to 53 %. These findings suggest that germination and outgrowth mechanisms in (Para)Geobacillus differ significantly from those in well-studied spore-formers, highlighting the need for further research.
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