Distinctive spore architecture and developmental biology of Turicibacter sanguinis reveal unexpected diversity among gut spore formers

Sporulation is a widespread but incompletely characterized trait among gut commensals, where it underpins microbial persistence, transmission, and ecological resilience. Most insights into spore biology derive from Bacilli and Clostridia, yet little is known about sporulation in phylogenetically distant gut-associated lineages. Turicibacter sanguinis, a strict anaerobe linked to host serotonin metabolism, lipid homeostasis, and neurodegenerative disease, represents one such understudied taxon. Here, we integrate ultrastructural, physiological, and comparative genomic analyses to define the sporulation and germination program of T. sanguinis. We show that T. sanguinis forms heat-resistant spores with a canonical core-cortex-coat architecture but displays previously undescribed features including a dual-layered outer envelope and bimodal electron-dense coat morphotypes. Developmental stages of sporulation follow canonical stages of Bacillus- and Clostridium-like sporulation while genomic analyses reveal a hybrid regulatory architecture combining Clostridial-type Spo0A initiation with Bacillus-like late-stage sigma factor control. Germination assays and genomic signatures further indicate a nutrient-responsive, Bacillus-like pathway involving Ger-family receptors, SpoVA-mediated Ca-DPA release, and CwlJ- and SleM-type cortex hydrolases. Together, these findings identify T. sanguinis as a distinct spore-forming lineage within the human gut microbiota and expand the known diversity of sporulation strategies across the Firmicutes.