Diversity and Prevalence of Clostridium innocuum in the Human Gut Microbiota

Clostridia are a polyphyletic group of Gram-positive, spore-forming anaerobes in the Firmicutes phylum that significantly impact metabolism and functioning of the human gastrointestinal tract. Recently, Clostridia were divided into two separate classes, Clostridia and Erysipelotrichia, based on phenotypic and 16S rRNA gene-based differences. While Clostridia include many well-known pathogenic bacteria, Erysipelotrichia remain relatively uncharacterized, particularly regarding their role as a pathogen versus commensal. Despite wide recognition as a commensal, the erysipelotrichial species Clostridium innocuum has recently been associated with various disease states. To further understand the ecological and potential virulent role of C. innocuum, we conducted a genomic comparison across 38 C. innocuum isolates and 194 publicly available genomes. Based on colony morphology, we isolated multiple C. innocuum cultivars from the feces of healthy human volunteers (n = 5). Comparison of the 16S rRNA gene of our isolates against publicly available microbiota data sets in healthy individuals suggests a high prevalence of C. innocuum across the human population (>80%). Analysis of single nucleotide polymorphisms (SNPs) across core genes and average nucleotide identify (ANI) revealed the presence of four clades among all available genomes (n = 232 total). Investigation of carbohydrate and protein utilization pathways, including comparison against the carbohydrate-activating enzyme (CAZyme) database, demonstrated inter- and intraclade differences that were further substantiated in vitro. Collectively, these data indicate genetic variance within the C. innocuum species that may help clarify its role in human disease and health. IMPORTANCE Clostridia are a group of medically important anaerobes as both commensals and pathogens. Recently, a new class of Erysipelotrichia containing a number of reassigned clostridial species has emerged, including Clostridium innocuum. Recent studies have implicated C. innocuum as a potential causative agent of diarrhea in patients from whom Clostridioides difficile could not be isolated. Using genomic and in vitro comparison, this study sought to characterize C. innocuum in the healthy human gut. Our analyses suggest that C. innocuum is a highly prevalent and diverse species, demonstrating clade-specific differences in metabolism and potential virulence. Collectively, this study is the first investigation into a broader description of C. innocuum as a human gut inhabitant.