This study investigated the clinical relevance of C. concisus in acute infectious diarrhoea in adults, aiming to elucidate its contribution to disease aetiology, phenotypic and genotypic antimicrobial susceptibility, the presence of virulence factors, and genetic diversity. A total of 142 isolates were analysed from both retrospective and prospective components of the study. C. concisus was identified as the sole intestinal pathogen in 24 of 246 patients (9.8 %), with AICC and/or AToCC patotypes present in 3,2 % of cases, while most isolates did not belong to known patotypes, suggesting the presence of additional, yet uncharacterised virulence determinants. Direct PCR detection from stool samples showed limited clinical utility. Phenotypic testing revealed high susceptibility to erythromycin (94-100 %) and tetracycline (99 %), whereas susceptibility to ciprofloxacin was slightly lower (86 %). Molecular analyses demonstrated that the presence of resistance-associated genes or mutations did not consistently correlate with phenotypic resistance. WGS confirmed pronounced genetic heterogeneity, the absence of clonal dissemination, and the presence of plasmids and secretion systems, indicating a complex epidemiology and the potential for horizontal gene transfer. Overall, these findings support the conclusion that C. concisus is an important yet often overlooked enteric pathogen, whose pathogenicity is heterogeneous and likely influenced by host interactions and additional, as yet unidentified, genetic factors.
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