Comparative Relatedness of Clostridioides difficile Strains Isolated from Humans and Companion Dogs in South Korea

<b>Background/Objectives</b>: <i>Clostridioides difficile</i> is an anaerobic Gram-positive bacterium and a leading cause of healthcare-associated diarrhea. In this study, <i>C. difficile</i> strains isolated from human patients with diarrhea and companion dogs in South Korea were compared to reveal the potential transmission between different hosts. <b>Methods</b>: A total of 304 <i>C. difficile</i> strains were isolated, including 217 human isolates and 87 dog isolates. The strains were characterized for antimicrobial susceptibility and genotypic features, including antimicrobial resistant genes and toxin genes. In addition, comparative genomic analyses were performed to investigate their genetic relatedness. <b>Results</b>: Although antimicrobial susceptibility test revealed no significant difference in overall resistance, human isolates had higher resistance to moxifloxacin and cefotetan, while dog isolates showed slightly higher resistance to clindamycin and ampicillin. Resistance to vancomycin (3.7%), rifampin (8.3%), and chloramphenicol (0.9%) was observed only in human isolates. Toxin genes (<i>tcdA</i> and <i>tcdB</i>) were found in 57.1% of human isolates and 43.7% of dog isolates, while binary toxin genes (<i>cdtA</i> and <i>cdtB</i>) were detected only in isolates from humans. Multilocus sequence typing (MLST) analysis identified 34 sequence types (STs) in human isolates and 16 in dog isolates. Among them, 15 STs were detected in the isolates from both origins; notably, ST203 and ST42 were the predominant taxa that were equally derived from humans and dogs. Although <i>tcdA</i> and <i>tcdB</i> have not been previously reported in ST203, they were detected in 7 out of 34 ST203 isolates. The whole genomes of 36 representative isolates belonging to ST42 and ST203 were classified according to the STs of the source origin. <b>Conclusions</b>: These results indicate that similar <i>C. difficile</i> strain populations are present in both humans and companion dogs, which is compatible with interspecies dissemination or circulation of shared strain populations, and may also reflect host adaptation.