Clostridium cluster IV and XIVa species, Clostridium spp. producing secondary BAs and pathogenic Clostridium spp.. According to the novel taxonomic arrangement criterion, the species of Clostridium cluster IV and XIVa are listed. What’s more, Clostridium spp. that can convert primary BAs to secondary BAs are presented here. Pathogenic Clostridium spp. listed in Fig. 1 also have some non-pathogenic strains, and most of them are commensal bacteria in gut.