Taxonomic Relationships among the Clostridia

The class Clostridia contains the order Clostridiales, within which the family Clostridiaceae contains 13 genera distributed among three paraphyletic clusters and a fourth clade represented by a single genus. The genus Clostridium has been extensively restructured, with many species moved to other genera, but it remains phylogenetically heterogenous. The genus currently contains 204 validly described species, of which approximately half are genuinely Clostridium. The main pathogenic clostridial species such as Clostridium botulinum, Clostridium chauvoei, Clostridium haemolyticum, Clostridium perfringens and Clostridium tetani belongs to the genus Clostridium because they share common ancestry with the type species Clostridium butyricum. The taxonomy of C. botulinum is unique since it is currently defined as C. botulinum only by the ability to produce one or more botulinum toxins; however, strains that can do this belong to at least four Clostridium species.