Clostridia are prokaryotic bacteria belonging to the phylum Firmicutes, the Gram-positive (mostly), low G + C bacteria that currently contains three classes, “Bacilli”, “Clostridia”, and “Erysipelotrichia”. 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 first clostridial cluster contains the genus Clostridium and four other genera. 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 (http://www.bacterio.net), of which approximately half are genuinely Clostridium.

The main pathogenic clostridial species, Clostridium botulinum, Clostridium chauvoei, Clostridium haemolyticum, Clostridium novyi, Clostridium perfringens, Clostridium septicum, and Clostridium tetani, clearly belong to the genus Clostridium because they share common ancestry with the type species Clostridium butyricum. These species belong to the phylogenetic group described by Collins et al. (1994) as “cluster I”, and are Clostridium sensu stricto. 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. This situation is complex and taxonomically confusing, since strains of other species, such as C. butyricum which may produce botulinum toxin and cause human botulism, have been given their own species designation (that is, not C. botulinum). To compound the inconsistency around species designation in the taxonomy of Clostridium, C. novyi type A and Clostridium haemolyticum belong to the same genospecies as C. botulinum

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