Clostridium is a large genus of obligate anaerobes belonging to the Firmicutes 
phylum of bacteria, most of which have a Gram-positive cell wall structure. The 
genus includes significant human and animal pathogens, causative of potentially 
deadly diseases such as tetanus and botulism. Despite their relevance and many 
studies suggesting that they are not a monophyletic group, the taxonomy of the 
group has largely been neglected. Currently, species belonging to the genus are 
placed in the unnatural order defined as Clostridiales, which includes the class 
Clostridia. Here, we used genomic data from 779 strains to study the taxonomy 
and evolution of the group. This analysis allowed us to 1) confirm that the 
group is composed of more than one genus, 2) detect major differences between 
pathogens classified as a single species within the group of authentic 
Clostridium spp. (sensu stricto), 3) identify inconsistencies between taxonomy 
and toxin evolution that reflect on the pervasive misclassification of strains, 
and 4) identify differential traits within central metabolism of members of what 
has been defined earlier and confirmed by us as cluster I. Our analysis shows 
that the current taxonomic classification of Clostridium species hinders the 
prediction of functions and traits, suggests a new classification for this 
fascinating class of bacteria, and highlights the importance of phylogenomics 
for taxonomic studies.