Clostridium is a genus of Gram-positive bacteria. This genus includes several significant human pathogens, including the causative agents of botulism and tetanus. The genus formerly included an important cause of diarrhea, Clostridioides difficile, which was reclassified into the Clostridioides genus in 2016.[1] They are obligate anaerobes capable of producing endospores. The normal, reproducing cells of Clostridium, called the vegetative form, are rod-shaped, which gives them their name, from the Greek κλωστήρ or spindle. Clostridium endospores have a distinct bowling pin or bottle shape, distinguishing them from other bacterial endospores, which are usually ovoid in shape. Clostridium species inhabit soils and the intestinal tract of animals, including humans.[2] Clostridium is a normal inhabitant of the healthy lower reproductive tract of females.[3]
The genus, as traditionally defined, contains many organisms not closely related to its type species. The issue was originally illustrated in full detail by a rRNA phylogeny from Collins 1994, which split the traditional genus (now corresponding to a large slice of Clostridia) into twenty clusters, with cluster I containing the type species and its close relatives.[4] Over the years, this has resulted in many new genera being split out, with the ultimate goal of constraining Clostridium to cluster I.[5]
"Clostridium" cluster XIVa and "Clostridium" cluster IV efficiently ferment plant polysaccharide composing dietary fiber,[6] making them important and abundant taxa in the rumen and the human large intestine.[7] As mentioned before, these clusters are not part of current Clostridium,[4][8] and use of these terms should be avoided due to ambiguous or inconsistent usage.[9]
Clostridium contains around 250 species that include common free-living bacteria, as well as important pathogens.[10][11] The main species responsible for disease in humans are:[12]
Bacillus and Clostridium are often described as Gram-variable, because they show an increasing number of gram-negative cells as the culture ages.[16]
Clostridium and Bacillus are both in the phylum Firmicutes, but they are in different classes, orders, and families. Microbiologists distinguish Clostridium from Bacillus by the following features:[2]