Identification of Clostridium species

The genus Clostridium belongs to the family Clostridiaceae and it currently contains 203 species and 5 subspecies, with only a few species being pathogenic to humans. Of these species, 21 have been reclassified to other genera, 5 have been reclassified within the genus and 1 has been de-accessioned.

Clostridium are phylogenetically heterogeneous and are Gram positive but can decolourise easily and appear Gram negative or Gram variable, spore formers and non-spore formers, rods and cocci and anaerobic and non-anaerobic bacteria.

Medically significant Clostridium strains tend to be Gram positive rods (some are Gram variable), 0.3 – 2.0 x 1.5 – 20.0μm which are often arranged in pairs or short chains, with rounded or sometimes pointed or square ends. They are commonly pleomorphic and vary considerably in their oxygen tolerance. Some species such as Clostridium novyi type A and Clostridium haemolyticum may require extended incubation on pre-reduced or freshly prepared plates and total handling in an anaerobic chamber. Conversely, Clostridium tertium, Clostridium histolyticum and Clostridium carnis are aerotolerant and will form colonies on blood agar plates incubated in an atmosphere of air with 5-10% added CO₂.

Virtually all of the members of the genus, except Clostridium perfringens, are motile with peritrichous flagellae and form oval or spherical endospores that may distend the cell. They may be saccharolytic or proteolytic and are usually catalase negative. Many species produce potent exotoxins.

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In 1994 the heterogeneity of this species was confirmed by 16S rRNA gene sequencing. This has been reaffirmed by the work of Yutin et al that 16S rRNA and ribosomal protein sequences are better indicators of evolutionary proximity than phenotypic traits. This genus like several others has undergone a number of revisions with the increasing availability of genomic data. An analysis of proteins from a number of members of this genus suggested another revision3. The main findings from the proposal suggested that:

The Selenomonas-Megasphaera-Sporomusa group are still members of the genus Clostridium

Clostridium difficile and its close relatives are placed within the family Peptostreptococcaceae. Under this proposal, the species Clostridium difficile would become Peptoclostridium difficile

Members of the family Ruminococcaceae belong to the genus Clostridium

It was also proposed to create six new genera to accommodate the 78 validly described species that fell outside the family Clostridiaceae. These genera are: Erysipelatoclostridium, Gottschalkia, Lachnoclostridium, Peptoclostridium, Ruminiclostridium and Tyzzerella

The type species is Clostridium butyricum.

For more about the identification of Clostridium species, see a special technical note from Public Health England.

Posted by Tim Sandle