Studies show they are not a monophyletic group. In fact, they are highly polyphyletic. Their relationships are not certain. Most are put in the Clostridiales, but is not a natural group. It is likely to be redefined in the future.
Most species of the genus Clostridium are saprophytic organisms found in many places, most notably the soil. However, the genus does contain some human pathogens. The toxins produced by some members of the Clostridium genus are among the most dangerous known. Examples are tetanus toxin (known as tetanospasmin) produced by C. tetani and botulinum toxin produced by C. botulinum.
Notable species of this class include:
- Clostridium perfringens (Gangrene, food poisoning)
- Clostridium difficile (Pseudomembranous colitis)
- Clostridium tetani (Tetanus)
- Clostridium botulinum (Botulism)
- Clostridium acetobutylicum
- Clostridium haemolyticum
- Clostridium novyi
- Clostridium oedematiens
Heliobacteria are also members of the class Clostridia.