Clostridia

The Clostridia are a class of Firmicutes, including "Clostridium" and other similar genera.
The Clostridia are obligate anaerobes. They lack aerobic respiration, and oxygen is toxic to them. Species of the genus "Clostridium" are all Gram-positive and can form spores.
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:
Heliobacteria are also members of the class Clostridia.