Clostridia species are very large Gram positive rods. Under proper environmental conditions, they are capable of forming endospores.

On blood agar at 24 hours, most clostridia produce flat, gray, spreading colonies somewhat resembling those of Bacillus sp. They are also usually hemolytic. The organism pictured here is Clostridium tetani, the cause of tetanus. Clostridium perfringens produces raised, entire colonies with double zone beta-hemolysis similar to that of Staphylococcus aureus. In thioglycolate, it grows almost throughout the tube since it is the most aero-tolerant of the clostridia.