Which statement accurately describes clostridia?

• A. Isolates typically will not grow on phenylethyl alcohol agar.
• B. Clinically significant clostridia are found in the normal biota of the colon and in the soil.
• C. Tetanus is caused by ingesting pre-formed toxin and can be prevented by boiling food prior to eating.
• D. C. tetani spores will form in the presence of oxygen; therefore, anaerobiosis in a wound is not required to cause tetanus.

Answer: (B)

Clostridia are anaerobic, spore-forming Gram-positive rods that are commonly found in soil and in the human colon. This environmental and intestinal presence explains why some clostridial species are clinically significant, capable of causing disease when they gain access to a suitable anaerobic environment, such as a wound, or when the gut flora balance is disrupted.

That makes the statement about their distribution accurate: many clinically important clostridia are part of the normal biota of the colon and are also prevalent in soil.

The other statements don’t fit as well. Growth on phenylethyl alcohol agar isn’t a defining feature of clostridia, since their recovery depends on anaerobic conditions and media used; the statement as written isn’t a reliable descriptor. Tetanus is not caused by ingesting a pre-formed toxin, and boiling food wouldn’t prevent tetanus—that description mixes up tetanus with botulism, which is the disease associated with pre-formed toxin in food. In tetanus, toxin is produced in an anaerobic wound, and vaccination is the preventive measure. Finally, Clostridium tetani spores form under anaerobic conditions, not in the presence of oxygen; oxygen actually inhibits spore formation, so that statement is incorrect.