Expert: John Locke - 3/3/2010

Question
Hi,
I have one question left on my microbiology homework and I just can't get it figured out. Thanks so much for taking the time to look it over.

Clostridium perfringens, an obligate anaerobe, is capable of utilizing the carbohydrates released from injured tissues as an energy source. During the infectious process, large amounts of gas accumulate in the infected tissues. Would you expect this gas to be CO2? Explain.

Thank you so much!

Answer
Thanks for using AllExperts. The key to understanding this question is realizing that Clostridium, as an obligate anaerobe, cannot use oxygen for its metabolism of carbohydrates and as a result cannot use metabolic pathways that require oxygen. As noted in the question, Clostridium prefers a low oxygen tension environment in which to grow and it does not use oxygen in its metabolic pathways. Remember that oxygen is used in aerobic metabolism as part of the electron transport chain and carbon dioxide is evolved during the Krebs Cycle oxidation of pyruvate (see here for a review: http://www.wiley.com/legacy/college/boyer/0470003790/animations/tca/tca.htm). The two are inextricably linked--all organisms that use aerobic metabolism have some form of reducing pathway like the citric acid cycle and some form of electron transport like the ETC. Organisms that are obligate anaerobes rely instead upon glycolosis and fermentation to obtain ATP.

Fermentation schemes vary dramatically between different organisms. In humans, the fermentation pathway involves the oxidation of pyruvate into lactic acid (this occurs, for example, during maximal exercise in skeletal muscle that is not receiving enough oxygen). In yeast, the fermentation pathway involves the oxidation of pyruvate into ethanol, a trait utilized for the production of alcoholic beverages. C. perfringens uses pathways that oxidize pyruvate into hydrogen gas, butyric acid, and glyoxylate. You would not immediately expect carbon dioxide to be produced during this sequence of reactions.

Unfortunately, things are rarely so simple in biology. Carbon dioxide is produced during the fermentation of carbohydrates by C. perfringens, along with nitrogen, hydrogen, and oxygen. The majority of the gas is not carbon dioxide, however, and it is not produced via the Krebs cycle. To answer your homework question, you would probably not expect carbon dioxide to be produced as a consequence of carbohydrate fermentation--but in reality, this does occur because Clostridium has a varied fermentation pathway, some of which produces carbon dioxide.