Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!


Can you watch placidly the horrible struggles of lock-jaw? … If you can, you had better leave the profession: cast your diploma into the fire; you are not worthy to hold it.

—Jacob M. Da Costa (1833-1900): College and Clinical Record

The bacteria discussed in this chapter are united by a common requirement for anaerobic conditions for growth. Organisms from multiple genera and all Gram stain categories are included. Most of them produce endogenous infections adjacent to the mucosal surfaces, where they are members of the indigenous microbiota. The clostridia form spores that allow them to produce diseases such as tetanus and botulism after environmental contamination of tissues or foods. Another anaerobic genus of bacteria, Actinomyces, is discussed in Chapter 28.




Anaerobes not only survive under anaerobic conditions, they require them to initiate and sustain growth. By definition, anaerobes fail to grow in the presence of 10% oxygen, but some are sensitive to oxygen concentrations as low as 0.5% and are killed by even brief exposures to air. However, oxygen tolerance is variable, and many organisms can survive briefly in the presence of 2% to 8% oxygen, including most of the species pathogenic for humans. The mechanisms involved are incompletely understood, but clearly represent a continuum from species described as aerotolerant to those so susceptible to oxidation that growing them in culture requires the use of media prepared and stored under anaerobic conditions.

✺ Anaerobes require low oxygen to initiate growth

Oxygen tolerance is a continuum

Anaerobes lack the cytochromes required to use oxygen as a terminal electron acceptor in energy-yielding reactions and thus to generate energy solely by fermentation (see Chapter 21). Some anaerobes do not grow unless the oxidation–reduction potential is extremely low (–300 mV); because critical enzymes must be in the reduced state to be active, aerobic conditions create a metabolic block.

✺ Low redox potential is required

Another element of anaerobiosis is the direct susceptibility of anaerobic bacteria to molecular oxygen. For most aerobic and facultative bacteria, catalase and/or superoxide dismutase neutralize the toxicity of the oxygen products hydrogen peroxide and superoxide. Most anaerobes lack these enzymes and are injured when these oxygen products are formed in their microenvironment. As discussed in the following text, many of the more virulent anaerobic pathogens are able to produce antioxidant enzymes like catalase or superoxide dismutase.

Antioxidant defense typically lacking

Pathogens often produce catalase and superoxide dismutase


The anaerobes indigenous to humans include almost every morphotype and hundreds of species. Typical biochemical and cultural tests are used for classification, although this is difficult because the growth requirements of each ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.