Aerotolerant anaerobes

Meaning: Aerotolerant anaerobes are prokaryotes that make ATP only by fermentation (or anaerobic processes), never by aerobic respiration. They can tolerate oxygen because they have defenses against reactive oxygen species, such as superoxide dismutase, so O₂ does not poison them. Growth is about the same whether O₂ is present or absent. Lactic acid bacteria such as Lactobacillus and Streptococcus are standard examples.

On the MCAT: Passages show bacteria growing in test tubes of liquid medium and ask you to classify them by where growth appears. Obligate aerobes grow only at the top, where O₂ is highest. Obligate anaerobes grow only at the bottom. Facultative anaerobes grow throughout but densest at the top, because they prefer aerobic respiration, which yields more ATP. Aerotolerant anaerobes grow evenly throughout the tube. Microaerophiles grow in a band near the top, where O₂ is present but low.

Common confusion: Students confuse aerotolerant anaerobes with facultative anaerobes because both survive with O₂. The difference is that facultative anaerobes switch to using O₂ for respiration when it is available, so growth improves with O₂. Aerotolerant anaerobes never use O₂, so their growth does not improve. Obligate anaerobes typically lack adequate defenses against reactive oxygen species and are harmed by O₂.