Oxygen comprises one fifth of our atmosphere, and we take for granted that this a good thing. After all, aerobic creatures like ourselves could not exist without free O2. Not all life shares this feeling. Oxygen and its derivatives (known as “reactive oxygen species” or ROS) can wreak havoc on the biochemistry of many microbes. For some obligate anaerobes such as methane-producing archaea, even a small trace of oxygen poisons them irreversibly (see, for example Kiener and Leisinger 1983). Others fall somewhere in the middle. Microaerophiles (e.g. Helicobacter pylori, the cause of gastric ulcers) require small amounts of oxygen but are unable to tolerate full atmospheric concentrations (Bury-Moné et al., 2006).
Why are these organisms sensitive to oxygen?