Bacteria exist in aerobic and anaerobic variants; aerobes require oxygen while anaerobes may not require oxygen for metabolism.

Microorganisms vary in their requirements for oxygen. Some bacteria require oxygen for survival, while others do not. Bacteria that can survive and grow in the presence of oxygen are called obligate aerobes. In contrast, bacteria that do not require oxygen are anaerobes. There are different types of anaerobes. Obligate anaerobes cannot survive in an oxygen-containing environment. The presence of oxygen leads to cell death. Facultative anaerobes can toggle between aerobic metabolism and anaerobic metabolism if it is not. Microaerophiles need oxygen to grow, but high concentrations of oxygen poison them. Finally, aerotolerant anaerobes are oxygen-tolerant. They cannot use oxygen for metabolism, but the presence of oxygen does not harm them.

Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycolate broth.

Aerobes and Anaerobes. The growth of bacteria with varying oxygen requirements in thioglycolate tubes; obligate aerobes (A), obligate anaerobes (B), facultative anaerobes (C), aerotolerant anaerobes (D), microaerophiles (E).

In this experiment, obligate aerobes gather at the top of the tube where the oxygen concentration is highest. Obligate anaerobes gather at the bottom of the tube where the oxygen concentration is lowest. Facultative anaerobes gather mostly at the top because aerobic respiration generates more ATP. Aerotolerant organisms can be found evenly spread throughout the test tube. Microaerophiles gather in the upper part of the test tube, but not the very top.

Key Points

• Obligate aerobes survive and grow with oxygen.

• Obligate anaerobes cannot survive in an oxygen-containing environment; the presence of oxygen leads to cell death. 

• Facultative anaerobes can toggle between aerobic metabolism can switch to anaerobic metabolism.

• Aerotolerant anaerobes are oxygen-tolerant; they cannot use oxygen for metabolism, but it does not harm them. 

• Microaerophiles need oxygen to grow, but high concentrations of oxygen poison them.

Key Terms

thioglycolate broth: an enriched, differential medium used primarily to determine the oxygen requirements of microorganisms

obligate aerobe: an organism that requires oxygen to grow

anaerobe: an organism that grows without air, or requires oxygen-free conditions to live

obligate anaerobes: microorganisms killed by average atmospheric concentrations of oxygen

facultative anaerobe: an organism that can perform aerobic respiration if oxygen is present, but is capable of switching to fermentation or anaerobic respiration if oxygen is absent

microaerophiles: microorganisms that are not killed outright by the presence of oxygen but can tolerate only sub-atmospheric levels of oxygen

aerotolerant anaerobes: bacteria which live by fermentation alone whether or not O₂ is present in their environment