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Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1: Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest. 2: Obligate anaerobes are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest. 3: Facultative anaerobes can grow with or without oxygen because they can metabolise energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more ATP than either fermentation or anaerobic respiration. 4: Microaerophiles need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top. 5: Aerotolerant organisms do not require oxygen and cannot utilise it even if present; they metabolise energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube. Both facultative anaerobes and aerotolerant organisms will undergo fermentation in the absence of oxygen, but the facultative anaerobes will switch to aerobic metabolism when oxygen is present (a phenomenon known as the Pasteur effect). The Pasteur effect is sometimes used to distinguish between facultative anaerobes and aerotolerant organisms, in the lab.
Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, some capable of surviving in up to 8% oxygen, others losing viability unless the oxygen concentration is less than 0.5%. An important distinction needs to be made here between the obligate anaerobes and the microaerophiles. Microaerophiles, like the obligate anaerobes, are damaged by normal atmospheric concentrations of oxygen. However, microaerophiles metabolise energy aerobically, and obligate anaerobes metabolise energy anaerobically. Microaerophiles therefore require oxygen (typically 2–10% O2) for growth. Obligate anaerobes do not.
Dissolved oxygen increases the redox potential of a solution, and high redox potential inhibits the growth of some obligate anaerobes. For example, methanogens grow at a redox potential lower than -0.3 V.
Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form disulfide, thus inactivating certain enzymes (e.g. nitrogenase). Organisms may not be able to grow with these essential enzymes deactivated.
Growth may be inhibited due to a lack of reducing equivalents for biosynthesis, because electrons are exhausted in reducing oxygen.
The energy yield of anaerobic respiration and fermentation (i.e. the number of ATP molecules generated) is less than in aerobic respiration. This is why facultative anaerobes, which can metabolise energy both aerobically and anaerobically, preferentially metabolise energy aerobically. This is observable when facultative anaerobes are cultured in thioglycollate broth.