Obligate anaerobe
[3] The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors including oxidative stress and enzyme production.
[4] Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known as reactive oxygen species (ROS), highly reactant products that are damaging to microbes, including obligate anaerobes.
[1][2][3][5] The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O2) is thought to reflect the quantity of superoxide dismutase and catalase being produced.
In the presence of oxygen, this mutation resulted in the inability to properly synthesize certain amino acids or use common carbon sources as substrates during metabolism.
Examples of alternative electron acceptors include sulfate, nitrate, iron, manganese, mercury, and carbon monoxide.
[8] Fermentation differs from anaerobic respiration in that the pyruvate generated from glycolysis is broken down without the involvement of an electron transport chain (i.e. there is no oxidative phosphorylation).
[9] Examples of obligately anaerobic bacterial genera include Actinomyces, Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus, Porphyromonas, Prevotella, Propionibacterium, and Veillonella.
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.