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    For most organisms, an adequate supply of oxygen enhances metabolism and growth. The oxygen acts as the hydrogen acceptor in the final steps of energy production catalyzed by the flavoproteins and cytochromes. Because the use of oxygen generates two toxic molecules, hydrogen peroxide (H2O2) and the free radical superoxide (O) bacteria Require two enzymes to detoxify these molecules when oxygen is utilized. The first is superoxide dismutase,which catalyzes the reaction and the second is catalase, which catalyzes the reaction. The response to oxygen is an important criterion for classifying bacteria and has great practical significance because specimens from patients must be incubated in a proper atmosphere for the bacteria to grow.
    (1) Some bacteria, such as M. tuberculosis, are obligate aerobes; that is, they require oxygen to grow because their ATP-generating system is dependent on oxygen as the hydrogen acceptor.
    (2) Other bacteria, such as E. coli, are facultative anaerobes; they utilize oxygen, if it is present, to generate energy by respiration, but they can use the fermentation pathway to synthesize ATP in the absence of sufficient oxygen.
    (3) The third group of bacteria consists of the obligate anaerobes, such as Clostridium tetani, which cannot grow in the presence of oxygen because they lack either superoxide dismutase or catalase, or both. Obligate anaerobes vary in their response to oxygen exposure; some can survive but are not able to grow, whereas others are killed rapidly.

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