The oxygen tolerance of bacteria is related to their defence mechanisms against free radicals: superoxide dismutase, catalase, glutathione peroxidase and NADH oxidase being the main enzymes involved5,30.
Superoxide Dismutase (SOD) is bacterial main defence against free radicals. This enzyme catalyses the dismutation reaction which eliminates the superoxide and turns it into hydrogen peroxide which is less toxic :
2O2" + 2H+ ^ H2O2 + O2 All SODs are metalloproteins. MnSOD (manganese SOD) is located in the mitochondria, whereas Cu & ZnSOD (copper and zinc SOD) is present in the cytoplasm. These destroy the free radicals produced in the cells where they are located because generally O2* does not pass through biological membranes. There is also a extracellular form of Cu & ZnSOD5,30. Hydrogen peroxide (H2O2) is eliminated by the enzymes catalase and glutathione peroxidase - the latter requiring reduced glutathione and selenium. These induce the breakdown of hydrogen peroxide into water and molecular oxygen, thus avoiding hydroxyl radicals from being produced :
H2O2 ^ 2H2O+ O2 All aerobic organisms with cytochrome systems have superoxide dismutase and catalase, whereas strict anaerobes have no superoxide dismutase, nor usually catalase. Aerotolerant anaerobes have no catalase but do have SOD. Some authors31 state that the deficit in superoxide dismutase is the main reason for which strict anaerobes cannot tolerate air. However, enzymatic activity has been detected in a number of strict anaerobes. In a study on 22 strains of pathogenic anaerobes, Tally was able to make a correlation between aerotolerance and SOD levels32. The latter can be a virulence factor that enables anaerobes to survive in oxygenated tissues until more favourable conditions enable them to start developing again5. Most Bacteroides have superoxide dismutase and this presence of superoxide dismutase can account for the aerotolerance of Bacteroides. However, some bacteria, such as Lactobacillus plantarum33 are without any SOD, catalase or peroxidase, yet tolerate air. Archibald & Fridovich have shown how intracellular manganese is able to trap the superoxide radical anion thus compensating for the absence of the enzymes involved in detoxifying the oxygen reduction derivatives34. NADH oxidase allows NADH to be turned into NAD with the hydrogen combining with oxygen to form water : 4 NADH + O2 ^ 4 NAD + H2O
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