Anaerobic bacteria, reductive metabolism

Certain anaerobic bacteria can reductively dechlorinate PCBs in sediments (EHC 140). Higher chlorinated PCBs are degraded more rapidly than lower chlorinated ones, which is in contrast to the trend for oxidative metabolism described earlier. Genetically engineered strains of bacteria have been developed to degrade PCBs in bioremediation programs. 

Holliger C, G Wohlfarth, G Diekert (1999) Reductive dechlorination in the energy metabolism of anaerobic bacteria. EEMS Microbiol Rev 22 383-398. 

Other bacteria. Intestinal bacteria may play a critical role in the metabolic activation of certain nitroaromatic compounds in animals (119) and several reports have appeared on the metabolism of nitro PAHs by rat and human intestinal contents and microflora (120-123). Kinouchi et al. (120) found that 1-nitropyrene was reduced to 1-aminopyrene when incubated with human feces or anaerobic bacteria. More recently, Kinouchi and Ohnishi (121) isolated four nitroreductases from one of these anaerobic bacteria (Bacteroides fragilis). Each nitroreductase was capable of converting 1-nitropyrene into 1-aminopyrene, and one form catalyzed the formation of a reactive intermediate capable of binding DNA. Howard ej al. (116) confirmed the reduction of 1-nitropyrene to 1-aminopyrene by both mixed and purified cultures of intestinal bacteria. Two additional metabolites were also detected, one of which appeared to be 1-hydroxypyrene. Recently, similar experiments have demonstrated the rapid reduction of 6-nitro-BaP to 6-amino-BaP (123). 

Table I also shows the great diversity of organisms in which iron—sulfur proteins have been detected. Thus far there is no organism which when appropriately examined has not contained an iron-sulfur protein, either in the soluble or membrane-bound form. Iron-sulfur proteins catalyze reactions of physiological importance in obligate anaerobic bacteria, such as hydrogen uptake and evolution, ATP formation, pyruvate metabolism, nitrogen fixation, and photosynthetic electron transport. These properties and reactions can be considered primitive and thus make iron-sulfur proteins a good place to start the study of evolution. These key reactions are also important in higher organisms. Other reactions catalyzed by iron-sulfur proteins can be added such as hydroxylation, nitrate and nitrite reduction, sulfite reduction, NADH oxidation, xanthine oxidation, and many other reactions (Table II).

Although most drugs are metabolized by oxidative processes reduction may be a clinically important pathway of drug metabolism. In most cases these metabolic transformations are carried out by reductase enzymes in intestinal anaerobic bacteria. In the case of... 

Several reactions of CO2 reduction to CH4 by methanogens resemble those of CO2 reduction to by acetogens. Most acetogens, which are anaerobic bacteria (eubacteria), catalyze in their energy metabolism the formation of acetate from CO2 and H2 according to the following equation ... 

Internal Sources and Atmospheric Exchange of Methane. Methane is produced by specialized groups of obligate anaerobic bacteria (22, 23). The formation of methane as a metabolic product results either from the microbial reduction of CO2 with molecular H2, or via the fermentation of acetic acid. More structurally complex substrates may also serve as electron acceptors/donors, but the end result of methanogenesis is to produce methane and CO2 as end products (23). 

Hofman T, Lees H (1953) The biochemistry of nitrifying organisms 4. The respiration and intermediary metabolism of Nitrosomonas. Biochem J 54 579-583 Holliger C, Wohlfarth G, Diekert G (1999) Reductive dehalogenation in the energy metabolism of anaerobic bacteria. FEMS Microbiol Rev 22 383-398 Hollocher TC, Tate ME, Nicholas DJD (1981) Oxidation of ammonia by Nitrosomonas europaea definitive lsO-tracer evidence that hydroxylamine formation involves a monooxygenase. J Biol Chem 256 10834-10836... 

As an alternative to the use of mutants, metabolic inhibitors may be used to interrupt metabolic pathways even transient intermediates may then be accumulated and provide evidence for the details of the consecutive steps. A wide range of compounds has been used in investigations on electron transport pathways and bioenergetics, but these lie beyond the scope of this account. Examples that have been used in metabolic studies with bacteria include molybdate as an inhibitor of anaerobic sulfate reduction, methyl fluoride and difluoromethane (Miller et al. 1998) as inhibitors of the aerobic... 

In water bacteria use oxidation-reduction reactions to obtain the energy that they need for their own growth and reproduction. Some bacteria require oxygen for their metabolic needs and are called aerobic bacteria. Other anaerobic bacteria extract their oxygen from sources such as N03", S042 and other matter represented as CH20. The most common bacterially mediated reaction in water is the oxidation of organic matter ... 

Reductive biotransformations of several compounds such as polyhalogenated, keto, nitro and azo derivatives, are catalysed by a variety of enzymes which differ according to the substrates and the species. The liver cytochrome P-450-dependent drug metabolizing system is capable of reducing Af-oxide, nitro and azo bonds, whereas the cytosolic nitrobenzene reductase activity is mainly due to cytochrome P-450 reductase, which transforms nitrobenzene into its hydroxylamino derivative. NADPH cytochrome c reductase is also able to catalyse the reduction of nitro compounds. These metabolic conversions may also be brought about by gastrointestinal anaerobic bacteria. 

Fungi, which are active in upland environments, cease to exist in wetland soils. This is primarily due to the absence of oxygen and alteration in soil pH (acid to neutral) under anaerobic conditions. Overall, microbial biomass decreases under saturated soil conditions. The metabolic activities of anaerobic bacteria depend on alternate electron acceptors, such as oxidized forms of nitrogen, iron, manganese, and sulfur. Under wetland soil conditions, rates of many microbially mediated reactions decline, and some reactions may be eliminated and replaced by new ones. New microbial reactions are involved in the reduction of oxidized compounds during respiratory processes, resulting in the production of reduced compounds. 

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