Microbial oxidation mechanism

Epidemiological studies and intervention trials with food and beverages rich in flavonoids are not conclusive although flavonoids were recognized to display numerous antioxidant, anti-inflammatory, anti-tumoral, and anti-microbial activities. The antioxidant capacity of flavonoids has been largely reported in numerous in vitro and ex vivo systems. Numerous reviews "" have been published on the antioxidant properties of flavonoids. Degenerative diseases are largely associated with oxidative mechanisms that may be counteracted by flavonoids. 

Evangelou, V. P. 1995b. Pyrite Oxidation and its Control. Acid Mine Drainage, Surface Chemistry, Molecular Oxidation Mechanisms, Microbial Role, Kinetics, Control, Ameliorates, Limitations, Microencapsulation. CRC/Lewis Press, Boca Raton, FL. 

Because Mn(II) is stable at pH <8, significant contributions from microorganisms to Mn(II) oxidation has been relatively easy to demonstrate, and a number of Mn(II) oxidizing bacteria have been isolated. Microbial oxidation is considered the primary mechanism of Mn(II) oxidation in circumneutral freshwater (Ghiorse, 1984 Nealson et al., 1988). The sheathed bacterium Leptothrix discophora is perhaps the most-studied species of Mn(II)-oxidizing bacteria, and rate laws have been developed to describe Mn(II) oxidation as a function of pH, temperature, dissolved O2, and Cu concentration (Zhang et al., 2002). Bacillus sp. 

Figure 1 Comparison of the microbial direct versus indirect oxidation mechanism for sulfide (source Sand et al., 2001).

However, the mechanism for direct oxidation is poorly understood (Silverman and Ehrlich, 1964). Iron is also made available for microbial oxidation after dissociation of the sulfide complexes by a chemical oxidation of the sulfide moiety of the mineral. A strong chemical oxidizing agent is the Fe ion itself. Singer eind Stumm (1970) showed that, under acidic conditions and in the absence of bacteria, Fe was a much more effective catalyst of pyrite oxidation than was ferrous iron. However, in the presence of bacteria, the rate of pyrite oxidation in the presence of Fe was higher the reduced iron was biologically oxidized to ferric iron which then oxidized the pyrite ... 

Various aspects of the mechanisms of microbial oxidation of sulfur have been referred to earlier, but it is clear that further investigation of such aspects as the nature of the frequently-observed close attachment of the cells to the sulfur surface and the penetration or otherwise of cell membranes by elemental sulfur (Kaplan and Rittenberg, 1962) would be of interest. Elemental sulfur occurs in a number of solid allotropic forms, its chemical activity is profoundly affected by a number of impurities, and it is photosensitive (Meyer, 1968). There is a paucity of information on the effects of variation of these factors on the amenability or otherwise of elemental sulfur to microbial attack. 

Van der Linden AC, Thijsse GJE. 1965. The mechanisms of microbial oxidations of petroleum hydrocarbons. In Nord FF, ed. Advances in enzymology and related subjects of biochemistry. Volume XXVII. New York, NY Interscience Publishers, 469-546. 

Carbonate cement haloes associated with hydrocarbon pools are well documented, and commonly attributed to the microbial oxidation of crude oil or methane in different geological settings (Gould Smith, 1978 Smith, 1978 Faber Stahl, 1984 Oehler Sternberg, 1984 Hovland et al., 1987 O Brien Woods, 1995). However, a number of observations point towards this type of precipitation mechanism not being appropriate in the context of the Angel Field and Gidgealpa Field areas ... 

Several researchers have also suggested that bacteria mediate mercury reduction [54,55]. SicUiano et al. [56] recently examined the role of microbial reduction and oxidation processes in regulating DGM diel (over a 24h period) patterns in freshwater lakes. The authors demonstrate that photochemi-cally produced hydrogen peroxide regulates microbial oxidation processes and may account for the diel patterns observed in DGM data. Overall, the mechanisms responsible for mercury reduction and the relative contributions of biotic and abiotic processes are still unclear but solar radiation appears to be a common instigator of photo-reduction. 

Several mechanisms have been proposed to explain the antimicrobial properties of tannins. The antimicrobial effects of phenolic compounds are probably related to the inhibition of bacterial enzymes, alterations in cell wall permeability, an increase in the hydrogen ion activity of the microbial environment, a reduction in the surface and/or interfacial tension and perhaps chelation of essential minerals, particularly iron with a concomitant impairment of the microbial oxidative metabolic system (Chung et al. 1998). The antimicrobial activities of tannins are ascribed to the interactions of goats tannins with the extracellular enzymes secreted and... 

Nocardia and P. aeruginosa were shown to break the cw-PI chain by an oxidative mechanism since aldehyde groups were found to accumulate during microbial degradation. This is always the first product formed during the abiotic peroxidation of cw-PI and the evidence suggests that the bacteria initiate a radical-chain peroxidation. This will be discussed further in the context of polyolefin biodegradation. 

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