Microbial mechanisms of deterioration of inorganic substrates - a general mechanistic overview.
A review with many refs. Generally, all types of microorganisms (bacteria and cyanobacteria, algae, fungi, and lichens) are able to attack and degrade materials. Sometimes, the phys. presence of microbial cells is sufficient to cause damage. Generally, deterioration is caused by the excretion of metabolic intermediates and/or end-products as well as exoenzymes. Depending on the use of a material, discoloration of a resin or total destruction of a material may result in a serious microbiol. influenced (corrosion) damage. Although many microorganisms are known to participate in these processes, their action may be summarized by nine main categories: (1) phys. presence of microbial cells->connection of elec. contacts (chips); (2) attack by mineral acids like sulfuric, nitric, carbonic acid->hydrolysis of materials; (3) attack by org. acids like acetic, citric, oxalic, gluconic, and other acids->hydrolysis of materials; (4) attack by org. solvents like acetic of butyric acid or alcs. like ethanol or propanol or ketons->swelling and hydrolysis of materials; (5) salt stress because of reaction products of (2) and (3)->retaining water in porous materials causes increased susceptibility against freeze-thaw attack and furthers crystn.->swelling attack; (6) prodn. of noxious compds. like hydrogen sulfide, nitrogen oxides->prodn. of mineral acids or pptn. of metal sulfides and oxidants/reductants; (7) effect of biofouling and biofilm->exopolymers cause localized corrosion cells; retainment of water in porous materials; hydrophobic effects on surfaces; reduced heat transfer efficiency; reduced (flow) velocity or increased pressure; (8) attack by exoenzymes->cleavage of insol. org. compds. to small, water sol. mols.; and (9) prodn. of chelating agents of emulsifying compds.->increased soly. of \"insol.\" and/or hydrophobic substances. Usually, the deterioration of a material is caused by a combined action of the above mentioned factors. However, chem. methods often fail to properly detect the mechanisms because some compds. may be subject to metabolic turnover (org. acids, nitrogen compds., etc.). Microbiol. analyses are necessary for elucidating the mechanism of attack.
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