Microbial mechanisms of deterioration of inorganic substrates—A general mechanistic overview

Abstract

Generally, all types of microorganisms—bacteria and cyanobacteria, algae, fungi, and lichens—are able to attack and degrade materials. Sometimes, the physical 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 microbiologically influenced (corrosion) damage. Although many microorganisms are known to participate in these processes, their action may be summarized by nine main categories: 1. (1) physical presence of microbial cells→connection of electric contacts (chips); 2. (2) attack by mineral acids like sulfuric, nitric, carbonic acid→hydrolysis of materials 3. (3) attack by organic acids like acetic, citric, oxalic, gluconic, and other acids→hydrolysis of materials 4. (4) attack by organic solvents like acetic of butyric acid or alcohols like ethanol or propanol or ketons→swelling and hydrolysis of materials 5. (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 crystallisation→swelling attack 6. (6) production of noxious compounds like hydrogen sulfide, nitrogen oxides→production of mineral acids or precipitation of metal sulfides and oxidants/reductants 7. (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. (8) attack by exoenzymes→cleavage of insoluble organic compounds to small, water soluble molecules; and 9. (9) production of chelating agents of emulsifying compounds→increased solubility of ‘insoluble’ and/or hydrophobic substances. Usually, the deterioration of a material is caused by a combined action of the above mentioned factors. However, chemical methods often fail to properly detect the mechanisms because some compounds may be subject to metabolic turnover (organic acids, nitrogen compounds, etc.). Microbiological analyses are necessary for elucidating the mechanism of attack.