MICROBIAL DETERIORATION OF MATERIALS - FU NDAMENTALS - MICROBIAL DESTRUCTION MECHANISMS

Generally, all kinds of microorganisms - bacteria and cyanobacteria, a lgae, fungi, and lichens - are able to attack and degrade materials. T he deterioration is caused by the excretion of metabolic intermediates and/or endproducts as well as exoenzymes. Depending on the use of a m aterial, a discoloring of a resin or a total destruction of a material , may result in a serious microbiologically influenced corrosion damag e. Although many microorganisms are known to participate in these proc esses, the ways of action may be summarized by seven main categories: 1) attack by mineral acids like sulfuric, nitric, carbonic acid --> hy drolysis of materials 2) attack by organic acids like acetic, citric, oxalic, gluconic, and other acids --> hydrolysis of materials and chel atization of cations 3) saltstress because of reactionproducts of 1) a nd 2) --> retaining water in porous materials causes increased suscept ibility against freeze-thaw attack and furthers crystallisation --> sw elling attack 4) production of noxious compounds like hydrogen sulfide , nitrogen oxides --> production of mineral acids or precipitation of metal sulfides 5) effect of biofilm --> exopolymers cause localized co rrosion cells as well as retainment of water in porous materials (s. a bove); in addition hydrophobic effects on surfaces 6) attack by exoenz ymes --> cleavage of insoluble organic compounds to small, water solub le molecules 7) production of wetting agents --> increased solubility of hydrophobic substances Usually, the deterioration of a material is caused by a combined action of the above mentioned factors. However, c hemical methods often fail to detect the mechanisms properly because s ome compounds may be subject to a metabolic turnover (organic acids, n itrogen compounds etc.). Microbiological analyses are necessary for cl earing up the mechanism of attack.