Microbial extracellular slimes

Stabilisation by climate control. The most desirable method to prevent damage induced by the repeated cycles of crystallisation and hydration would probably be environmental control. However, neither the selection nor the maintenance of such an ideal environment is possible if looked at realistically. Predictions of salt crystallisation and hydration from mixed salt solutions are more or less impossible, taking into account all the different parameters that influence the process. Sawdy provides a brilliant overview of the subject and considers relative humidity, temperature, air movement, type and structure of the porous support, salt mixture composition and salt concentration. It is necessary to consider not only consolidation treatments of the plaster or the paint layer, which as such may influence the transition behaviour of the salts, but also the influence of microbial extracellular slimes on the porosity of the system. 

The ability of the stone-colonizing microflora to cover and even penetrate material surface layers by the excretion of organic extracellular polymeric substances (EPS) leads to the formation of complex slimes, or biofilms, in which the microbial cells are embedded. Phototrophic organisms usually initiate colonisation by establishing a visible, nutrient-rich biofilm on new stone from which they can penetrate the material below to seek protection from high light intensities or desiccation. Stone EPS trap aerosols, dust and nutrients, minerals,... 

At present, the discovery of new polysaccharides relies on screening of the extracellular polysaccharides produced by microorganisms. Fungi and yeasts are also potential sources of new polysaccharides. The production of microbial polysaccharide has the advantages of controlled cost, abundant supply and ease of modification of the chemical structure. These new polysaccharides with new properties may generate new market opportunities. Microbial polysaccharides can be classified as extracellular structural or intercellular storage forms. Extracellular polysaccharide can be either exocellular capsules of the cell wall or loose slime components that accumulate outside the cell wall and then diffuse into the medium. 

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