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The breakdown (’weathering’) of rocks is a well-known phenomenon which is not only caused by weather factors such as temperature changes, wind erosion, freezing and thawing, extraction by rain and snow melt water, but also by chemical pollution (acid rain) and microbial activities. Microorganisms are found on the rock surfaces, in cracks and in some cases even within the pore space of sandstone or granite. The present review discusses methods for the study of microbial diversity, frequency, physiological activity, and contributions to rock deterioration. Many different rock types are affected: sandstone, granite, gneiss, amphibolite, limestone, basalt, dolerite, or even bricks and their glazes. Microbial rock degradation is not dependent on climate or location nor on annual seasons. It has been observed in hot and cold deserts, in subtropical and tropical climates, in the mediterranean region, in Europa, America and Asia. Recent research has focussed on the deterioration of building stones, monuments, and especially churches in densely populated areas. Most research has been descriptive inasmuch as effects and the microorganisms involved were studied. Little is known about the weathering mechanisms, and exact data on weathering rates are missing.

This review describes non-destructive methods for the study of monuments and other valuable structures. Also discussed are more destructive methods that can be employed where rock samples can be collected. While some of the methods and techniques have already been published, others were specifically developed and successfully tried out by the authors. Non-destructive would be the washing, scraping, or printing off of rock surface microorganisms for cultivation and identification purposes. Non-destructive would also be the demonstration of in situ physiological activity of surface microcolonies by applying fluorogenic substrate analogues. The local intensity of microbial outfall from the air has been studied by exposing agar media. For destructive studies the rock samples have to be collected aseptically and fragmented to release the microorganisms. Rock-coring is described as a possible method for sampling. Enrichment and cultivation of rock microorganisms are more successful with oligotrophic media and in dim light to allow photosynthetic primary producers to excrete organic substances which are favoured by the heterotrophs. Cell counting after release of the rock microflora can be achieved by staining all DNA-containing cells with fluorescent dyes such as DAPI or acridine orange. Viable cell counts will be lower than total counts because media or incubation conditions are selective for the different microorganisms present. Microbial biomass can be estimated by determining, in extracts, phospholipid phosphate. Finally, methods are described for studying exudates produced by microbial growth on or in rock samples.