What About Biofilms on the Surface of Stone Monuments?

Bacteria can live either as free planktonic cells in bulk solution, or as sessile cells attached to a surface. In addition to their attachment status, sessile bacteria are part of sessile communities termed biofilms. A biofilm can be defined as a microbial community attached to a solid surface composed of cells organized as microcolonies embedded in an organic polymer matrix of microbial origin. Thus, a biofilm is made of microbial cells and extracellular polymeric substances. The biofilm mode of life provides several advantages to microorganisms including resistance to environmental stresses, increased communication and genetic exchange between cells. Microbial biofilm development can be observed on virtually all kinds of stone monuments such as castles, caves, churches/cathedrals, fountains, temples, tombs/catacombs, etc., and can be associated with problems of conservation. Several types of autotrophic and heterotrophic microorganisms are usually observed on stone monuments such as bacteria, fungi, algae and lichens. The European standard EN15898 defines the main general terms used in the field of conservation of cultural property such as alteration, deterioration, weathering, treatment and cleaning. The term biodeterioration is not defined in the EN15898 standard, but can be defined as “any undesirable change in the properties of a material caused by the vital activities of organism”. The biodeterioration of stone materials corresponds to an aesthetic action (production of pigments), to a biochemical action (stone dissolution or salt crystallization driven by cell metabolism) and to a physical action (mechanical pressure during growth). This review has two main objectives: to summarize the actual knowledge about basics of microbial colonization and biodeterioration of stone monuments and to realize a quantitative and qualitative analysis of publications in scientific journals on this topic.