An overview of techniques for the characterization and quantification of microbial colonization on stone monuments

Biodeterioration can be defined as any undesired change of the properties of a material caused by biological activity of living organisms. The biodeterioration of stone materials is related to the production of pigments (aesthetic action), to cell metabolism (biochemical action) and to the mechanical action of the biomass colonizing the material during its growth (physical action). Quantification of the sessile biomass and characterization of microbial communities colonizing stone are essential first steps to ensure the diagnosis of biodeterioration processes and to implement control strategies and appropriate treatment. Different destructive and non-destructive approaches can be used to sample stone specimens from monuments: scraping, swab using, and cutting. Different analytical methods can be used depending on the type of microorganism sought: determination of chlorophyll content and color analysis for pigmented microorganisms; measurement of in situ physiological activity of surface microcolonies by applying fluorogenic substrate analogues or confocal laser scanning microscopy observations after CTC staining for active biomass; scanning or transmission electron microscopy observation for biofilm visualization; enzyme-linked immunosorbent assay for the investigation of both microorganisms that can and cannot be cultured; classical microbiological methods, which consist in cultivation of microorganisms on synthetic media; and molecular methods for the study of microbial biodiversity based on the polymorphism of molecular markers using PCR, hybridization, classical or high throughput sequencing. The aim of this review is to present basics of the different biodeterioration mechanisms and to focus on the main techniques that can be used to characterize and quantify the biodeterioration biomass.