Stone Consolidation by Bacterial Carbonatogenesis: Evaluation of in situ Applications

Precipitation of calcium carbonate minerals by bacteria, the so-called bacterial carbonatogenesis, is a promising method for the consolidation of decayed stone. Despite extensive laboratory testing that has demonstrated the efficacy of this method, little is, however, known regarding the medium- and/or long-term performance of this bacterial conservation treatment once appliedin situ, on stone buildings. Here, we report on the evaluation of the performance of bacterial consolidation treatments applied in three different historical buildings placed in an urban environment and built using a highly porous, easily decayed calcarenite stone. Peeling tape tests show a significant long-lasting (up to four years) strength improvement following treatment with either aMyxococcus xanthusbacterial culture or a sterile nutritional solution that selectively activates the carbonatogenic bacteria inhabiting the stone. Total color changes, measured before and after treatment using a spectrophotometer, are systematically below the acceptable value of ΔE≤ 5. Culture-dependent analysis of the microbiota shows that 100% of the culturable bacteria collected before and after treatment is carbonatogenic, and the total count of fungi spores remains constant or diminishes, while the population of acid-producing bacteria decreases over time after treatment application. Culture-independent microbial analyses show that no deleterious microbiota develops after treatment, being carbonate-producing Proteobacteria, Firmicutes and Actinobacteria the most abundant phyla both before and after treatment. Overall these results show that thein situapplication of the bacterial consolidation method shows no detrimental side effects and is highly effective in the medium- and long-term.