Stone-isolated carbonatogenic bacteria as inoculants in bioconsolidation treatments for historical limestone

Stone consolidation treatments that use bacterial biomineralization are mainly based on two strategies: (1) the inoculation of a bacterial culture with proven carbonatogenic ability and/or (2) the application of a culture medium capable of activating those bacteria able to induce the formation of calcium carbonate, from amongst the bacterial community of the stone. While the second strategy has been demonstrated to be effective and, unlike first strategy, it does not introduce any exogenous microorganism into the stone, problems may arise when the bacterial community of the stone is altered, for instance by the use of biocides in the cleaning process. In this study we isolate bacteria that belong to the natural microbial community of the stone and which have proven biomineralization capabilities, with the aim of preparing an inoculum that may be used in stone consolidation treatments wherein the natural community of those stones is altered. With this aim, outdoor experiments were undertaken to activate and isolate bacteria that display high biomineralization capacity from altered calcarenite stone. Most of the bacteria precipitated calcium carbonate in the form of calcite. The selected bacteria were phylogenetically affiliated with members of Actinobacteria, Gamma-proteobacteria and Firmicutes. Furthermore, the capability of these selected carbonatogenic bacteria to consolidate altered calcarenite stone slabs was studied in in vitro experiments, both in the presence and the absence of Myxococcus xanthus, as a potential reinforcement for the bacterial biomineralization. Herein, Acinetobacter species, belonging to the microbial community of the stone, are proposed as powerful carbonatogenic bacteria that, inoculated under appropriate conditions, may be used as inoculum for calcareous stone conservation/consolidation in restoration interventions where the microbial community of the stone is altered.