## 2 Bioconsolidation Techniques
### Microbial Cells in Bioconsolidation Treatments  
When considering the application of selected bacteria, a good option is to use organisms isolated from the autochthonous population, which are obviously already adapted to the stone environment and are less likely to interact negatively with the endogenous community [24; 32; 33]. The first such bacterial strain to be so isolated was a _Bacillus cereus_, which produced carbonate crystals in the alkaline conditions caused by breakdown of amino acids to ammonia [34]. It was successfully tested in the field in 1999 (see Table 1 [35]). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Organisms** & **Isolation Source/ Habitat** & **Testing Method** & **Reference** \\ \hline _Bacillus, Pseudomonas, Breviluchern,_ & Degraded limestone & Crystal formation by isolated strains & \\ _Streptomyces,_ & Isolated during the & on solid medium; crystal & \\ _Streptomyces_ & & course of the study & identification by FTIR, XRD and SEM & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of carbonatogenic microorganisms that have been tested for MICP potential. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Organisms** & **Isolation Source/ Habitat** & **Testing Method** & **Reference** \\ \hline \hline \multirow{4}{*}{_Basillus sp., Microoccus sp._} & Black crusts on limestone buildings & Bio-cementation on limestone slabs; Bio-consolidation of fragmented stones (Church of Santa Maria dei \\  & course of the study & Miracoli, Venice); SEM, EDX \\ \hline \multirow{4}{*}{_Basillus cereus_} & Natural carbonate rock Isolated during the course of the study & First in situ consolidation attempt (Saint Medard Church, Thouars, France); SEM, water absorption, surface roughness, colorimetry \\ \hline \multirow{4}{*}{_Basillus subtilis_ LMG 3589} & Belgian Co-ordinated Collections of Microorganisms & Consolidation of deteriorated _Globigeria_ limestone in Malta; drilling resistance, water absorption, salt deterioration, porosity \\  & INQCSS: Instituto Nacional de Controle de & \\  & Bacillus subtilis, & Brazilian Culture & Production of CaCO\({}_{3}\) in growth medium \\  & Pseudomonas putida & Collection (Rio de Janeiro, Brazil) & \\ \hline \multirow{4}{*}{_Pseudomonas, Pantota, Cupriaréidus_} & Ancient marble quarry in Athens, Greece & SEM, ERD and FTIR analyses of treated marble from same quarry \\  & & & \\  
One published example is that of [PERSON] et al. [45], who effectively treated deteriorated limestone with biocalcifying _B. subtilis_ isolated from Maltese hypogea. The treatment conferred uniform bioconsolidation to a depth of 30 mm and the treated stone had high resistance to salt deterioration and low water absorption, with a preserved pore network. Further work on this technology is still necessary. Research by one of the authors is currently underway on a limestone wall in Campeche, southern Mexico (Figure 1). Bioconolidation of the damaged limestone blocks is underway using a suspension of a locally isolated carbonatogenic bacterial strain applied by spraying. Frames have been set to study the efficiency of consolidation in several quadrants, to determine variability of the results. Efficiency is being measured by surface hardness and cohesion, using peeling tests. Preliminary measurements made two months after application revealed a slight increase of surface hardness and less flake detachment. The experimental study will be continued for up to one year and both short- and medium-term effects of the treatment will be determined under the natural climatic conditions of this region, classified as Koppen climate As (Tropical Savannah). Many other microorganisms have been tested for their calcifying and consolidation activities, including fungi (see Table 1). An important proportion of microorganisms reported in Table 1 are well characterized isolates that are deposited in established culture collections, while a significant number of bacteria were isolated during the course of the respective studies.