Bacterially induced carbonate mineralization has been proposed as a
new method for the restoration of limestones in historic buildings and
monuments. We describe here the formation of calcite crystals by
extracellular polymeric substances isolated from Bacillus firmus
and Bacillus sphaericus. We isolated bacterial outer structures
(glycocalix and parietal polymers), such as exopolysaccharides (EPS) and
capsular polysaccharides (CPS) and checked for their influence on calcite
precipitation. CPS and EPS extracted from both B. firmus and
B. sphaericus were able to mediate CaCO3 precipitation
in vitro. X-ray microanalysis showed that in all cases the formed
crystals were calcite. Scanning electron microscopy showed that the shape
of the crystals depended on the fractions utilized. These results suggest
the possibility that biochemical composition of CPS or EPS influences the
resulting morphology of CaCO3. There were no precipitates in
the blank samples. CPS and EPS comprised of proteins and glycoproteins.
Positive alcian blue staining also reveals acidic polysaccharides in CPS
and EPS fractions. Proteins with molecular masses of 25–40 kDa and
70 kDa in the CPS fraction were highly expressed in the presence of
calcium oxalate. This high level of synthesis could be related to the
binding of calcium ions and carbonate deposition.