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In this work we investigated the mechanism of calcium carbonate deposition at the nanoscale on a crystalline bacterial surface resulting in calcite decoration. For this purpose we selected the bacterium Geobacillus stearothermophilus DSM 13240, whose structure has not yet been described in literature. Step by step we mineralised the surface of both the isolated S-layer and the whole bacterium. Mineralisation was analysed by TEM and high-resolution TEM and electron holography. Mineralisation initiates by covering parts of single unit cells, and then proceeds to complete coverage of the sheets. Biomineralisation of S-layers is commonly observed, mostly for calcite. It plays a role in the mineral formation in geology and is therefore a matter of discussion of geological groups and publications. The mineralisation of the outermost bacterial layer ("skin") is understood as a defence reaction to oversaturation of the ions in the surrounding water. By precipitation of calcite the concentration of the Ca2+ and the (CO3)2- ions is lowered, and the bacterium can survive by stripping off its mineralised surface. Other minerals that precipitate on bacterial surfaces are SrSO4 and SrCO3. However, the mineralisation of S-layers by calcite gives the highest impact, because it occurs naturally in sea water as well as in fresh water to a large extent, giving rise to geological formations. Therefore the mechanism of the nucleation and the process of calcite mineral formation on bacterial S-layer are of high interest. |
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