Building a Cell House from Cellulose: The Case of the Soil Acidobacterium Acidisarcina polymorpha SBC82(T)

Acidisarcina polymorpha SBC82(T) is a recently described representative of the phylum Acidobacteriota from lichen-covered tundra soil. Cells of this bacterium occur within unusual saccular chambers, with the chamber envelope formed by tightly packed fibrils. These extracellular structures were most pronounced in old cultures of strain SBC82(T) and were organized in cluster-like aggregates. The latter were efficiently destroyed by incubating cell suspensions with cellulase, thus suggesting that they were composed of cellulose. The diffraction pattern obtained for 45-day-old cultures of strain SBC82(T) by using small angle X-ray scattering was similar to those reported earlier for mature wood samples. The genome analysis revealed the presence of a cellulose biosynthesis locus bcs. Cellulose synthase key subunits A and B were encoded by the bcsAB gene whose close homologs are found in genomes of many members of the order Acidobacteriales. More distant homologs of the acidobacterial bcsAB occurred in representatives of the Proteobacteria. A unique feature of bcs locus in strain SBC82(T) was the non-orthologous displacement of the bcsZ gene, which encodes the GH8 family glycosidase with a GH5 family gene. Presumably, these cellulose-made extracellular structures produced by A. polymorpha have a protective function and ensure the survival of this acidobacterium in habitats with harsh environmental conditions.