Characterization of S-layers from mesophilic bacillaceae and studies on their protective role towards muramidases

Abstract

The structure of the S-layers from eight mesophilic Bacillaceae was investigated by freeze-etching of whole cells. The S-layer lattices from Bacillus alvei CCM 2051 and B. circulans CCM 1048 exhibited oblique symmetry. Square S-layer lattices were identified for B. polymyxa CCM 1459, B. circulans CCM 2048 and B. sphaericus strains CCM 2120 and CCM 2177. Two superimposed, structurally different S-layers were detected for B. brevis strains CCM 1089 and CCM 1463. In both organisms the outer S-layer lattice showed oblique and the inner S-layer lattice hexagonal symmetry. Labelling with polycationized ferritin (a net positively charged topographical marker for electron microscopy) revealed that the S-layer surface from all strains is not net negatively charged. In thin-section preparations all strains exhibited only a 5 nm thick peptidoglycan-containing layer. Whole cells from B. circulans CCM 2048 and from both B. brevis strains were rapidly lysed by the muramidases lysozyme (MW 14,600) and mutanolysine (MW 24,000) showing that their S-layers which allowed free passage for both enzymes must possess pores larger than 3.5 nm. By contrast, whole cells from B. circulans CCM 1048, B. alvei CCM 2051, B. polymyxa CCM 1459 and B. sphaericus strains CCM 2120 and CCM 2177 were resistant to both muramidases. It could be shown that these strains possess muramidase resistant peptidoglycan-containing layers. Therefore, the latter results did not confirm the claim in the literature that muramidase resistance of S-layer carrying mesophilic Bacillaceae is caused by the S-layer itself acting as a protective coat.