CRYSTALLINE SURFACE PROTEIN OF PEPTOSTREPTOCOCCUS-ANAEROBIUS

The surface ultrastructure of three anaerobic Gram-positive cocci freq uently encountered in oral infections, Peptostreptococcus micros, P. m agnus and P. anaerobius, was studied. The type strains of P. micros (D SM 20468) and P. anaerobius (ATCC 27337), several clinical isolates of both species and the type strain of P. magnus (DSM 20470) were includ ed. Thin-sectioned cells studied by electron microscopy revealed a hom ogeneous layer outside the peptidoglycan layer in P. anaerobius. In P. micros and P. magnus a more amorphous layer was present. No periodic structures were seen in negatively stained whole cells of these three species. However, in freeze-etched cells of P. anaerobius a crystallin e surface protein layer (S-layer) was detected. No periodicity was see n in any of the P. micros strains or the P. magnus type strain by the methods used, but a periodic pattern was observed in negatively staine d specimens of cell wall fragments of sonicated P. anaerobius cells. N o capsular material was visible outside the S-layer in P. anaerobius. The cells of the Peptostreptococcus spp. were extracted for 30 min wit h detergents and urea. One per cent SDS and 6 M urea both extracted a major 78 kDa protein from all strains of P. anaerobius. Extraction of P. micros and P. magnus cells did not reveal any major protein bands c omparable to that of P. anaerobius. Surface biotinylation of cells fol lowed by Western blotting and detection by alkaline-phosphatase-conjug ated extravidin showed strong staining of the 78 kDa band in P. anaero bius, further indicating that this molecule is located on the surface of the cell and is the S-protein. Another protein, of 127 kDa, was als o detected by surface biotinylation in all P. anaerobius strains. No d ifferences were detected in colony morphology or cell surface structur es between the four strains. Isoelectric focusing of the proteins of P . anaerobius revealed that the S-proteins of individual strains have d ifferent pI values, ranging from 5.3 to 6.9.