Streptococcus sobrinus and Streptococcus downei are closely related cariogenic species. A DNA fragment (1.2 kb) of a central region of the S. sobrinus dextranase gene (dex) was amplified by polymerase chain reaction and utilized for a DNA probe to differentiate S. sobrinus and S. downei. The dex probe was dot-hybridized to the reference strains (17 strains of 5 species) of mutans streptococci and other related strains (13 strains of 9 species). The probe hybridized only to both S. sobrinus and S. downei at a highly stringent condition, but to none of the other mutans streptococci nor other gram-positive cocci. To differentiate S. sobrinus and S. downei, Southern blot hybridization was carried out against restriction fragments of chromosomal DNAs with the dex probe. The probe showed species-specific hybridization patterns to the EcoRI, EcoRV, HindIII and PvuII-fragments. This result suggests that hybridization analysis with the dex DNA probe is useful for the detection and differentiation of S. sobrinus and S. downei.
The transverse cut and polished surfaces of a cow longbone were observed by backscattered electron (BSE) imaging, and then they were treated with a combined treatment of sodium hypochlorite (NaOCl) and ethylenediamine tetra-acetic acid (EDTA). The samples were observed again by BSE or by scanning electron microscopy (SEM). The combined treatment strongly eroded young hypocalcified osteons, and they were compared with older osteons. Some young osteons before treatment contained hypocalcified lines along the Haversian lamellas. The hypocalcified lines were more strongly eroded than the neighboring lines in the NaOCl-EDTA/ BSE method. In addition, a developing small osteon or a Haversian lamella showed a loose structure in the NaOCl-EDTA/SEM method. Thus, it was revealed that hypocalcified osteons and incremental lines, containing either a large or a smaller amount of collagen fibers, were strongly eroded by the combined treatment of NaOCl and EDTA. From such findings, it is suggested that the hypocalcified lines of spherical and linear laminate structures in human dentin are more strongly eroded than the hypercalcified lines in the NaOCl-EDTA/SEM method. Though we have previously suggested the positive correlation between the concentration of collagen fibers and the degree of calcification in the dentin, the correlation may be negative if the lower the concentration of collagen fibers is, the higher the degree of calcification as suggested by an author. On this correlation, further investigations will be necessary.