Comparison of the physicochemical surface properties of Streptococcus rattus with those of other mutans streptococcal species.

Mutans streptococci comprise a group of seven closely related, yet distinct species. The distinction between the four species used in this study, namely Streptococcus sobrinus, Streptococcus cricetus, Streptococcus rattus, and Streptococcus mutans, has been made only recently on the basis of DNA homologies. In order to determine if there is a difference in the physicochemical surface properties of these species, strains were characterized by contact angles, zeta potentials and isoelectric points (IEP), elemental surface compositions by X-ray photoelectron spectroscopy, and molecular moieties by infrared spectroscopy. Contact angles, particularly when measured with water, can be considered a measure of cell surface hydrophobicity; zeta potentials reflect the charge of the outermost cell surface; X-ray photoelectron spectroscopy yields the relative abundance of carbon, oxygen, nitrogen, and phosphorus over the outer 5 nm of the bacterial cell surface; infrared spectroscopy enables a molecular characterization in terms of proteins, phosphates, and polysaccharides. All four species were homogeneous with regard to their physicochemical surface properties. However, the S. rattus species were clearly different from the others on the basis of the low water contact angle (21 +/- 2 vs. 26-31 degrees), highly negative zeta potential and lack of IEP, and high oxygen/carbon (0.50 +/- 0.02 vs. 0.41-0.43) and phosphorus/carbon (0.016 +/- 0.001 vs. 0.006-0.008) surface concentration ratios. Amongst the other differences observed, each species had a characteristic pH dependence of their zeta potential measured in phosphate buffer, yielding an IEP of 1.7, 2.1, and 2.5 for S. cricetus, S. sobrinus, and S. mutans, respectively. However, a cluster analysis on the zeta potential data showed only an isolated cluster for the S. rattus species. Thus it is likely that the higher cariogenicity of S. sobrinus with respect to S. cricetus and S. mutans is, in addition to a higher acidogenicity, due to the smaller negative surface charge, giving an increased adhesion to negatively charged tooth surfaces. S. rattus is expected to be the least adherent strain in the oral cavity as it carries a relatively high negative surface charge.