`Soft-particle' analysis of the electrophoretic mobility of a fibrillated and non-fibrillated oral streptococcal strain: Streptococcus salivarius

Abstract

The electrophoretic mobility of microbial cell surfaces can be analysed in terms of a so-called soft layer model, in which the electrophoretic mobility is described as originating from the potentials over the surface charge layer and the membrane fixed charges. Often, the polyelectrolyte layer deforms under the influence of ionic strength variations. In the soft layer analysis of electrophoretic mobilities this is expressed in the softness 1/λ. Here, we determined the softness of two oral streptococcal strains, S. salivarius HB and HBC12 from particulate microelectrophoresis in KCl solutions of varying ionic strength. Electron microscopy of negatively-stained organisms and X-ray photoelectron spectroscopy showed that strain HB had several classes of proteinaceous fibrils with lengths up to 178 nm on its outermost surface, while variant HBC12 had a bald, peptidoglycan-rich outer surface. The fibrillated strain HB appeared as relatively soft (1/λ equals 1.4 nm) from analysis of its electrophoretic mobility, while the bald variant HBC12 was hard (1/λ equals 0.7 nm) due to its comparatively rigid, peptidoglycan-rich outer surface, characteristic to Gram-positive bacteria. The presence of proteinaceous fibrils on strain HB slightly shielded the membrane fixed charges on HBC12.