The influence of ionic strength and pH on diffusion of micro-organisms with different structural surface features

Abstract

Exact knowledge of microbial diffusion coefficients is a prerequisite for the application of mass transport theories to microbial deposition data. Microbial diffusion coefficients can be calculated on the basis of cell radii using the Einstein equation. This approach, however, does not take into account the additional effects of structural cell surface features such as fibrils or a fuzzy coat. Microbial diffusion coefficients of micro-organisms in suspension can be experimentally measured using dynamic light scattering. In this paper we compare experimental microbial diffusion coefficients with those calculated from radii for a variety of microbial strains suspended in 10 mM or 40 mM potassium phosphate solutions of different pH. Furthermore, the pH dependence of the microbial diffusion coefficients is related to the pH dependence of the microbial zeta potentials in similar ionic strength solutions. Microbial diffusion coefficients ranged from 2 × 1013to 5 × 1013 m2s−1and were generally higher at low pH (pH 2) than at high pH (pH 7) for strains with structural surface features. Experimental diffusion coefficients at pH 2 were approximately similar to those based on microscopic radii. This indicates that at pH 2 structural surface features are in a collapsed state, presumably due to the lack of stabilizing electrostatic repulsion between the cell surface structures as shown by the zeta potentials measured. Alternatively, a hydrodynamic radius of a micro-organism can be calculated from the experimental diffusion coefficients and the Einstein equation. Assuming that the maximal difference in hydrodynamic radius, observed over the pH range employed, represents the maximal length of structural cell surface features present, then the calculated fibrillar lengths for Streptococcus mitisBA and S. salivariusHB are in reasonable agreement with electron microscopical estimates.