Interactions of pathogens Escherichia coli and Streptococcus suis with clay minerals

The adsorption of two agricultural isolates of pathogens Escherichia coli and Streptococcus suis on common clay minerals (montmorillonite and kaolinite) was investigated as a function of pH and ionic strength (IS) using batch studies. Electrokinetic properties and interaction energies between the cells and minerals were quantified at different pH (4.0–9.0) and IS (1–100 mM KCl and CaCl2) by applying the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The adsorption isotherms of bacteria on clay minerals conformed to the Freundlich equation. More S. suis cells were found to be adsorbed by the minerals than E. coli, with calculated Kf values 2–8 times higher than those of E. coli. As for the examined two pathogens, their adsorption capacities on montmorillonite were greater than those on kaolinite. Increasing solution pH (≥ 4.0) or decreasing IS (≤ 20 mM) resulted in less adsorption as a result of the decrease in the measured zeta potentials of bacteria and minerals and subsequently higher electrostatic repulsion. This trend was generally in agreement with predictions from DLVO theory. Conversely, higher IS (≥ 50 mM) reduced cell adsorption, likely due to an additional non-DLVO type interaction (electrosteric repulsion) occurring in the presence of extracellular macromolecules. This finding provides important insight into the pathogenic behavior in soil and associated environments.