MICROBIAL CELL-SURFACE HYDROPHOBICITY - THE INVOLVEMENT OF ELECTROSTATIC INTERACTIONS IN MICROBIAL ADHESION TO HYDROCARBONS (MATH)

Microbial adhesion to hydrocarbons (MATH) is the most commonly used method to determine microbial cell surface hydrophobicity. Since, however, the assay is based on adhesion, it is questionable whether the results reflect only the cell surface hydrophobicity or an interplay of hydrophobicity and surface charge properties. In order to demonstrate the involvement of electrostatic interactions in MATH, hydrophobicities by MATH (kinetic mode) were measured in 10 mM potassium phosphate solutions at different pH's and compared with the zeta potentials of the microorganisms and of hexadecane droplets in the same solution. Two oral, microbial strains were involved: Streptococcus salivarius HB (a hydrophobic strain by MATH) and Streptococcus salivarius HB-C12 (a hydrophilic strain by MATH). The initial removal rates of S. salivarius HB-C12 by hexadecane were zero over the entire pH range (pH 2-pH 9) and its zeta potentials were negative in this pH range. S. salivarius HB, however, had an isoelectric point (IEP) at pH 3.2 and accordingly a positive zeta potential below IEP. Correspondingly, the initial removal rates found for this strain were high (2.6 min-1) below and around IEP and much lower (almost-equal-to 0.5 min-1) above IEP. Surprisingly, the hexadecane droplets also had highly negative zeta potentials above pH 4 and appeared uncharged in the pH range 2-3. Taking the product of the bacterial zeta(b) and hexadecane zeta(h) zeta potentials as a measure for electrostatic interactions, it was observed that the measured hydrophobicity of S. salivarius HB, but not of the hydrophilic strain S. salivarius HB-C12, depended on electrostatic interactions as well. The highest removal rates by hexadecane were found in the absence of electrostatic interactions, i.e. in the pH range close to the IEP's of the interacting particles. It is concluded that, in general, MATH does not measure cell surface hydrophobicity but an interplay of hydrophobicity and electrostatic interactions. The involvement of electrostatic interactions in MATH can be reduced by performing the test under ionic conditions in which either the cells or the hydrocarbon droplets (or both) are uncharged.