Implications of microbial adhesion to hydrocarbons for evaluating cell surface hydrophobicity 1. Zeta potentials of hydrocarbon droplets

Abstract

Microbial adhesion to hydrocarbons (MATH) is generally considered to be a measure of the organisms cell surface hydrophobicity. As microbial adhesion is a complicated interplay of long-range van der Waals and electrostatic forces and various short-range interactions, the above statement only holds when the MATH test is carried out under conditions where the interacting surfaces are uncharged. In the present study it is shown that the most commonly used hydrocarbons in MATH, aliphatic octane and hexadecane, and aromatic xylene and toluene, are highly negatively charged in solutions in which MATH is often carried out. Zeta potentials of the aliphatic hydrocarbon droplets were generally more negative than those of the aromatic hydrocarbons with values as negative as up to −60 mV at pH 7, but hovered around zero at acidic pH values for both types of hydrocarbons. For the aromatic hydrocarbons, zeta potentials hovered around zero up to pH 4–5, but for the aliphatic hydrocarbons, most notably octane, zeta potentials decreased sharply starting at pH 2–3. The different pH dependence of the zeta potentials of the aliphatic hydrocarbons as compared to those of the aromatic hydrocarbons is most likely due to the hydrogen-accepting capacity of the electron ring in the aromatic hydrocarbons and can have major implications for the adhesion of microorganisms to these hydrophobic surfaces in MATH.