Implications of microbial adhesion to hydrocarbons for evaluating cell surface hydrophobicity 2. Adhesion mechanisms

Abstract

Microbial adhesion to hydrocarbons (MATH) is generally considered to be a measure of the organisms cell surface hydrophobicity. Recent observations that the zeta potentials of hydrocarbons can be highly negative in the various solutions commonly used in MATH, have suggested that MATH may measure a complicated interplay of long-range van der Waals and electrostatic forces and of various short-range interactions. By carrying out the MATH test on two intrinsically hydrophobic (i.e. by water contact angle) and two intrinsically hydrophilic microbial strains, it was demonstrated that the hydrophobic strains were removed by the hydrocarbons in a pH dependent fashion, with maximal removal at pH values where the zeta potentials of the organisms and/or of the hydrocarbons are zero, that is in the absence of electrostatic repulsion. The hydrophilic strains were not removed by the hydrocarbons to any significant extent, because the attractive forces between water and the organisms are much stronger than those between the organisms and the hydrocarbon droplets, in line with the low water contact angles on these microorganisms. These observations clearly disqualify MATH as a hydrophobicity assay. Possibly, the maximal initial removal rates for microorganisms by a given hydrocarbon at pH values where electrostatic repulsion is absent, can be considered as a measure for microbial cell surface hydrophobicity.