Acid-base interactions in microbial adhesion to hexadecane and chloroform

Acid-base interactions play an important role in adhesion, including microbial adhesion to surfaces. Qualitatively acid-base interactions in microbial adhesion can be demonstrated by comparing adhesion to hexadecane (a negatively charged interface in aqueous solutions, unable to exert acid-base interactions) and chloroform (able to exert acid-base interactions) from aqueous suspensions of microorganisms. For 31 different microbial strains (4 Escherichia coli, 4 Enterococcus faecalis, 4 lactobacilli, 10 staphylococci, 9 streptococci) in low and high ionic strength potassium phosphate buffers, adhesion to chloroform was initially faster and in a stationary end-point more extensive than to hexadecane, due to acid-base interactions additionally operative from chloroform. However, a quantitative surface thermodynamical analysis of acid-base interactions based on water, formamide, methylene iodide and a-bromonaphthalene contact angles failed to explain the adhesion data obtained at low ionic strength, likely because of interference of electrostatic interactions. A weak relationship between initial removal rates and the acid-base interfacial free energy of adhesion was observed for the high ionic strength data, provided the analysis was confined to strains with an absolute zeta potential less than 15 mV. Similarly, a weak relationship was obtained between the Gibbs free energy of partitioning, calculated from the stationary end-point adhesion of the strains, and the acid-base interfacial free energy of adhesion. It is concluded that acid-base interactions play a definitive role in microbial adhesion, but the extent to which thermodynamically determined acid-base interactions eventually become operative in microbial adhesion may vary from strain to strain due to the presence of structural appendages and chemical heterogeneities on microbial cell surfaces.