Experimental and theoretical examination of surface energy and adhesion of nitrifying and heterotrophic bacteria using self-assembled monolayers.

Biofilm-based systems, including integrated fixed-film activated sludge and moving bed bioreactors, are becoming increasingly popular for wastewater treatment, often with the goal of improving nitrification through the enrichment of ammonia and nitrite oxidizing bacteria. We have previously demonstrated the utility of self-assembled monolayers (SAMs) as tools for studying the initial attachment of bacteria to substrata systematically varying in physicochemical properties. In this work, we expanded these studies to bacteria of importance in wastewater treatment systems and we demonstrated attachment rates were better correlated with surface energy than with wettability (water contact angle). Toward the long-term goal of improving wastewater treatment performance through the strategic design of attachment substrata, the attachment rates of two autotrophic ammonia-oxidizing bacteria (Nitrosomonas europaea and Nitrosospira multiformis) and a heterotroph (Escherichia coli) were evaluated using SAMs with a range of wettabilities, surface energies, and functional properties (methyl, hydroxyl, carboxyl, trimethylamine, and amine terminated). Cell attachment rates were somewhat correlated with the water contact angles of the SAMs with polar terminal groups (hydroxyl, carboxyl, trimethylamine, and amine). Including all SAM surfaces, a better correlation was found for all bacteria between attachment rates and surface free energy, as determined using the Lewis Acid-Base approach. The ammonia-oxidizers had higher adhesion rates on the SAMs with higher surface energies than did the heterotroph. This work demonstrated the successful application of SAMs to determine the attachment surface preferences of bacteria important to wastewater treatment, and it provides guidance for a new area of research aimed at improving treatment performance through rational attachment surface design.