Insight into the mechanisms underlying bacterial adhesion is critical to the formulation of membrane biofouling control strategies. Using AFM-based single-cell force spectroscopy, we investigated the interaction between Pseudomonas fluorescens, a biofilm-forming bacterium, and polysulfone (PSF) ultrafiltration (UF) membranes to unravel the mechanisms underlying early stage membrane biofouling. We show that hydrophilic polydopamine (PDA) coatings decrease bacterial adhesion forces at short bacterium-membrane contact times. Further, we find that adhesion forces are weakened by the presence of natural organic matter (NOM) conditioning films, owing to the hydrophilicity of NOM. Investigation of the effect of adhesion contact time revealed that PDA coatings are less effective at preventing bioadhesion when the contact time is prolonged to 2-5 s, or when the membranes are exposed to bacterial suspensions under stirring. These results therefore challenge the notion that simple hydrophilic surface coatings are effective as a biofouling control strategy. Finally, we present evidence that adhesion to the UF membrane surface is mediated by cell-surface macromolecules (likely to be outer membrane proteins and pili) which, upon contacting the membrane, undergo surface-induced unfolding.
Bibliographical noteFunding Information:
We gratefully acknowledge funding from the United States Geological Survey through the Minnesota Water Resources Center (grant no. MN WRC 2015MN362B), and from 3M Co. (Non-Tenured Faculty Award Program). Parts of this work were carried out in the Characterization Facility and Minnesota Nano Center at the University of Minnesota, which receive partial support from NSF through the MRSEC and NNIN programs, respectively. S.B.A. acknowledges the support of a Ling Graduate Fellowship in Environmental Engineering. A.H. acknowledges the support of the Norwegian Center for International Cooperation in Education Partnership Program with North America. We also acknowledge Nathan Karp for his contributions to the preliminary stages of this study. We thank the University Imaging Centers at the University of Minnesota for support, and Dr. Guillermo Marqueś for technical assistance.
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