1 Interdisciplinary Nanoscience Center, Faculty of Science, Aarhus University, Aarhus University2 Department of Biological Sciences, Microbiology, Faculty of Science, Aarhus University, Aarhus University3 iNano-School, Science and Technology, Aarhus University4 Department of Bioscience - Microbiology, Department of Bioscience, Science and Technology, Aarhus University5 Danish Technological Institute6 Interdisciplinary Nanoscience Center - INANO-Bioscience, iNANO-huset, Interdisciplinary Nanoscience Center, Science and Technology, Aarhus University7 iNano-School, Science and Technology, Aarhus University8 Department of Bioscience - Microbiology, Department of Bioscience, Science and Technology, Aarhus University9 Interdisciplinary Nanoscience Center - INANO-Bioscience, iNANO-huset, Interdisciplinary Nanoscience Center, Science and Technology, Aarhus University
L.Biofilm formation on surfaces in food production and processing can deteriorate the quality of food products and be a hazard to consumers. The food industry currently uses a number of approaches to either remove biofilm or prevent its formation. Due to the inherent resilience of bacteria in biofilm, a particularly attractive approach is the modification of surfaces with the aim to impede the first step in biofilm formation, namely bacterial adhesion. Surface properties such as hydrophobicity, roughness and predisposition for fouling by protein are recognised as important in bacterial adhesion. Sol-gel technology and the recent availability of organic modified silicas have lead to development of hybrid organic/inorganic glass ceramic coatings with specialised surface properties. In this study we investigate bacterial adhesion and the subsequent biofilm formation on stainless steel (SS) and compare it to two nanostructured sol-gel coatings with variable hydrophobicity. Test surfaces were characterised with respect to surface roughness by atomic force microscopy, surface hydrophobicity by contact angle (CA) measurements, protein adsorption by quartz crystal microbalance analyses, and bacterial adhesion. Adhesion of bacteria was quantified both as short-term adhesion (1-4h) and as early biofilm development (18-24h). Adhesion tests were performed with an enrichment culture from minced pork to reflect the bacterial community that colonise surfaces in abattoirs and butcher’s shops. The bacterial communities were identified by clone libraries and fluorescence in situ hybridization. We initially compared surfaces of relatively similar hydrophobicity (CA=60-79º) but different roughness. The roughness (Ra) was 300nm for SS type 2B, 6nm for electro polished SS, and 0.2 nm for sol-gel. There was a clear difference in initial bacterial adhesion between 2B SS and the smoother surfaces, while the difference between electro polished SS and sol-gel was small. Electro polished SS was then compared to two sol-gel coatings of similar roughness but variable hydrophobicity (CA=72-101º). Despite a substantial difference in surface hydrophobicity, we found no difference between the two sol-gel coatings in terms of protein adsorption, amount of adhered bacteria, or the community structure of the biofilm. The SS surface differed by adsorbing twice as much protein as the sol-gel coatings, but the amount of biofilm formation was not significantly different. However, the composition of the biofilm-forming community differed. The abundance of the primary surface coloniser - Pseudomonas sp. – was reduced by 25%. The enrichment cultures used for bacterial adhesion and biofilm formation assays were dominated by Gammaproteobacteria, and although the diversity in the enrichment culture was lower than in the bacterial community found on steel surfaces in a butchers shop, both Pseudomonas and Acinetobacter were abundant in clone libraries from both environments. This study showed that surface roughness and surface chemistry are more important in bacterial adhesion and biofilm formation than hydrophobicity within the specified range.