Wetting of dehydrated Pseudomonas f luorescens biofilms grown on glass substrates by an external liquid is employed as a means to investigate the complex morphology of these biofilms along with their capability to interact with external fluids. The porous structure left behind after dehydration induces interesting droplet spreading on the external surface and imbibition into pores upon wetting. Static contact angles and volume loss by imbibition measured right upon droplet deposition indicate that biofilms of higher incubation times show a higher porosity and effective hydrophilicity. Furthermore, during subsequent rotation tests, using Kerberos device, these properties dictate a peculiar forced wetting/spreading behavior. As rotation speed increases a long liquid tail forms progressively at the rear part of the droplet, which stays pinned at all times, while only the front part of the droplet depins and spreads. Interestingly, the experimentally determined retention force for the onset of droplet sliding on biofilm external surface is lower than that on pure glass. An effort is made to describe such complex forced wetting phenomena by presenting apparent contact angles, droplet length, droplet shape contours, and edges position as obtained from detailed image analysis.

<p>Biofilms are bacterial communities embedded in an extracellular matrix, able to adhere to surfaces. A deeper knowledge of the biofilm as a whole will aid the development of efficient methods to control deleterious biofilms (clinical biofilms, biofouling) or to enhance beneficial ones (waste-water treatment, bio-filtration). P. fluorescens has been widely studied, this strain produces bioactive secondary metabolites, and forms biofilms[1]. Several experimental set-ups have been widely used for in vitro biofilm cultivation of P. fluorescens, even if a deep characterization among different culture conditions is still lacking in the literature. This work, based on previous studies[2], is focused on the investigation of growth conditions on biofilm structure and properties. Growth kinetics of P. fluorescens biofilms was characterized in vitro under stagnant and flow-controlled conditions, using a rotating annular bioreactor. Two different supports in borosilicate glass and polycarbonate have been used. Bacterial growth kinetics has been measured through bio-turbidity analysis and TOC/DOC quantification. Biofilm morphology has been quantified through optical microscopy and image analysis by measuring the fraction of support surface covered by biofilm. The wetting properties of the biofilm layers have been investigated by using an innovative device, named Kerberos®, able to control centrifugal and gravitational forces acting on a single droplet placed on a surface[3]. The evolution of the droplet shape and position was measured as function of the imposed stress, to quantify wetting of different biofilm coated samples, following already assessed methodologies[4]. Different chemo-physical environments, investigated by changing growth medium, physical support, and imposed flow stress, induced different growth kinetics, biofilm morphology, and wetting properties. Accurate experimental measurements allowed us to estimate in a quantitative way the influence of investigated parameters on specific morphologic measurements.</p> <div><br /> <div> <p>[1] Brittan S. Scales and others, ‘Microbiology, Genomics, and Clinical Significance of the Pseudomonas Fluorescens Species Complex, an Unappreciated Colonizer of Humans’, Clinical Microbiology Reviews, 27.4 (2014), 927–48 <https://doi.org/10.1128/CMR.00044-14>.</p> </div> <div> <p>[2] Federica Recupido and others, ‘The Role of Flow in Bacterial Biofilm Morphology and Wetting Properties’, Colloids and Surfaces B: Biointerfaces, 2020 <https://doi.org/10.1016/j.colsurfb.2020.111047>.</p> </div> <div> <p>[3] Sotiris P. Evgenidis and others, ‘Kerberos: A Three Camera Headed Centrifugal/Tilting Device for Studying Wetting/Dewetting under the Influence of Controlled Body Forces’, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2017 <https://doi.org/10.1016/j.colsurfa.2016.07.079>.</p> </div> <div> <p>[4] Inmaculada Ríos-López and others, ‘Effect of Initial Droplet Shape on the Tangential Force Required for Spreading and Sliding along a Solid Surface’, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018 <https://doi.org/10.1016/j.colsurfa.2018.04.004>.</p> </div> </div>