Decisive Role of Polymer-Bovine Serum Albumin Interactions in Biofilm Substrates on "Philicity" and Extracellular Polymeric Substances Composition.

Abstract

Formation of extracellular polymeric substances (EPS) is a crucial step for bacterial biofilm growth. The dependence of EPS composition on growth substrate and conditioning of the latter is thus of primary importance. We present results of studies on the growth of biofilms of two different strains each, of the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae, on four polymers used commonly in indwelling medical devices ─polyethene, polypropylene, polycarbonate, and polytetrafluoroethylene─immersed in bovine serum albumin (BSA) for 24 h. The polymer substrates are studied before and after immersing in BSA for 9 and 24 h, using contact angle measurement (CAM) and field emission scanning electron microscopy (FE-SEM) to extract, respectively, the "philicity" φ (defined as -cos θ, where θ is the contact angle of the liquid on the solid at a particular temperature and ambient pressure) and spatial Hirsch parameter H (defined from the relation F(r) ∼ r2H, where F(r) is the mean squared density fluctuation at the sample surface). H = 0.5, <0.5, or >0.5 signifies no correlation, anticorrelation, and correlation, respectively. The substrates are seen to transform from large hydrophobicity to near amphiphilicity with the formation of a BSA conditioning surface layer, and the H-values distinguish the length scales of 100, 500, and 2000 nm, with the anticorrelation increasing with length scale. Biofilms of E. coli did not grow on bare PTFE and HDPE substrates. Biofilms grown on BSA-covered surfaces are studied with CAM, FE-SEM, Fourier transform infrared (FTIR), and surface-enhanced Raman spectroscopy (SERS). Both spectra and φ-values were independent of bacterial species but dependent on the polymer, while H-values show some bacterial variation. Thus, EPS composition and wetting properties of the corresponding bacterial biofilms seem to be decided by the interaction of the conditioning BSA layer with the specific polymer substrate.