Interfacial electrochemical electron transfer processes in bacterial biofilm environments on Au(111).

We have studied Streptococcus mutans (S. mutans) biofilm growth and growth inhibition on Au(111)-surfaces using atomic force microscopy (AFM) and interfacial electrochemistry of a number of redox probe molecules. AFM of the biofilm growth and growth inhibition on both mica and Au(111)-surfaces was followed by sampling at given times, drying the samples naturally, and imaging. The electrochemical investigations were based on single-crystal Au(111)-electrode surfaces to exclude polycrystallinity as a cause of inhomogeneous voltammetric broadening on the biofilm covered electrode surfaces. The redox couples were chosen for their positive (Ru(NH(3))(6), Co(terpy)(2), terpy = 2,2',2''-terpyridine) or negative (Fe(CN)(6), IrCl(6)) electrostatic charge. Co(NH(3))(6) and Co(phen)(3) (phen = 1,10-phenanthroline) were other inhibition factors investigated. The positively and negatively charged redox probe couples displayed antagonistic inhibition and voltammetric patterns. Ru(NH(3))(6) and the homologous compound Co(NH(3))(6) were the only probe compounds to effect growth inhibition. On the other hand, cyclic voltammetry (CV) of both Ru(NH(3))(6) (positively charged, biofilm growth inhibitor) and Co(terpy)(2) (positively charged, no biofilm growth inhibition) displayed fully reversible CV on biofilm covered electrodes, almost indistinguishable from CV at bare Au(111)-electrode surfaces. In comparison, CVs of Fe(CN)(6) and IrCl(6) (both negatively charged and no growth inhibition) were distorted from planar diffusion behavior on bare Au(111)-electrode surfaces toward spherical diffusion behavior on S. mutans biofilm covered Au(111)-electrode surfaces. DNAase treatment of the biofilm covered Au(111)-electrode surface partly restores planar diffusion CV of Fe(CN)(6) and IrCl(6). This is reflected in a decrease of the growth rate and the appearance of molecular-scale structures near the bacterial edges as imaged by AFM after DNAase treatment. A rationale for the different voltammetric behavior of positively and negatively charged redox probe molecules based on electrostatic properties of the local surface environment is offered.