Interfacial Electrochemical Electron Transfer Processes in Bacterial Biofilm Environments on Au(111)

We have studied Streptococcus mutans (S. mutans) biolilm 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(NH3)(6)](3+2+), [Co(terpy)(2)](3+/2+), terpy = 2,2',2 ''-terpyridine) or negative ([Fe(CN)(6)](3-/4-), [IrCl6](3-)/(4-)) electrostatic charge, [Co(NH3)(6)](3+2+) and [Co(phen)(3)](3+2+) (phen = 1,10-phenanthroline) were other inhibition factors investigated.

The positively and negatively charged redox probe couples displayed antagonistic inhibition and voltammetric patterns. [Ru(NH3)(6)](3+2+) and the homologous compound [Co(NH3)(6)](3+/2+) were the only probe compounds to effect growth inhibition. On the other hand, cyclic voltammetry (CV) of both [Ru(NH3)(6)](3+2+) (positively charged, biofilm growth inhibitor) and [Co(terpy)(2)](3+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)](3-/4-) and [IrCl6](3-/4-) (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 teatment of the biofilm covered Au(111)-electrode surface partly restores planar diffusion CV of [Fe(CN)(6)](3-/4-) and [IrCl6](3-/4-).

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.