# Electrochemical Control of Biofilm Formation and Approaches to Biofilm Removal
## 4 Electrochemical Biofilm Control
### Electrochemical Control of Bacterial Adhesion  
As mentioned above, the bacterial cell wall is formed by peptidoglycan, consisting of sugars and amino acids that form a mesh-like layer outside the plasma membrane, where the sugar component contains alternating residues of \(\beta\)-(1,4) linked \(N\)-acetylglucosamine (NAG), and \(N\)-acetylmuramic acid (NAM). It is evident that the individual organic molecules of peptidoglycan will undergo protonation and deprotonation at different pH of the environment. Due to the negative charge of the bacterial cell, the isoelectric point occurs at relatively low pH values because the negative charge is compensated by protons, namely _Mycobacterium_ 4.15, _Alcaligenes_ 3.25, _Clostridium_ 2.75, _Proteus_ 2.67, _Azotobacter_ 2.07, and _Streptococcus_ 1.9. On the verge of the new millennium, [PERSON] et al. [143] introduced a simple indirect voltammetric method to determine the number of bacterial cells (_Pseudomonas syringae_ pv. _atropurpurea_ NIAES 1309) that had been attached onto the surface of a carbon paste electrode (CPE). A simplified principle of this method is explained in Figure 3. At first, the CPE is immersed into a bacterial cell suspension for a certain time to allow the cells to attach to its surface, where the process takes place spontaneously (in open circle) or by applying an accumulation potential (\(E_{\text{dep}}\)) depending on the pH of the environment. In the second step, the CPE with accumulated bacteria cells is transferred into a solution of an organic dye (Hoechst) to adsorb onto the remaining free sites on the electrode surface. Finally, the CPE with the presence of bacteria cells and the adsorbed organic dye is subsequently immersed in detection media. There was a calculated difference in the peak heights corresponding to anodic oxidation signals of absorbed organic dye with (\(I_{\text{N}}\)) or without spontaneously adhered bacterial cells (\(I_{0}\)). Eleven years later, similar experiments were repeated with _Staphylococcus epidermidis_ in neutral media of phosphate buffer using Amido Black 10B dye. Herein, it was found that the cells of _G+ Staphylococcus epidermidis_ are negatively charged because positive potentials applied during the accumulation have enhanced their adsorption onto the surface of the CPE in comparison with that which occurred during the spontaneous adsorption process. Otherwise, the negative potentials from \(-0.1\) to \(-0.4\) V did not have any significant effect on the adsorption of _Staphylococcus epidermidis_ cells. Only the application of accumulation potentials lower than \(-0.6\) V resulted in the repulsion of bacterial cells from the CPE surface [144]. In 2012, this study was extended to clarify the effect of pH, incubation time, and solid-medium type on the adhesion of _Staphylococcus epidermidis_ cells onto the surface of a CPE. The spontaneous adhesion of _Staphylococcus epidermidis_ to the CPE was not observed in an alkaline environment. This phenomenon is in accordance with the previously reported data confirming an inhibition effect of alkaline media (with pH 8.5) on the adhesion of _Staphylococcus epidermidis_ and _Staphylococcus aureus_ cells [145]. In contrast, the adhesion of _Staphylococcus epidermidis_ cells to the surface of a CPE was slightly enhanced in an acidic environment (pH 5) [146]. However, it should be noted that the above-mentioned knowledge cannot be used in general because a decrease in G+ _Listeria monocytogenes_ adhesion was found in acidic conditions (pH 5) compared to a physiological pH of approximately 7 [147]. Some recent research suggests that the presence of divalent metal ions can significantly affect biofilm formation, either to increase the adhesion of motile cells [148] or to inhibit biofilm formation [149]. For example, it was experimentally confirmed that the presence of Ca\({}^{2+}\) and Mg\({}^{2+}\) ions increased the biofilm formation of _Sphingomonas paucimobilis_[150] isolated from an industrial environment.