# Electrochemical Control of Biofilm Formation and Approaches to Biofilm Removal
## 2 Biofilms
### Bacterial Adhesion and Biofilm Formation  
In general, gram-positive (G+) and gram-negative (G\(-\)) bacteria differ in wall structure, although their common feature is the presence of peptidoglycan (murein), which can represent up to 90% of the cell wall. The wall of G+ bacteria, with a thickness of approximately 20 nm, is simpler than in the case of G\(-\) bacteria. Unlike G\(-\) bacteria, almost all \(N\)-acetylmuramic acid residues are connected by a peptide bridge. Teichoic acid chains, which have the function of the main surface antigen, run through the peptidoglycan-can layer. This polysaccharide binds Mg\({}^{2+}\) and Ca\({}^{2+}\) cations, which are necessary for the integrity of the wall and membrane. With a few exceptions (_Mycobacteria_, _Corynebacteria_, and _Nocardiia_), it does not contain any lipids and proteins, except for _Streptococci_. This is also the reason why they form a microcapsule [83]. The wall of G\(-\) bacteria, with a thickness of approximately 15 nm, consists of an outer membrane and the periplasmic space, where a thin layer of peptidoglycan is deposited. The outer membrane then contains a bilayer of phospholipids and proteins attached to the peptidoglycan via lipoproteins. Lipopolysaccharide molecules are present on the outside of the membrane, which are composed of three parts, namely O-polysaccharide, core polysaccharide, and inner lipid A [84; 82].  
Many bacteria have fine protein fibers outside the cell wall which are commonly referred to as flagella, pili (fimbriae), and curli. The flagella mediate active motion, thus facilitating chemotaxis and phototaxis. In the past, some researchers reserved the term pilus for the appendage required for bacterial conjugation, although all types of pili are primarily composed from pili proteins, unlike the curli, differing from pili proteins by representing the coiled surface structures composed of a single type of subunit. They are synthesized in the absence of a cleavable signal peptide [85].  
From the point of view of biofilm formation, short pili (\"attachment pili\"), generally known as fimbbrae, are required for the formation of biofilm because they are responsible for the attachment of bacteria to host surfaces for colonization during infection. Curli belong to one of the unique amyloid fibers produced by certain bacteria of the family _Enterobacteriaceae_ and are involved in adhesion to surfaces, cell aggregation, and biofilm formation. It has been proven that curli also mediate host cell adhesion and invasion, and they are potent inducers of the host inflammatory response [86].  
Naturally, the first step of biofilm formation starts with the reversible adhesion of bacteria to a surface through nonspecific interactions (physical forces) between the bacterial wall and the substrate. It is assumed that if bacteria cells are to be located more than 50 nm from the surface, they will be affected only by weak electrostatic interaction (van der Waals forces). After that, irreversible adhesion can occur through the effects of specific (short-range) interactions (distance less than 5 nm from the surface) with the involvement of hydrogen bonding, ionic and dipole interactions, hydrophobic interactions, and bacterial structural adhesins [87]. The bacterial adhesion with subsequent attachment might be affected by several factors, which are illustrated below in Figure 2.