# Electrospinning of Chitosan for Antibacterial Applications--Current Trends
## 6 Antibacterial Effect of Chitosan Nanofibers
### Mechanism of Antibacterial Activity of Chitosan  
Studying the antimicrobial mechanism of chitosan, several theories have been proposed according to its characterized nature and electrostatic attraction. The first theory suggests that positive charges carried by the chitosan polymeric chain interact with negatively charged cell wall components such as phospholipids and lipopolysaccharidesaccharides [132] in the case of Gram-negative bacteria, and with teichoic acids linked to peptidoglycan present in the cell wall in Gram-positive bacteria, creating cavities and leading to increased perforation and altered permeability of the membrane that results in the leakage of intracellular components, ending up in bacterial lysis (breakdown) [133]. Figure 5 shows the mechanism of attack of chitosan on the bacterial cell wall. Furthermore, the amine groups responsible for the chelating effect of chitosan will cause displacement of metal ions such as Ca++ from anionic sites of the membrane, eventually destabilizing these trace metals that are essential for the bacterial growth, leading to their efflux from the cytoplasm to the external medium. Simultaneously, high-molecular-weight chitosan surrounds and encloses the bacterial cell and blocks the cell exchange with the extracellular matrix and absorption of essential nutrients, preventing cellular metabolism, while electronegative sites of the membrane compete for the remaining Ca++, leading to subsequent leakage of the intracellular material [134; 135] (see Figure 5a,b). In contrast, low-molecular-weight chitosan can penetrate the cell, causing impairment of the physiological activities of the bacteria and preventing the replication of DNA, which leads to cell death.  
The antibacterial properties of chitosan nanofibers may be based on the action mechanism of partial solubilization of chitosan in the medium. Short chitosan chains can penetrate the plasma membrane and the cell wall, while longer chains can encapsulate bacteria and prevent the exchange of cells with the extracellular medium [136]. Further forcing the antibacterial activity of chitosan nanofibers, the surface characteristics and cell stages were considered by another research group in response to the treatment with chitosan nanofibers. An impermeable envelope of chitosan around the bacteria could be observed, which may have blocked the absorption of vital components into the cells, and the cytoplasmic membranes of _E. coli_ and _S. aureus_ cells completely collapsed after 30 min contact time. It was hypothesized that chitosan nanofibers' bactericidal action is the  
Figure 5: (**a**) Mechanism of attack of chitosan on the bacterial cell wall of Gramm-positive and (**b**) Gramm-negative bacteria. Adapted from [136], originally published under a CC-BY 4.0 license.  
consequence of membrane permeability and perforation, involving a complicated mix of various bactericidal effects. First, bacterial growth is inhibited by membrane capture and perforation. Then, partially solubilized chitosan chains restrict cell exchange and nutrient absorption. Finally, chitosan particles induce osmotic shock by chelating trace elements required for bacterial survival. However, no indication of membrane penetration could be deduced. Since bacteria require adhesion to surfaces to grow and multiply better, the researchers concluded that the bacteria migrate to the surfaces of the nanofibers [94; 137].  
The capacity of chitosan to attract and capture bacteria is, therefore, the initial stage in the nanofibers' mode of action, which is followed by membrane perforation, cytoplasmic compound leakage, and ultimately cell lysis and disintegration.