# Chitosan Nanocomposite Coatings for Food, Paints, and Water Treatment Applications
## 2 Chitosan and its Properties
### Antimicrobial Properties  
[PERSON] and [PERSON] first reported the antifungal properties of chitosan and suggested that chitosan does not only possess fungicidal properties, but is also more effective on a wider range of fungi than chitin [55]. Since then, several researchers have evaluated the antimicrobial nature of chitosan against different microorganisms and their action mechanisms, but no clear consensus on the mechanism of antimicrobial activity of chitosan has been reached [56; 57; 58; 59; 60]. Several mechanisms have been proposed to explain the antimicrobial properties of chitosan. These include: (i) interactions between the positively charged amine groups of chitosan and the negatively charged microbial cell membranes, leading to leakage of cellular constituents; (ii) activation of several defence mechanisms in the host tissue by chitosan molecules acting as a water-binding agent and hindering several enzymes by blocking their active centres; (iii) chitosan as a chelating agent, selectively binding metals and then preventing the microbial growth; (iv) chitosan (high-molecular-weight) forms an impervious polymeric layer on the cell surface that alters cell permeability and ultimately blocks the entry of nutrients into the cell; (v) penetration of chitosan into microbial cytosol that may bind with DNA, resulting in alterations for the synthesis of mRNA and proteins, mostly prevalent with low-molecular-weight chitosan; (vi) adsorption and flocculation of electro-negative materials in the cell, hampering the physiological properties of microorganisms, triggering cell death.  
The most widely accepted mechanisms for antimicrobial activity of chitosan is the interaction between the positively charged amine groups of chitosan and the negatively charged microbial cell membranes. In acidic solutions (pH < 6.3) chitosan has a polycationic nature and the positively charged amino groups of chitosan interact with negatively charged components on microbial cell membranes, causing extensive alterations to the cell surface and membrane barrier properties, leading to leakage of intracellular contents that results in cell death. [PERSON] et al. have demonstrated this hypothesis through a mechanistic study of chitosan nanoparticles against Gram-positive bacteria, _S. aureus_[56]. As chitosan has pKa \(\sim\)6.3, in mildly acidic conditions it is protonated, leading to a reduction in osmotic stability affecting membrane disruption that may efficiently lead to alterations in cell permeability and leakage of intracellular contents, ultimately imbibing rupture of cell. In addition, positively charged chitosan may interact with the negatively charged teichoic acids in the cell wall of gram-positive microbes leading to the formation of small pores on the cell wall and subsequently leading to leakage of the intracellular components [56]. [PERSON] et al. suggested that increased permeability of the outer membrane of _Escherichia coli_ is the primary reason for antibacterial activity of CH against such Gram-negative bacteria. Increased permeability leads to the release of cellular contents followed by cell lysis, as shown through microstructural analysis using transmission electron microscopy (TEM) (Figure 4) [61].