The antimicrobial activity of chitosan is affected by the pH, MW, and DDA, the physicochemical characteristics of the C2-NH2, C3-OH (secondary hydroxyl), and C6-OH (primary hydroxyl) functional groups of chitosan also significantly influence the antimicrobial properties. However, the antimicrobial effect of chitosan is observed only in acidic environments. Thus, due to the low solubility and lack of a positive charge at neutral pH, a large number of chitosan derivatives modified with amine (N-modified) and hydroxyl (O-modified) groups by acylation, carboxylation, alkylation, and quaternization have been developed. Carboxymethyl chitosan-zinc complex exhibits a better cidal effect against microbes, both N- and O-modified (O-quaternary ammonium N-acyl thiourea) chitosan had greater bacterial growth inhibition than singly modified chitosan and pure chitosan. Kindly check the following RG link:

Article Antimicrobial Actions and Applications of Chitosan

Quaternization of chitosan's nitrogen atom is the most common way to make water-soluble chitosan derivatives, particularly at physiological pH. Recent studies have shown that chitosan derivatives with a permanent positive charge on nitrogen atoms side-bonded to the polymer backbone have excellent antimicrobial activity. Kindly check the following RG link:

Article Antimicrobial Activity of Chitosan Derivatives Containing N-...

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Please see my paper in attached file.