How does chitosan prevent bacterial and fungal growth?

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Chitosans are well known for their anti-microbial activities. They are not bactericidal or fungicidal (i.e., they do not kill bacteria and fungi) but instead, they are bacteriostatic and fungistatic (i.e., they prevent bacterial and fungal growth). There are a number of hypotheses as to how chitosans achieves these effects, such as chelation of trace elements required for microbial growth, or formation of a chitosan layer surrounding the microbial cells and blocking nutrient uptake. However, the most convincing hypothesis assumes membrane damage due to the polycationic nature of the chitosan polymers interacting with the polyanionic surface of phospholipidic membranes. This interpretation is underlined by the fact that the highest antimicrobial activities are shown by chitosans with very low degree of acetylation and, consequently, many free amino groups carrying positive charge. Also, antimicrobial activities of chitosans increase with decreasing pH of the solution which also increases the positive charge.