A bioactivity matrix for antimicrobial activities of chitosans: A review

Chitosans have prominent antimicrobial activities, inhibiting growth of both bacteria and fungi, but molecular structure-function relationships of these activities are still only partially understood. Structurally, chitosans differ in their degree of polymerization (DP), fraction of acetylation (F_A), and pattern of acetylation (PA). How these structural parameters are influencing antimicrobial activities is still a matter of debate. A comprehensive screening of all pertinent reviews and original publications dealing with antimicrobial activities of chitosans published in five selected years from 2000 to 2020 was performed. This screening of 2929 publications in total yielded 134 original papers, that contained data suitable for a thorough analysis of the influence of DP and F_A on chitosans' antimicrobial activities. Despite many differences between the studies, e.g. in the purity and quality of the chitosans, the microbial species, or the bioassay used, a partial consensus picture emerged. The strongest antimicrobial activity was observed for chitosans with a low to intermediate Mw. Larger polymers had lower activities, and chitosan oligomers were almost inactive. Less clearly, a trend was observed for decreasing activities with increasing F_A. Possible reasons for identifying only a partial rather than a comprehensive consensus picture are discussed.