Influence of acetylation degree and molecular weight of homogeneous chitosans on antibacterial and antifungal activities.

The results given in the literature are conflicting when considering the relationship between antimicrobial activity and chitosan characteristics. To be able to clarify, we prepared fifteen homogeneous chitosans with different acetylation degrees (DA) and molecular weights (MW) by reacetylation of a fully deacetylated chitin under homogeneous conditions. They were tested at different pH values for their antimicrobial activities against four Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Salmonella typhi), four Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis and Micrococcus luteus) and three fungi (Aspergillus niger, Fusarium oxysporum and Alternaria solani). Chitosans markedly inhibited growth of most bacteria and fungi tested, although the inhibitory effect depends on the type of microorganism and on the chitosan characteristics (DA and MW) with minimum inhibitory concentrations in the range of 0.001 to 0.1 w%. Considering chitosan efficiency on bacteria, our series of data clearly show that the lower DA and the lower pH give the larger efficiency. Antibacterial activity was further enhanced for Gram-negative bacteria with decreasing MW, whereas, opposite effect was observed with the Gram-positive. Concerning the antifungal activity, the influence of chitosan characteristics was dependent on the particular type of fungus. Fungal growth decreased with increasing MW for F. oxysporum and decreasing DA for A. solani, but no MW or DA dependences were observed with A. niger.