Beyond single parameters: How structural combinations determine chitosans' antibacterial effects

Chitosans are versatile and promising functional biopolymers with diverse bioactivities that are strongly influenced by three characteristic structural parameters: the degree of polymerization (DP), the fraction of acetylation (F_A), and the pattern of acetylation (PA). However, limited understanding of these structure-function relationships and of the underlying mechanisms compromises reproducibility and effective applications. To address this, we have produced a set of 16 different chitosans from a single parent chitosan (DP 1237, F_A 0.01) yielding chitosans with defined DP and F_A, and random (Bernoullian) PA. We analyzed the antimicrobial activity of selected chitosans against four phytopathogens, Pseudomonas syringae pv. tomato, Clavibacter michiganensis, Fusarium graminearum, and Ustilago maydis, observing strong antimicrobial activities for all chitosans. Detailed analysis including more chitosans against P. syringae revealed no consistent trend linking a low F_A or a specific DP alone to enhanced antibacterial activity. Instead, the influence of F_A on antibacterial efficacy was dependent on DP, and vice versa. There seems to be a complex interplay between DP and F_A, and single-parameter interpretations seem insufficient. Further studies will have to show whether this conclusion can be generalized beyond P. syringae.