Factors Determining the Antimicrobial Effectiveness of Chitosan: A Critical Analysis of the Impact of Molecular Weight and Degree of Deacetylation

Despite chitosan’s proven biocidal potential, there is a lack of consensus regarding the influence of its key physicochemical parameters, degree of deacetylation (DD) and molecular weight (MW), on its antimicrobial efficacy. The aim of this study was to quantitatively synthesize available literature data and provide an exploratory statistical analysis of the trends regarding the influence of chitosan DD and MW on the minimum inhibitory concentration (MIC). A PRISMA-guided literature review (2016–2026) across four major databases extracted 127 independent in vitro experiments, Gram-positive/negative bacteria and fungi. Physicochemical correlations were analyzed using multiple linear regression. Insoluble polymer fractions (here set as DD < 60%) critically interfere with MIC determination due to restricted diffusion and were strictly excluded from analyses. For the optimized group of soluble chitosans (n = 106, R2 = 0.36), DD was the dominant factor determining biocidal activity (p < 0.0001)—every 1% increase in DD reduces the logMIC by an average of 0.040. In contrast, molecular weight did not emerge as a statistically significant predictor within this model (p = 0.156), suggesting that the degree of deacetylation remains the primary driver of antimicrobial efficacy among soluble chitosans. In summary, maximizing chitosan antimicrobial activity requires high DD polymers, and rigorous standardization (excluding insoluble fractions) is essential for the proper design of new biomaterials.