Molecular weight and pH affect chitosan antibacterial activity. (A, B) Chitosan of low (50-190 kD), medium (190-310 kDa), or high (310-375+ kDa) molecular weight or (C, D) chitosan at different pH was incubated with (A, C) P. acnes or (B, D) S. aureus for 3 hours to assess antibacterial activity. Bacteria was then enumerated by colony-forming unit assay to determine concentration, which was plotted as a percentage of untreated sample. Error bars represent SEM (n = 3 for A, C, D, n = 4 for B, independent experiments). *p < 0.05, **p < 0.01, and ***p < 0.001 ( sign test across all concentrations for A, C, D, paired t-test for B).