Growth of Bacillus subtilis in Phosphate Limited Media Reduces Susceptibility to Antibacterial Activity of Chitosan

Chitosan is a commercially available derivative of chitin that has been extensively studied for its antimicrobial properties. Recent studies point to the disruption of cell membranes as a possible mode of action that is facilitated by a strong positive charge from protonation of free amino groups in acidic to neutral conditions. This study investigated the impact of cell surface charge on susceptibility to chitosan by using phosphate limited media to induce a conversion of cell wall moieties. Based on previous studies, phosphate limitation would cause the conventional teichoic acid moieties on Bacillus subtilis to convert to the less negatively charged teichuronic acid moieties. Exposure to chitosan after using phosphate limited media resulted in increased resistance to cell death as determined by a permeability staining assay. While this experiment pointed to electrostatic interaction between negatively charged teichoic acids and positively charged chitosan as a possible reason for the strong antimicrobial activity of chitosan, quantitative analyses were not performed to assess the magnitude of the effect on charge.