Gallic Acid-Grafted-Chitosan Inhibits Foodborne Pathogens by a Membrane Damage Mechanism

In this study, antimicrobial activity of gallic acid-grafted-chitosans (gallic acid-g-chitosans) against five Gram-positive and five Gram-negative foodborne pathogens was evaluated. The minimum inhibitory concentrations (MICs) of gallic acid-g-chitosans ranged from 16 to 64 μg/mL against Gram-positive bacteria and ranged from 128 to 512 μg/mL against Gram-negative bacteria. These activities were higher than those of unmodified chitosan. The bactericidal activity of gallic acid-g-chitosan (I), which showed the highest antimicrobial activity, was evaluated by time-killing assay with multiples of MIC, and it was recognized to depend on its dose. The integrity of cell membrane, outer membrane (OM), inner membrane (IM) permeabilization experiments, and transmission electron microscopy (TEM) observation were conducted for elucidation of the detailed antimicrobial mode of action of gallic acid-g-chitosan. Results showed that treatment of gallic acid-g-chitosan (I) quickly increased the release of intracellular components for both Escherichia coli and Staphylococcus aureus. In addition, gallic acid-g-chitosan (I) also rapidly increased the 1-N-phenylanphthylamine (NPN) uptake and the release of β-galactosidase via increasing the permeability of OM and IM in E. coli. TEM observation demonstrated that gallic acid-g-chitosan (I) killed the bacteria via disrupting the cell membrane.