Purpose: The objective of this study was to characterize the antimicrobial synergy between lysozyme and metal chelating active packaging.
Methods: Polypropylene films were surface modified by photoinitiated graft polymerization of acrylic acid (PP-g-PAA) onto the food contact surface of the films to impart chelating activity based on charge interactions. To quantify synergistic antimicrobial activity between lysozyme and the chelating active packaging film, a microtiter assay was developed using a 96 well microarray microtiter plate, in which swatches of film are assembled at the bottom of the plate and function as the bottom of each well.
Results: PP-g-PAA exhibited a carboxylic acid density of 113 ± 5.4 nmol cm-2 and an iron chelating activity of 53.7 ± 9.8 nmol cm-2. The antimicrobial interaction of lysozyme and PP-g-PAA against Listeria monocytogenes depended on the media composition. PP-g-PAA decreased lysozyme activity at low ionic strength (2.48 log increase at 64.4mM total ionic strength) and enhanced lysozyme activity at moderate ionic strength (5.22 log reduction at 120mM total ionic strength). These results support the hypothesis that, at neutral pH, synergy between carboxylate metal chelating films (pKabulk 6.45) and lysozyme (pI 11.35) is optimal in solutions of moderate ionic strength. These findings suggest that chelating active packaging with high metal affinity based on ligand-specific interactions, not charge-based interactions, may enable enhanced synergy with membrane-disrupting antimicrobials across a wider range of pH values and ionic strengths.
Significance: This work demonstrates the potential application of metal chelating active packaging films to enhance activity of food grade antimicrobials.