The synergistic antibacterial effect and mechanism of ε-polylysine and punicalagin in microorganism inhibition

Endogenous protease degradation and pH dependence limit the practical application of ε-polylysine (ε-PL) as an antimicrobial agent in raw meat preservation. To expand the usage of ε-PL in raw meat, the study investigated the synergistic inhibitory effects of ε-PL and punicalagin (PU) against Staphylococcus aureus ( S. aureus) and Escherichia coli ( E. coli ) using micro checkboard method. The synergistic effects were quantified using fractional inhibitory concentration index (FICI) and fractional bactericidal concentrations index (FBCI). Both ε-PL and PU at the ratio of 1/4 MIC ε-PL + 1/4 MIC PU showed FICI of 0.5 ≤ 0.5 and FBCI of 0.266 ≤ 0.5 for both S. aureus and E. coli , suggesting that the two have a synergistic effect. The actual inhibitory zone diameters of ε-PL/PU for S. aureus and E. coli were significantly higher than the theoretical summed values ( p < 0.05). The colony number of S. aureus and E. coli were reduced by 1000 times by ε-PL/PU after 8 h and 12 h, compared to single treatments. Mechanistic studies revealed that ε-PL/PU synergistically disrupted cell membranes, increased leakage of alkaline phosphatase and proteins, and inhibited ATP production. ε-PL/PU demonstrated synergistic antioxidant effects in DPPH, ABTS, and lipid oxidation assays. Through application to the beef preservation, ε-PL/PU inhibited the rise in pH, microbial growth, maintained texture, and extended shelf life from 4 d to 8 d at 4°C, and 1 d to 3 d at 20°C and 30°C, respectively. Thus, ε-PL/PU is a potential synergistic strategy for food preservation.