Synergistic antibacterial activity and mechanism of ε-polylysine hydrochloride and sodium diacetate against Serratia liquefaciens

Serratia liquefaciens is a psychrotrophic spoilage bacterium known for biofilm formation and thermoresistant enzyme production. This study investigated the synergistic antimicrobial mechanism of ε-polylysine hydrochloride (PLH) and sodium diacetate (SDA) against S. liquefaciens B2107-1. Individually, both agents inhibited planktonic growth and biofilm formation in a concentration-dependent manner. PLH alone or combined with SDA caused severe membrane damage (including wrinkling, shrinkage, depression, and rupture), while SDA alone induced only mild morphological alterations. Membrane disruption was further confirmed by nucleic acid and protein leakage, and fluorescence microscopy. Both compounds induced bacterial apoptosis-like death, and their combination synergistically enhanced antimicrobial efficacy. Further analyses revealed that the PLH + SDA combination compromised membrane integrity through increased potassium ion efflux, suppressed ATPase activity, and a marked synergistic decrease in membrane potential. Moreover, it significantly inhibited key metabolic enzymes (pyruvate kinase, succinate dehydrogenase, malate dehydrogenase), disrupting energy production. Activities of antioxidant enzymes (catalase, superoxide dismutase) were also markedly suppressed. We propose that PLH disrupts the membrane, promoting SDA entry, which subsequently intensifies metabolic interference and oxidative stress. This “outside-in” synergy positions PLH + SDA as a promising eco-friendly food preservative.