Synergistic effect of propyl gallate and antibiotics against biofilms of Serratia marcescens and Erwinia carotovora in vitro

Serratia marcescens NJ01 and Erwinia carotovora , two pathogenic bacteria involved in vegetable spoilage, can persist in leafy vegetables and subsequently form biofilms. The aim of this study was to evaluate the effect of propyl gallate (PG) on the susceptibility of antibiotics against the biofilms of S. marcescens NJ01 and E. carotovora and uncover its anti-virulence mechanism. PG notably inhibited the production of AHLs and enhanced the susceptibility of antibiotics against the biofilm formation of S. marcescens NJ01 and E. carotovora and significantly improved the disrupted effect on the preformed biofilms of S. marcescens NJ01. PG exposure decreased the minimum inhibitory concentrations of these five antibiotics and reduced the production of virulence factors. Further investigation indicated that PG exposure resulted in the downregulation of genes involved in virulence, biofilm formation, and antioxidant enzymes of S. marcescens NJ01. The suppression of antioxidant enzymes caused the enhancement of oxidative stress, increased membrane permeability, and ultimately promoted the susceptibility of bacterial/biofilm cells to antibiotics. The tomato and lettuce infection assay indicated that PG could notably attenuate the pathogenicity of S. marcescens NJ01 and E. carotovora . These findings suggest that PG has the potential to function as an antibiotic accelerant to defend against spoilage pathogens.