Antimicrobial peptide zp37 inhibits Escherichia coli O157:H7 in alfalfa sprouts by inflicting damage in cell membrane and binding to DNA

Due to frequent outbreaks of sprouts-associated foodborne illness, sprouts have been considered as a “high risk” food. In this study, the antimicrobial peptide zp37 (GIKAKIIIKIKK-NH₂), which possesses a helical structure, was found to exhibit a minimum inhibitory concentration (MIC) value of 16 μM on E. coli O157:H7, effectively killing such strain in vitro and reducing its population size by 94.7% in sprouts upon storage for seven days. The effect of zp37 on cell membrane was investigated, with results showing that zp37 caused membrane depolarization and weak membrane permeabilization, which in turn resulted in cellular deformation observable by scanning electron microscope (SEM). Peptide zp37 was labeled by FITC to track the cellular location of zp37. Fluorescence microscopy studies showed that a larger amount of FITC-zp37 entered the bacterial cells when exposed to a higher concentration of the labeled peptide. Confocal microscopy showed that FITC-zp37 was detectable in cell membrane and cytoplasm. Upon entering the cytoplasm, zp37 was found to bind to the DNA of E. coli O157:H7, causing DNA aggregation and precipitation. Moreover, in vitro (HEK293 cell) and in vivo (Galleria mellonella) assays demonstrated that zp37 exhibited low toxicity.