Hurdle Effects of Ethanolic Plant Extracts with Antimicrobials Commonly Used in Food against Foodborne Pathogenic Escherichia coli

Escherichia coli (E. coli) O157:H7 is a major foodborne pathogen that causes severe human infections. Plant extracts, glycine, and sodium acetate (NaOAc) exert antimicrobial effects that can be used to control pathogenic E. coli. However, their combinations have not been investigated. Thus, this study investigates the combination of ethanolic plant extracts with glycine and NaOAc against E. coli at various pH and temperature levels. Clove and rosemary extracts exhibited significant (p ≤ 0.05) antimicrobial activity against E. coli. At neutral pH, the combination of plant extracts with 1.0% glycine or 0.1% NaOAc reduced the minimum inhibitory concentration of clove from 0.4% to 0.2%; at pH 5.5, clove (0.1%) and rosemary (0.2%) extracts supplemented with NaOAc (0.1%) showed an additive effect. The population of E. coli O157:H7 in phosphate-buffered saline with 0.2% clove extract, 2% glycine, and 2% NaOAc showed a more than 5 log reduction after incubation at 15 °C for 96 h, while the combination of 0.1% clove extract with 2% NaOAc at pH 5.5 completely inhibited E. coli within 24 h at 35 °C. Thus, the combination of plant extracts with glycine and NaOAc could serve as a promising hurdle technology in controlling the growth of E. coli.