Antimicrobial Effects of Plant Compounds against Virulent Escherichia coli O157:H7 Strains Containing Shiga Toxin Genes in Laboratory Media and on Romaine Lettuce and Spinach

Abstract

Escherichia coli strains produce Shiga-toxins Stx-1 and Stx-2 that contribute to their virulence. The objective was to evaluate antimicrobial activities of plant essential oils (oregano, cinnamon, lemongrass), their active components (carvacrol, cinnamaldehyde, citral) and plant-extracts (green tea polyphenols, apple skin, black tea, decaffeinated black tea, grapeseed and pomace extracts) against E. coli O157:H7 strains containing Stx-1 and Stx-2 genes, as determined by Multiplex Polymerase Chain Reaction, in vitro and on leafy greens. Antimicrobials at various concentrations in sterile PBS were added to bacterial cultures (~3-4 logs CFU/ml), mixed thoroughly, and incubated at 37˚C. Surviving bacteria were enumerated at 0, 1, 3, 5 and 24 h. The most effective essential oil (oregano oil; 0.5%) and plant extract (green tea; 3%) were evaluated against E. coli O157:H7 on romaine lettuce and spinach stored at 4˚C for 7 days. Microbial survival was a function of the concentration of an-timicrobials and incubation times. All antimicrobials reduced bacterial population to below detection levels in vitro; however, essential oils and active components exhibited greater activity than plant extracts. Oregano oil and green tea reduced E. coli O157:H7 on lettuce and spinach to below detection. Plant-based antimicrobials have the potential to protect foods against E. coli O157: H7.