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

Authors: REYNA-GRANADOS, JAVIER - University Of Arizona; JOENS, LYNN - University Of Arizona; LAW, BIBIANA - University Of Arizona; Friedman, Mendel; RAVISHANKAR, SADHANA - University Of Arizona

Submitted to: Food and Nutrition Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2021
Publication Date: 4/22/2021
Citation: Reyna-Granados, J.R., Joens, L.A., Law, B., Friedman, M., Ravishankar, S. 2021. 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. Food and Nutrition Sciences. 12:392-405. https://doi.org/10.4236/fns.2021.124030.
DOI: https://doi.org/10.4236/fns.2021.124030

Interpretive Summary: Escherichia coli O157:H7 is one of the most important foodborne pathogenic bacterium responsible for hemorrhagic colitis (HC) and hemolytic-uremic syndrome (HUS) in affected individuals. The findings of this study indicate that essential oils and their active components demonstrated better effectiveness as natural antimicrobials against E. coli O157:H7 than polyphenolic powder extracts, except for green tea and grapeseed extract, which showed strong activity. All the essential oils showed strong antimicrobial activity after 1 h (complete reduction with no detectable survivors) at all test concentrations and exposure times. The effectiveness of the antimicrobials was both concentration- and time-dependent. Exposure to oregano oil and carvacrol resulted in no detectable survivors at all concentrations at 0 min. Exposure to green tea and grapeseed extracts resulted in no detectable survivors at 3 h at all tested concentrations. All plant extracts exhibited storage time-dependent complete reduction with no detectable survivors after 24 h at all tested concentrations. Oregano oil (0.5%) and green tea polyphenols (3%) reduced the population of E. coli O157:H7 on contaminated lettuce and spinach to below detection limits.

Technical 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 antimicrobials 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.