Pre-harvest biocontrol of Listeria and Escherichia coli O157 on spinach and lettuce by lactic acid bacteria

Authors: Yin, Hsin Bai; CHEN, CHI-HUNG - Oak Ridge Institute For Science And Education (ORISE); Gu, Ganyu; Nou, Xiangwu; Patel, Jitu

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2022
Publication Date: 2/7/2022
Citation: Yin, H., Chen, C., Gu, G., Nou, X., Patel, J.R. 2022. Pre-harvest biocontrol of Listeria and Escherichia coli O157 on spinach and lettuce by lactic acid bacteria. International Journal of Food Microbiology. https://doi.org/10.1016/j.ijfoodmicro.2022.110051.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2022.110051

Interpretive Summary: Consumption of fresh produce contaminated with disease-causing bacteria have been implicated in foodborne illnesses in recent years. Contaminated irrigation water and biological soil amendments, animals, and other vectors could contaminate fresh produce at the farm. Control of these pathogens at the farm level is important since post-harvest chemical wash of fresh produce may have limited decontamination efficacy. We evaluated lactic acid bacteria (LAB) as a natural antimicrobial spray to control pathogens at the farm prior to harvest of lettuce and spinach. Escherichia coli O157:H7 and Listeria monocytogenes (growth chamber study) or their non-pathogenic surrogate bacteria (farm study) were spray-inoculated on fresh produce leaves and then treated with LAB spray. At 0. 3. and 5 days post-treatments, fresh produce leaves were harvested and analyzed for surviving populations of inoculated bacteria. The LAB treatment significantly killed L. monocytogenes and E. coli O157:H7 (growth chamber) and non-pathogenic E. coli O157:H12 (farm study) on lettuce leaves immediately after treatment. Similarly, significant reduction in L. innocua was observed on spinach leaves harvested after 5 days at the farm. The findings are useful to organic leafy green growers in using biocontrol LAB treatment to mitigate the food safety risks of pathogen contamination at the farm.

Technical Abstract: Recent outbreaks linked to the contaminated lettuce highlight the need for identifying effective natural approaches to improve produce safety at pre-harvest level. Efficacy of a mixture cocktail of lactic acid bacteria (LAB) including Lactococcus lactis, Lactobacillus plantarum, Lactobacillus johnsonii, and Lactobacillus acidophilus, as pre-harvest biocontrol against Listeria and Escherichia coli O157 on spinach and lettuce was investigated. Bacterial pathogens L. monocytogenes and E. coli O157:H7 and the non-pathogenic surrogates L. innocua and E. coli O157:H12 were used to spray-inoculate cultivars of lettuce and spinach grown in growth chamber and in field, respectively. Inoculated plants were spray-treated with water or a mixture cocktail of LAB. On day 0, 3, and 5 post-inoculation, four samples from each group were collected and bacterial populations were determined by serial dilution and spiral plating on selective agars. LAB treatment exhibited an immediate antimicrobial efficacy against L. monocytogenes and E. coli O157:H7 on “Green Star” lettuce by ~ 2 and ~ 1 log reductions under growth chamber conditions, respectively (P<0.05). The antimicrobial effect of LAB against E. coli O157:H7 on “New Red Fire” lettuce remained effective thorough out the 5-day period in growth chamber (P<0.05). Further, treatment of LAB delivered an effective bactericidal effect against E. coli O157:H12 immediately after application on the field-grown lettuce plants (P<0.05). Approximately 1 log reductions against L. innocua reduction was observed on “Matador” and “Palco” spinach on day 5 (P<0.05). Result of this study support that LAB could potentially be applied as biocontrol agents for controlling Listeria and E. coli O157 contamination on the leafy greens at the pre-harvest level.