The recurring contamination of leafy greens with pathogenic Escherichia coli bacteria is a critical public health issue that has impacted consumers and the food industry alike. The high susceptibility of leafy greens to E. coli contamination is exacerbated by various factors, from field exposure to processing practices. This article delves into the technical aspects of E. coli contamination in leafy greens processing facilities, including environmental control measures, sanitation practices, and strategies to mitigate bacterial load.

Understanding E. coli Contamination of Leafy Greens

E. coli outbreaks associated with leafy greens have been well-documented, with Shiga toxin-producing E. coli (STEC), particularly O157, being a prevalent strain. The contamination can originate from various sources, including water used for irrigation, proximity to livestock, soil, and even the workers handling the produce. Once introduced, E. coli can persist through multiple stages of processing due to the unique morphology of leafy greens, such as rough surfaces and folds, which provide bacterial attachment points and protection from disinfectants.¹

A recent study by Gurtler et al.² emphasized the role of contaminated irrigation water as a significant contributor to leafy greens contamination. The researchers' findings indicated that fields exposed to contaminated water sources had a higher E. coli prevalence on produce compared to those with uncontaminated sources. Hence, proactive measures to control contamination in the field and throughout the processing environment are essential.

Key Sanitation Practices in Leafy Greens Facilities

Sanitation plays a fundamental role in reducing E. coli contamination risk in processing environments. Proper cleaning and sanitization, especially on food contact surfaces, are critical for minimizing cross-contamination. The U.S. Food and Drug Administration's (FDA's) Food Safety Modernization Act (FSMA) underscores sanitation controls as an essential preventive measure.

Key sanitation practices to prevent E. coli contamination of leafy greens include:

• Surface sanitization: Biofilms pose a significant challenge in leafy greens facilities. E. coli, along with other pathogens, can form biofilms on equipment surfaces, making them more resistant to standard cleaning protocols. The use of rotating sanitizers and biocides has shown efficacy in disrupting biofilms³ and killing planktonic cells within these structures. Common sanitizers include chlorine-based compounds, quaternary ammonium compounds, and peracetic acid. Chlorine, at concentrations of 50–200 ppm, has been widely used in produce facilities and has demonstrated a significant reduction in E. coli populations; however, continuous exposure may allow for adaptive resistance, necessitating rotational protocols.⁴
• Hygiene training and monitoring: Worker hygiene is a key factor in contamination control within processing facilities. Studies such as Choi et al.⁵ have shown that handling leafy greens with inadequate personal hygiene can facilitate the transfer of pathogens, even from sanitized surfaces. Therefore, ensuring strict adherence to handwashing protocols, the use of gloves, and the use of appropriate personal protective equipment is essential. Additionally, regular microbiological monitoring and environmental sampling are necessary to verify the efficacy of sanitation practices. Real-time PCR and ATP bioluminescence are common methods used in pathogen detection on surfaces and equipment.
• High-pressure processing (HPP) and ultraviolet (UV) treatment: HPP and UV treatment⁶ have emerged as nonthermal alternatives that effectively reduce microbial load without compromising the sensory qualities of leafy greens. HPP operates by applying pressures up to 600 MPa, which disrupts microbial cell membranes and reduces E. coli populations by up to 5 log units. UV-C light at wavelengths of 200–280 nm has also been shown to damage bacterial DNA and is particularly effective on flat surfaces. However, due to the irregular morphology of leafy greens, HPP may provide more consistent inactivation than UV treatment.

Reducing E. coli Exposure in Leafy Greens Facilities 

Minimizing E. coli contamination requires an integrated approach, combining facility design, worker training, and water management. The following strategies are pivotal in reducing E. coli exposure in leafy greens processing facilities:

1. Controlled water systems and filtration: Ensuring that water used in washing and cooling is free from contaminants is crucial in controlling E. coli exposure. The implementation of reverse osmosis or UV water filtration systems can effectively reduce bacterial counts. Enhanced by chlorine dioxide or ozonated water systems, these methods have shown significant reductions in contamination levels.^7,8 Additionally, recirculated water should be closely monitored for microbiological load and changed frequently to prevent microbial accumulation.
2. Cross-contamination control: The facility layout can influence the risk of cross-contamination. Proper segregation of raw and processed products, along with controlled traffic flow, is essential. A "dirty-to-clean" workflow helps reduce contamination from high-risk areas. Well-defined traffic patterns and limited access to processing areas are effective in reducing the introduction of pathogens. Airflow within the facility is also a critical consideration; implementing HEPA filters can reduce airborne bacteria, especially in high-risk zones.
3. Advanced microbial interventions: Incorporating microbial interventions, such as organic acids⁹ or bacteriophages, has gained traction. Lactic acid and peracetic acid, for example, provide an antimicrobial effect and are less likely to lead to resistance compared to traditional sanitizers. Bacteriophages specific to E. coli O157 have shown promise in reducing bacterial counts on leafy greens in processing environments, providing an additional safety measure.¹⁰

Takeaway

The challenge of mitigating E. coli in leafy greens calls for a rigorous, multifaceted approach that includes stringent sanitation practices, facility design optimization, and the use of advanced microbial control methods. By integrating high-standard sanitation practices with emerging technologies such as HPP, UV treatment, and bacteriophages, leafy greens facilities can effectively reduce contamination risk. Continued research and collaboration among food safety professionals, regulators, and industry stakeholders are crucial to refining these approaches and ensuring the safety of leafy greens in the marketplace.

References

1. Beuchat, L.R. "Pathogenic Microorganisms Associated with Fresh Produce." Journal of Food Protection 59, no. 2 (February 1996): 204–216. https://www.sciencedirect.com/science/article/pii/S0362028X2205270X?via%3Dihub.
2. Gurtler, J.B. and K.E. Gibson. "Irrigation water and contamination of fresh produce with bacterial foodborne pathogens." Current Opinion in Food Science 47 (October 2022): 100889. https://www.sciencedirect.com/science/article/abs/pii/S2214799322000911?via%3Dihub.
3. Carmichael, I., I.S. Harper, M.J. Coventry, P.W.J. Taylor, J. Wan, and M.W. Hickey. "Bacterial Colonization and Biofilm Development on Minimally Processed Vegetables." Journal of Applied Microbiology 85, no. S1 (December 1998): 45S–51S. https://enviromicro-journals.onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2672.1998.tb05282.x.
4. Davidson P.M. and M. Harrison M. "Resistance and Adaptation to Food Antimicrobials, Sanitizers, and Other Process Controls." Food Technology 56, no. 11 (November 2022). https://www.ift.org/news-and-publications/food-technology-magazine/issues/2002/november/features/resistance-and-adaptation-to-food-antimicrobials-sanitizers-and-other-process-controls.
5. Choi, J., H. Norwood, S. Seo, S.A. Sirsat, and J. Neal. "Evaluation of Food Safety Related Behaviors of Retail and Food Service Employees While Handling Fresh and Fresh-Cut Leafy Greens." Food Control 67 (September 2016):  199–208. https://www.sciencedirect.com/science/article/abs/pii/S0956713516300901.
6. Singh, H., S.K. Bhardwaj, M. Khatri, K.H. Kim, and N. Bhardwaj. "UVC Radiation for Food Safety: An Emerging Technology for the Microbial Disinfection of Food Products." Chemical Engineering Journal 417 (August 2021): 128084. https://www.sciencedirect.com/science/article/abs/pii/S1385894720342005.
7. Allende, A. and J. Monaghan. "Irrigation Water Quality for Leafy Crops: A Perspective of Risks and Potential Solutions." International Journal of Environmental Research and Public Health 12, no. 7 (July 2015): 7457–7477. https://www.mdpi.com/1660-4601/12/7/7457.
8. Gombas, D., Y. Luo, J. Brennan, et al. "Guidelines To Validate Control of Cross-Contamination During Washing of Fresh-Cut Leafy Vegetables." Journal of Food Protection 80, no. 2 (February 2017): 312–330. https://www.sciencedirect.com/science/article/pii/S0362028X22096879?via%3Dihub.
9. Huang, Y. and H. Chen H. "Effect of Organic Acids, Hydrogen Peroxide and Mild Heat on Inactivation of Escherichia coli O157:H7 on Baby Spinach." Food Control 22, no. 8 (August 2011): 1178–1183. https://www.sciencedirect.com/science/article/abs/pii/S0956713511000302.
10. ‌Ferguson, S., C. Roberts, E. Handy, and M. Sharma. "Lytic Bacteriophages reduce Escherichia coli O157:H7 on Fresh Cut Lettuce Introduced Through Cross-Contamination." Bacteriophage 3, no. 1 (2013). https://www.tandfonline.com/doi/full/10.4161/bact.24323.