Wanted to make sure you saw this!
On Sat, Apr 11, 2026 at 12:46 PM Kit Redwine <[email protected]> wrote:
Bagged salads have become a dietary staple for convenience-seeking consumers. Yet this modern food product carries a microbial risk that public health agencies have struggled to manage over the past decade. Between 2015 and 2024, health authorities in the United States identified eight distinct listeriosis outbreaks connected to packaged salad products, leading the U.S. Food and Drug Administration (FDA) to initiate recalls for roughly 240 such items due to possible Listeria monocytogenes contamination (The Centers for Disease Control and Prevention [CDC], Emerging Infectious Diseases, December 2025).
Unlike many foodborne bacteria that cause rapid gastrointestinal distress, Listeria operates on a longer timeline, symptoms may emerge weeks after exposure, and it kills a much higher proportion of infected individuals than more familiar pathogens like Salmonella. Listeriosis has a hospitalization rate of 94% and a mortality rate of about 16%, making it the third leading cause of death from foodborne illness in the United States (U.S. Food & Drug Administration, January 2020). For pregnant women, older adults, and those with compromised immunity, eating a contaminated salad can result in hospitalization, loss of pregnancy, or death.
The persistence of Listeria in fresh produce is not merely a story of isolated facility failures. It arises from the bacterium’s intrinsic biological traits, the structural realities of centralized produce processing, and the inherent limits of current cleaning technologies. Understanding how Listeria reaches leafy greens, where it hides, and why it resists removal is fundamental to reducing the toll of this dangerous infection.
Why Listeria Poses a Unique Threat to Raw Produce
Listeria monocytogenes possesses biological characteristics that make it unusually suited to contaminate fresh fruits and vegetables. The most notable is its ability to multiply at refrigeration temperatures. While growth slows at or below 40 degrees Fahrenheit, the bacterium does not cease reproduction entirely. For leafy greens transported or stored above 41 degrees Fahrenheit, a common occurrence in complex supply chains, conditions actively support bacterial population increases.
The pathogen is also widely distributed in natural settings. Scientists consider Listeria a soil-associated organism, commonly found in dirt, surface water, and decomposing plant matter. This environmental prevalence means contamination can originate at the very start of the growing cycle, before any harvesting or processing occurs. Compounding the issue, most Listeria isolates collected from produce operations carry genetic markers linked to hypervirulence, meaning they possess the full capacity to cause severe human disease (Medical Xpress, December 2018).
Perhaps most critically, fresh produce lacks any lethal processing step. Unlike meat, which is cooked, or milk, which is pasteurized, leafy greens are consumed without any heating. Once contamination reaches the final product, no washing method can reliably eliminate the pathogen. No rinsing procedure can remove all harmful germs from leafy greens. This fundamental vulnerability underlies every documented outbreak linked to fresh produce.
How Listeria Enters the Produce Supply Chain
The pathogen can infiltrate the produce system at several distinct stages, each requiring different prevention strategies.
Contamination during growth represents the earliest possible entry point. Irrigation water serves as a primary vehicle, particularly when water sources become contaminated with animal feces. A well-documented E. coli outbreak in 2018 involving romaine lettuce, while caused by a different pathogen, illustrates the principle: contaminated agricultural water spread bacteria across an entire growing region, sickening 210 people across 36 states and killing five (Centers for Disease Control and Prevention, June 2018). The same mechanism applies to Listeria, which can be transported from animal operations to crop fields via runoff or flooding.
Wildlife activity also introduces pathogens. Birds, deer, and other animals moving through produce fields deposit feces that may contain Listeria. Once the bacterium contacts leaf surfaces, it can attach firmly and, in some cases, enter the plant tissue through natural openings or damaged areas, making surface washing ineffective.
Processing facility contamination operates through a different but equally significant mechanism. Bagged salads undergo centralized washing, cutting, mixing, and packaging. When a single contaminated batch enters such a facility, the equipment can spread the pathogen across much larger volumes than the original contaminated supply. This consolidation means a single contamination event can affect consumers nationwide.
A two-year investigation examined Listeria contamination patterns in produce processing facilities, including lines handling cut iceberg lettuce and salad bowls (FoodSafetyTech, April 2024). Using whole genome sequencing to track bacterial isolates, the research confirmed that processing environments can serve as reservoirs where Listeria persists despite routine cleaning and sanitizing.
Long-term residence in facilities may be the most troubling contamination pathway. Once Listeria enters a produce facility, it can establish itself in drains, floor cracks, and equipment crevices, surviving for months or years. A 2022 study performed whole-genome sequencing on 169 Listeria isolates collected from 13 produce packinghouses and three fresh-cut facilities. The researchers found highly related isolates reappearing at least two months apart in seven of the 16 operations, with two facilities showing reisolation of strains collected at least one year apart (Applied and Environmental Microbiology, October 2022).
Critically, the study identified isolates collected during preproduction periods, after sanitation but before production began, that were highly related to isolates collected during active production. This finding provided direct evidence that certain Listeria strains survive standard sanitation protocols. The research concluded that Listeria presence in produce operations results from both repeated reintroduction and genuine long-term persistence of strains within facilities.
Cross-contamination between facilities adds another layer of complexity. The same study identified closely related isolates in different operations, suggesting that contamination can spread between facilities or that common suppliers introduce the same strains to multiple sites. For produce companies sourcing from several facilities or sharing equipment across locations, this interconnectedness creates additional avenues for pathogen transmission.
Lessons from Major Outbreaks
The history of Listeria outbreaks connected to fresh produce reveals a troubling pattern of long-term persistence and repeated contamination from identifiable sources.
The 2025 study published in the CDC’s Emerging Infectious Diseases journal examined two genetically unrelated listeriosis outbreaks linked to packaged salads. Combined, these events caused 30 illnesses, 27 hospitalizations, and four deaths across the United States and Canada over an eight-year span. The first outbreak (designated Outbreak A) ran from 2014 to 2022, involving 20 U.S. cases and two Canadian cases. The second (Outbreak B) occurred from 2016 to 2021, with ten U.S. cases.
Outbreak A was investigated three separate times before its source was finally identified. Initial clusters detected in 2019 and 2020 lacked sufficient evidence to pinpoint a food vehicle. In October 2021, routine sampling by the Georgia Department of Agriculture detected Listeria in a brand’s packaged garden salad produced at a North Carolina facility, triggering a voluntary recall. Subsequent sampling by Michigan public health authorities found a highly related strain in iceberg lettuce produced by the same firm’s Arizona facility.
The firm conducted a root-cause analysis that ultimately identified the outbreak strain on a harvest rig used across multiple facilities. This discovery demonstrated that contamination can persist outside of processing environments, on equipment used in the field. The firm permanently removed the rig from service, enhanced its sanitation protocols, and implemented whole-genome sequencing testing as part of its routine pathogen surveillance.
Outbreak B involved a cluster of ten cases linked to water and sediment samples from the Salinas Valley of California. Investigators reviewed genetic data and found that clinical isolates matched strains previously detected in Salinas Valley watersheds as part of a USDA study. This finding underscored the value of widespread microbiologic sampling of natural environments and highlighted the role that watersheds can play as long-term reservoirs of Listeria contamination, as detailed in the [Emerging Infectious Diseases report](https://wwwnc.cdc.gov/eid/article/31/1/24-1234_article).
A more recent outbreak in February 2023 involved 19 people infected with the outbreak strain across 16 states, with illness onset dates ranging from July 2018 to March 2023 (Centers for Disease Control and Prevention, August 2024). Epidemiologic data showed that leafy greens were a likely source, with 93% of interviewed patients eating leafy greens, 93% eating iceberg lettuce, and 71% eating romaine lettuce. A case-case analysis showed that people in this outbreak were eight times more likely to eat iceberg lettuce and five times more likely to eat romaine lettuce than people sick with Listeria who were not part of the outbreak. Eighteen people were hospitalized, though no deaths were reported.
Obstacles to Effective Prevention
Several factors explain why Listeria continues to contaminate fresh produce despite industry efforts and regulatory oversight.
Biofilm formation poses a major challenge. Mature Listeria biofilms exhibit reduced susceptibility to sanitizers commonly used in the fresh food supply chain. Once the pathogen establishes a biofilm on equipment surfaces or drains, standard cleaning protocols may fail to eliminate it, allowing the bacterium to persist and reseed contamination over months or years.
The viable but non-culturable (VBNC) state further complicates detection. Some water disinfection treatments, such as chlorine and chlorine dioxide used in the fresh-cut industry, can induce Listeria cells to enter an intermediate state where they remain alive but cannot be cultured using standard laboratory methods. These VBNC cells can later revive during shelf-life, becoming infectious again.
Supply chain complexity creates additional challenges. Packaged salads involve multiple touch points, harvesting, washing, cutting, mixing, packaging, distribution, each offering opportunities for contamination to enter or spread. The same facilities often process products for multiple brands, meaning contamination at one facility can affect products sold under dozens of brand names.
Investigation difficulties mean outbreaks are often detected late or not at all. Listeria outbreaks can be difficult to investigate due to relatively low case numbers, long incubation periods that make food recall difficult, and older patients who may have poor food recall or become too sick to provide detailed food history. Environmental sampling, rather than patient epidemiology, often plays a major role in identifying the food vehicle.
Practical Guidance for Consumers
For individual consumers, the limitations of industry prevention mean that personal food safety practices remain essential. First, consumers should understand that no washing method can remove all germs from leafy greens. The safest produce is cooked; the next safest is washed. When eating raw leafy greens, wash hands, utensils, and surfaces before and after preparation. Clean leafy greens before eating or cutting them, discarding outer leaves on whole heads and any torn or bruised leaves. Rinse under running water and use hands to gently rub the surface of leaves, then dry with a clean cloth or paper towel.
For packaged greens labeled “ready to eat,” “triple washed,” or “no washing necessary,” additional washing is not recommended and may introduce cross-contamination from kitchen surfaces. Keep leafy greens separate from raw meat, poultry, and seafood. Refrigerate leafy greens within two hours, or within one hour if exposed to temperatures above 90 degrees Fahrenheit.
For individuals at higher risk of severe Listeria infection, including pregnant women, older adults, and immunocompromised individuals should take additional precautions. These include avoiding raw sprouts of any kind, which are particularly high-risk, and considering cooking leafy greens before consumption. For pregnant women, Listeria infection can cross the placenta and cause miscarriage, stillbirth, or severe illness in newborns.
Analysis and Next Steps
Several developments in the understanding of Listeria contamination in fresh produce have reshaped the food safety landscape in recent years. What is new includes the definitive documentation, through whole-genome sequencing, that Listeria strains can persist in produce facilities for a year or more, surviving standard sanitation practices . Also new is the recognition that contamination can originate outside processing environments entirely, as demonstrated by the harvest rig identified in Outbreak A, and that natural environments such as the Salinas Valley watersheds can serve as long-term reservoirs for outbreak strains.
This matters because fresh produce is consumed raw by millions of Americans daily, and for vulnerable populations the consequences of Listeria infection are disproportionately severe. In the 2023 outbreak, sick individuals ranged in age from less than one to 96 years, with a median age of 72, and 63% were female.. Pregnant women, older adults, and immunocompromised individuals cannot simply avoid fresh produce, leafy greens are an important part of a healthy diet, but they may not be aware of the specific risks they face.
The populations affected by these dynamics extend beyond those who become severely ill. Families lose loved ones. Pregnancies end in miscarriage or stillbirth. Survivors of severe listeriosis may face long-term health complications. The economic costs of outbreak investigations, product recalls, and healthcare utilization are borne by the broader public.
What to do now requires coordinated action across multiple levels. For consumers, particularly those in vulnerable groups, understanding that “pre-washed” does not mean “sterile” is essential. Following guidance for handling leafy greens and paying attention to recall notices can reduce individual risk. For pregnant women, avoiding raw sprouts and considering cooking leafy greens are prudent precautions.
For the produce industry, the research provides clear evidence that environmental monitoring programs must extend beyond processing facilities to include harvest equipment, irrigation water sources, and adjacent land uses. Whole-genome sequencing should be integrated into routine pathogen surveillance to enable rapid identification of persistent strains and effective root-cause analysis. Facilities that identify persistent Listeria strains must consider both persistence and reintroduction as possible root causes, as standard sanitation alone may not eliminate established biofilms.
For regulators, the findings support continued investment in routine surveillance sampling at the state and local level, as such sampling was critical to identifying the source of Outbreak A. Washing does not reliably eliminate pathogens and should be supported with continued research into alternative interventions.
For researchers, important questions remain about the efficacy of different sanitizers against mature Listeria biofilms, the conditions that induce and reverse the VBNC state, and the development of rapid detection methods that can identify viable Listeria regardless of cultural state. Investigations into alternative processing methods, such as high-pressure processing or antimicrobial rinses that penetrate leaf surfaces, could inform future industry standards.
The challenge of Listeria in fresh produce will not be solved by any single intervention. It requires a sustained, multi-layered approach that addresses contamination at every stage of the supply chain, from the irrigation water in the field to the harvest rig in the processing facility to the consumer’s kitchen counter. The pathogen’s ability to persist, to survive sanitation, and to grow at refrigeration temperatures means that vigilance cannot be episodic. It must be continuous.
