In the wake of a 2025 multistate outbreak linked to prepared pasta meals that resulted in dozens of hospitalizations and multiple deaths, a complex question confronts food safety regulators, manufacturers, and consumers: what is the acceptable amount of a deadly pathogen in food? The investigation, which traced Listeria monocytogenes contamination to pre-cooked pasta from a single supplier, triggered a cascade of recalls affecting products from major national retailers including Trader Joe’s, Kroger, and Albertsons. As of late October 2025, this outbreak had sickened 27 people across 18 states, led to 25 hospitalizations, caused six confirmed deaths, and resulted in one pregnancy loss. This public health crisis has brought renewed scrutiny to a long-standing scientific and regulatory debate: the divergence between a “zero-tolerance” approach to Listeria adopted by the United States and alternative, risk-based approaches implemented by other developed nations.
The American Standard: An Unforgiving Policy
In the United States, federal food safety authorities maintain what is known as a “zero-tolerance” policy for Listeria monocytogenes in ready-to-eat (RTE) foods. This means that no detectable level of this pathogen is permitted in products intended for consumption without further cooking, such as deli meats, soft cheeses, and the prepared pasta meals implicated in the recent outbreak. This policy is enforced by both the Food and Drug Administration (FDA) and the U.S. Department of Agriculture Food Safety and Inspection Service (USDA-FSIS).
The scientific rationale for this stringent position hinges on the particular virulence of the Listeria bacterium. Unlike many foodborne pathogens, Listeria monocytogenes can thrive in cold temperatures, surviving and even multiplying in refrigerated environments between +2°C and 4°C. This characteristic makes it a persistent threat in both food processing plants and home refrigerators. Furthermore, the severe consequences of infection, known as listeriosis, drive the conservative approach. Although relatively rare, with about 1,250 estimated cases annually in the U.S., listeriosis has a high mortality rate, making it the third leading cause of death from foodborne illness with an estimated 172 deaths per year. The case-fatality rate is approximately 20%, and nearly 25% of pregnancy-associated cases result in fetal loss or newborn death.
The regulatory basis for the USDA’s zero-tolerance policy is codified in the Code of Federal Regulations, which states that an “RTE product is adulterated if it contains L. monocytogenes, or if it comes into direct contact with a food contact surface that is contaminated with L. monocytogenes”. For the FDA, the policy is outlined in its Compliance Policy Guides, which have evolved over several decades in response to scientific understanding and outbreak events. The policy applies with particular force to products that support the growth of Listeria, requiring manufacturers to control the pathogen through Hazard Analysis Critical Control Point (HACCP) plans and rigorous sanitation procedures.
A Different Calculus: Risk-Based Approaches Abroad
While the U.S. holds fast to its zero-tolerance stance, other major economies have adopted what they describe as more pragmatic, risk-based approaches that incorporate recent scientific evidence. These frameworks often establish a specific numerical limit, typically 100 colony-forming units per gram (CFU/g), for Listeria in certain ready-to-eat foods where growth is unlikely throughout the product’s shelf life.
The European Union has been a prominent adopter of this alternative framework. In a significant regulatory update, the EU recently passed Regulation (EU) 2024/2895, which amends its existing microbiological criteria for Listeria and is set to take full effect in July 2026. This updated regulation maintains the 100 CFU/g limit for RTE foods placed on the market but tightens the “not detected in 25g” criterion. This stricter standard now applies to all RTE foods that support Listeria growth, unless the manufacturer can convincingly demonstrate to competent authorities that the pathogen will not exceed 100 CFU/g at any point during the product’s entire shelf life. The EU’s policy is grounded in scientific recognition that ingestion levels below 100 CFU/g are generally not injurious to health for the general adult population, though foods able to support growth beyond this level require stringent risk mitigation measures.
Similarly, Canada updated its “Policy on Listeria monocytogenes in Ready-to-Eat Foods” in 2023, creating a nuanced categorization system based on a food’s potential to support Listeria growth. Canada’s Category 1 includes RTE foods which support the growth of Listeria, where detection may trigger a serious “Health Risk 1” concern. Category 2 is divided into two subgroups: RTE foods in which growth may be limited to levels not exceeding 100 CFU/g (2A) and those in which growth will not occur (2B). Like the EU, the Canadian policy considers both the potential for pathogen growth and the presence or levels of Listeria, while also factoring in the intended consumers, giving special consideration to foods destined for vulnerable populations.
The following table summarizes the key differences in regulatory approaches across these jurisdictions:
| Jurisdiction | Core Policy Principle | Key Regulatory Details | Effective Timeline |
| United States | Zero-Tolerance | No detectable Listeria monocytogenes allowed in RTE foods | Long-standing policy |
| European Union | Risk-based with Stricter Verification | “Not detected in 25g” unless manufacturer proves <100 CFU/g throughout shelf life | July 2026 |
| Canada | Categorization-Based | Health concern determined by food category (1, 2A, 2B) based on growth potential | 2023 Policy Update |
The Scientific Foundation for a Threshold
The regulatory divergence between zero-tolerance and risk-based approaches stems from different interpretations of scientific evidence regarding the dose required to cause human illness. Proponents of the risk-based approach point to research indicating that low levels of Listeria monocytogenes are frequently present in various food products and environments, yet the incidence of listeriosis remains relatively low compared to other foodborne illnesses. This suggests that exposure to low levels of the pathogen does not invariably lead to disease.
Quantitative microbial risk assessments conducted over the past two decades have attempted to model the relationship between the number of Listeria cells ingested and the probability of illness. These models have informed the international trend toward establishing a 100 CFU/g threshold for many RTE foods. Scientific opinion from organizations like the European Food Safety Authority (EFSA) indicates that more than 90% of invasive listeriosis cases are caused by the ingestion of RTE foods containing greater than 2,000 CFU Listeria monocytogenes/g, with only a minority of cases linked to lower concentrations. This research supports the premise that maintaining the pathogen below 100 CFU/g throughout a product’s shelf life provides an adequate margin of safety for most consumers.
The focus on a product’s ability to support Listeria growth is equally scientifically grounded. Listeria monocytogenes is notable for its ability to grow at refrigeration temperatures, with its growth limits influenced by multiple factors including temperature, water activity, pH, and the presence of organic acids like lactic acid. Predictive microbiology models have been developed to determine the probability of Listeria growth under specific combinations of these constraints, allowing manufacturers and regulators to categorize products based on their inherent risk. Foods formulated with growth inhibitors or intrinsic characteristics that prevent Listeria multiplication (such as low pH or water activity) are considered lower risk under the alternative frameworks.
The Pendulum Swing: A Policy in Flux
The current U.S. zero-tolerance policy represents the latest point in an evolving regulatory history, not a static position. The FDA previously considered moving toward a risk-based approach in the early 2000s. In 2008, the agency drafted a Compliance Policy Guide that proposed allowing Listeria monocytogenes at levels below 100 CFU/g in RTE foods that do not support its growth. This draft guidance represented a significant potential shift, acknowledging the scientific evidence that low levels might not pose a significant public health risk.
However, this movement toward a tolerance limit was short-lived. Several factors contributed to a reversal of this position. A joint workshop in 2011 between the Interagency Risk Assessment Consortium and the Joint Institute for Food Safety and Applied Nutrition focused on dose-response modeling issues for Listeria and highlighted the uncertainties in establishing a safe threshold for all consumer groups. Additionally, a pivotal risk assessment model from 2003, which had suggested a 100 CFU/g limit might be protective, was later re-evaluated. When the model was run with different, arguably more realistic, assumptions about the proportion of highly contaminated foods and the variability of consumer susceptibility, the estimated number of illnesses increased, casting doubt on the safety of the proposed limit.
By January 2017, the FDA had published a Revised Draft Guidance that eliminated the suggested 100 CFU/g regulatory limit, completing a full circle back to a zero-tolerance position for all RTE foods, regardless of their growth potential. This evolution demonstrates the complex interplay between emerging science, public health priorities, and regulatory pragmatism. As noted in one analysis of FDA’s policy history, the agency’s position has “gone full circle, from a ‘zero tolerance’ for all RTE foods, to a proposed regulatory limit in RTE foods in which Listeria monocytogenes cannot grow, and back to a zero tolerance in all RTE foods”.
Vulnerable Populations and the Precautionary Principle
At the heart of the zero-tolerance argument lies the profound vulnerability of specific population groups to Listeria infection. Unlike many foodborne pathogens that cause generally mild illness in healthy adults, Listeria monocytogenes disproportionately and severely affects certain segments of the population. The groups at highest risk include older adults, pregnant women and their newborns, and people with weakened immune systems (such as those undergoing cancer treatment, organ transplant recipients, or individuals with conditions like HIV/AIDS). For these individuals, listeriosis can lead to invasive and life-threatening conditions including sepsis, meningitis, and encephalitis. In pregnant women, infection can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn, even if the mother experiences only mild, flu-like symptoms.
This stark disparity in risk underpins the U.S. regulatory preference for the precautionary principle. The argument centers on the ethical and public health responsibility to protect the most vulnerable consumers, for whom even low levels of Listeria might pose a significant danger. Quantitative risk assessment models have confirmed that a majority of listeriosis cases occur in identifiable at-risk subpopulations. One study focusing on RTE delicatessen meats and hot dogs predicted that approximately 80% of listeriosis cases would occur in at-risk subpopulations comprising only 20% of the total population. This uneven distribution of risk strengthens the case for the most protective regulatory standard, as the consequences of failure are most severe for those least able to withstand infection.
The vulnerability argument is explicitly recognized in international regulations as well. The European Union’s framework maintains that “infants and immunocompromised individuals must not be exposed to foods containing Listeria monocytogenes at any concentration, as even trace levels pose significant health risks”. This creates a dual standard where general population foods may have a tolerance, but foods specifically intended for these vulnerable groups are held to a stricter, often zero-tolerance, criterion.
Practical Impacts: From Processing Plants to Grocery Shelves
The choice between regulatory philosophies has tangible consequences throughout the food supply chain, affecting manufacturing practices, testing protocols, product formulation, and market dynamics. The U.S. zero-tolerance policy creates a powerful incentive for RTE food producers to implement rigorous environmental monitoring and control programs within their processing facilities. Since the presence of Listeria monocytogenes at any level renders the product adulterated and subject to recall, manufacturers invest significant resources in sanitation, hygienic facility and equipment design, and frequent testing of both the production environment and finished products.
This approach aims to prevent contamination entirely, but it also carries economic and operational burdens. The constant threat of regulatory action and costly recalls for any detectable contamination creates a high-stakes environment for food businesses. A single positive test result, even at very low levels that may not cause illness, can trigger widespread product withdrawals, damaging brand reputation and creating financial strain. The recent pasta outbreak illustrates this dynamic, where contamination at a single supplier, Nate’s Fine Foods, necessitated recalls across multiple national brands and retailers, demonstrating how a localized contamination event can ripple through the complex modern food supply chain.
In contrast, risk-based approaches allow manufacturers of certain shelf-stable RTE foods or products with formulations that inhibit Listeria growth to focus their resources differently. Under these frameworks, manufacturers can invest in challenge studies to scientifically demonstrate that their products will not support Listeria growth beyond 100 CFU/g during the entire shelf life under reasonable storage conditions. This pathway provides regulatory flexibility but requires significant scientific expertise and investment in predictive microbiology. For food business operators who succeed in demonstrating this control, the “not detected in 25g” criterion no longer applies before the product leaves their control, shifting the regulatory focus to ensuring the 100 CFU/g limit is not exceeded during the product’s shelf life.
Analysis & Next Steps
The transatlantic divergence in Listeria policy represents more than a technical regulatory discrepancy; it embodies a fundamental philosophical disagreement about how to manage an unavoidable public health risk in a complex food system. What is new in this ongoing debate is the recent regulatory tightening in the European Union, which took effect in December 2024 and will be implemented in July 2026. This development signals that even risk-based approaches are evolving toward greater stringency, particularly in verification requirements, narrowing the practical gap between the two regulatory philosophies. Simultaneously, the June 2025 prepared pasta outbreak provides a sobering, real-time case study of the consequences of Listeria contamination in a globally interconnected food network, reminding regulators and industry of the persistent threat the pathogen poses.
This debate matters because the regulatory framework directly influences public health outcomes, international trade, and industry innovation. Consumers, particularly those in vulnerable groups, are directly affected by the level of protection these standards provide. Food businesses operating in multiple international markets face significant compliance challenges, as they must navigate conflicting regulatory requirements, potentially reformulating products or maintaining separate production protocols for different export destinations. The tension between these approaches will likely intensify as new food categories, particularly plant-based ready-to-eat products, emerge and present novel challenges for Listeria control.
Moving forward, the path is not necessarily toward global harmonization but toward clearer, more scientifically defensible regulations within each framework. For consumers, especially those in at-risk groups, the necessary response remains vigilance: carefully following storage instructions, heeding recall notices, and understanding that ready-to-eat foods carrying a zero-tolerance standard are not inherently safer than those governed by a 100 CFU/g limit. The crucial factor is that the food safety system, whether based on prohibition or managed tolerance, is effectively designed, implemented, and verified to deliver a consistent level of public health protection from processing plant to dinner plate. The goal shared by all competing regulatory philosophies is the same: to reduce the incidence of a severe and preventable foodborne illness that continues to claim lives despite decades of scientific advancement and regulatory attention.
