Why is Listeria monocytogenes uniquely difficult to control in aged cheese?

Listeria monocytogenes is difficult to control in aged cheese because several of its biological properties overlap directly with the conditions that cheese aging creates. It grows at temperatures as low as -0.4°C, meaning refrigeration-temperature aging caves do not inhibit it. It tolerates sodium chloride concentrations up to approximately 10%, far above the salt levels in most aged cheeses. It forms biofilms on food contact surfaces that are approximately 100 times more resistant to common sanitizers than free-floating cells. And in soft-ripened cheese styles, the rising surface pH during ripening creates near-optimal conditions for Listeria growth. No single one of these properties defines its persistence in aged cheese. The combination of all four does.

What temperature does Listeria monocytogenes grow at?

Listeria monocytogenes grows across a temperature range of -0.4°C to approximately 45°C, with an optimum around 30 to 37°C. This minimum growth temperature of -0.4°C (31.2°F) distinguishes it from virtually every other significant foodborne pathogen, most of which cannot grow at refrigeration temperatures. Hard cheeses typically age at 10 to 15°C — well within Listeria's growth range, though growth at that temperature is slow. Soft and surface-ripened cheeses often age under similar conditions with higher humidity, providing an environment the organism can colonize over weeks to months.

How does Listeria monocytogenes compare to E. coli O157:H7 in aged cheese?

The two organisms present fundamentally different risk profiles in aged cheese. E. coli O157:H7 survives through an acid tolerance response (ATR) triggered by the initial pH drop of cheesemaking, persisting in hard aged cheeses beyond 270 days. Its risk is primarily driven by whether the organism was present in the raw milk. Listeria monocytogenes survives through cold tolerance, salt resistance, low minimum water activity, and biofilm formation — and its risk is often primarily environmental rather than a function of milk quality. Listeria colonizes aging rooms and equipment and can re-contaminate cheese during aging even when source milk tests negative. In hard aged cheeses, Listeria's risk is lower than in soft-ripened styles because the combined physical conditions are less favorable to growth. E. coli O157:H7's risk is consistent across hard cheese styles regardless of moisture level.

What is the case fatality rate of listeriosis?

The case fatality rate for invasive listeriosis is approximately 20 to 30%, among the highest of any common foodborne pathogen. The CDC estimates approximately 1,600 listeriosis cases in the United States annually, resulting in roughly 260 deaths. Listeriosis disproportionately affects pregnant women, newborns, adults over 65, and immunocompromised individuals. In pregnant women, infection can cause miscarriage, stillbirth, or severe illness in the newborn. The long incubation period of up to 70 days complicates both individual case identification and outbreak investigation, often delaying the association of illness with a contaminated food source.

A worker trimming mold from aging cheese at the Tillamook cheese plant in Tillamook County, Oregon, October 1941, photographed by Russell Lee for the U.S. Farm Security Administration.

Why Listeria Monocytogenes Is Uniquely Difficult to Control in Aged Cheese

Q: Does standard cheese salting eliminate Listeria monocytogenes?

No. Listeria monocytogenes tolerates sodium chloride concentrations up to approximately 10%, which substantially exceeds the salt concentrations present in most aged hard cheeses. Cheddar and Gouda styles typically contain 1.5 to 2.5% salt. The minimum water activity for Listeria growth is approximately 0.92, lower than the minimum for Campylobacter (approximately 0.99) and within the range of many semi-hard cheeses. This means moisture reduction through aging does not reliably render hard cheeses inhospitable to Listeria the way it does for organisms with higher moisture requirements. Salt and desiccation are more effective against other dairy pathogens than against Listeria monocytogenes.

Q: How do Listeria biofilms persist in cheese aging environments?

Listeria monocytogenes forms biofilms on food contact surfaces — structured bacterial communities enclosed in a self-produced matrix that adheres to surfaces and increases resistance to sanitizers, temperature stress, and desiccation. Biofilm-associated Listeria cells are approximately 100 times more resistant to common sanitizers than free-floating cells. In cheese aging facilities, aging caves, wooden boards, floor drains, cracks in concrete, and food contact surfaces can become colonized with Listeria biofilms that persist through routine sanitation cycles. Research on persistent Listeria strains has documented the same genotype recovered from a single food processing facility across multiple consecutive years of environmental sampling. Once established, Listeria can re-contaminate cheese surfaces continuously during aging regardless of the microbial status of the original milk.

Q: Does the 60-day aging rule address Listeria monocytogenes?

No. The 60-day aging requirement codified in 21 CFR Part 133 was designed for Mycobacterium bovis and Brucella abortus, not for Listeria monocytogenes. The organism was first described in 1926 and was not recognized as a significant foodborne pathogen in the United States until the 1980s — decades after the rule was codified. In hard aged cheeses, extended aging suppresses rather than eliminates Listeria, and the overall risk is lower than in soft-ripened styles. In soft and surface-ripened styles, the 2015 FDA/Health Canada risk assessment found that aging conditions can actively support Listeria growth on the rind. The properties that drive Listeria risk in aged cheese are features of the aging environment as much as they are features of the milk, making facility sanitation, environmental monitoring, and surface hygiene the primary control points for this organism.

Q: How long is the incubation period for listeriosis?

The incubation period for invasive listeriosis typically ranges from one day to 70 days, with symptoms most commonly appearing one to four weeks after exposure. The median incubation period of approximately three weeks is one of the longest of any common foodborne pathogen. This extended window makes cheese-linked outbreak investigations particularly difficult, as patients are rarely able to accurately recall food consumed three to four weeks before symptom onset. The CDC estimates approximately 1,600 listeriosis cases in the United States annually, resulting in roughly 260 deaths. The case fatality rate of approximately 20 to 30% is among the highest of any common foodborne pathogen.

June 25, 2026 · 8 min read

The dairy pathogens covered in this cluster each present a distinct challenge. Campylobacter jejuni is eliminated by the basic conditions of cheesemaking before aging even begins. E. coli O157:H7 survives because it has a molecular mechanism that turns the acidification process against itself. Listeria monocytogenes is different from both. It does not merely tolerate the conditions of aged cheese production. In several respects, those conditions suit it.

L. monocytogenes grows at refrigeration temperatures, tolerates salt concentrations that exceed those of most aged cheeses, forms biofilms that resist sanitizers, and thrives on the rising surface pH of soft-ripened styles. These properties do not represent a single adaptive trick like the acid tolerance response of O157:H7. They represent a biological profile that overlaps extensively with the environment cheese aging creates.

Why Listeria monocytogenes Grows at Refrigeration Temperatures Other Pathogens Cannot Tolerate

The most consequential property of L. monocytogenes in the context of aged cheese is its cold tolerance. The organism grows across a temperature range of -0.4°C to approximately 45°C, with an optimum around 30 to 37°C. Unlike virtually every other significant foodborne pathogen, it can multiply just below the freezing point of water.

This has specific implications for aging. Hard cheeses typically age at 10 to 15°C. Soft and surface-ripened cheeses often age at higher humidity and similar temperatures. Neither range prevents Listeria growth. Aging conditions that inhibit or kill Campylobacter outright, and that contribute to the eventual decline of E. coli O157:H7 over months, represent a temperature range within which L. monocytogenes can grow, albeit more slowly than at optimum.

The CDC estimates approximately 1,600 listeriosis cases in the United States annually, resulting in roughly 260 deaths. The case fatality rate of approximately 20 to 30% is among the highest of any common foodborne pathogen. Symptoms of invasive listeriosis typically appear one to four weeks after exposure, though the incubation period can extend as long as 70 days, one of the longest windows of any common foodborne pathogen. The extended incubation makes cheese-linked outbreak investigations particularly difficult, as patients are rarely able to accurately recall food consumed three to four weeks before symptom onset. The combination of cold tolerance and high lethality makes L. monocytogenes the organism of primary concern in the extended cold-chain environments that define commercial cheese aging.

Why Standard Cheese Salting Does Not Eliminate Listeria monocytogenes

Listeria monocytogenes tolerates sodium chloride concentrations up to approximately 10%, substantially higher than the concentrations present in most aged hard cheeses. Cheddar and Gouda styles typically contain 1.5 to 2.5% salt. Surface-ripened and washed-rind cheeses may carry somewhat higher concentrations, but rarely approach the organism’s tolerance ceiling.

The minimum water activity for L. monocytogenes growth is approximately 0.92, which is lower than the minimum for Campylobacter (approximately 0.99) and roughly equivalent to the water activity of many semi-hard cheeses. This means that moisture reduction through aging does not reliably render hard cheeses inhospitable to Listeria in the way it does for more moisture-sensitive organisms.

The practical consequence is that the two primary physical hurdles of cheesemaking, salt and desiccation, are more effective against other dairy pathogens than against L. monocytogenes. The organism is not impervious to salt, but the concentrations at which most cheese is made fall well within its growth range.

Biofilm Formation and Environmental Persistence

The third distinguishing property of Listeria monocytogenes in the cheese environment is its capacity to form biofilms on food contact surfaces. A biofilm is a structured community of bacteria enclosed in a self-produced matrix of polysaccharides, proteins, and DNA that adheres to surfaces and substantially increases resistance to sanitizers, temperature stress, and desiccation. Biofilm-associated Listeria cells have been documented to be approximately 100 times more resistant to common sanitizers than their planktonic (free-floating) counterparts.

In a cheese aging environment, this property has direct operational significance. Aging caves, wooden boards, floor drains, cracks in concrete, and food contact surfaces can become colonized with L. monocytogenes biofilms that persist through routine sanitation cycles. Once established in an aging facility, the organism can re-contaminate cheese surfaces continuously during the aging process, regardless of the microbial status of the original milk.

This mechanism distinguishes the Listeria risk profile from that of E. coli O157:H7 in an important way. O157:H7 in raw milk cheese is primarily a function of whether the organism was present in the raw milk and how it behaves during aging. Listeria contamination in aged cheese is often an environmental problem, not merely a milk quality problem. Positive tests for Listeria in finished aged cheese have been traced to contaminated processing equipment and aging environments even when the source milk tested negative. Research on persistent Listeria strains has documented the same genotype recovered from a single food processing facility across multiple consecutive years of environmental sampling. The organism’s persistence in the aging environment is not a transient contamination event but a colonization problem that sanitation alone may be insufficient to resolve without structural intervention.

Why Soft-Ripened Cheese Creates Near-Optimal Conditions for Listeria Growth

The Listeria risk in aged cheese is not uniform across styles. Hard aged cheeses with low moisture, significant salt content, and stable low pH present a more hostile environment for L. monocytogenes growth than soft and surface-ripened styles. The mechanism lies in what happens to surface pH during ripening.

In soft-ripened cheeses such as Brie, Camembert, and surface-washed styles, the initial lactic acid fermentation drops the surface pH to approximately 4.5 to 5.0. This is within the range of Listeria‘s minimum growth pH of approximately 4.4, and initially suppresses growth. However, as ripening progresses, surface molds and bacteria consume lactic acid and produce alkaline compounds. The result is a surface pH that rises from roughly 4.5 back toward 6.0 to 7.5 over the course of days to weeks.

This pH reversal is not incidental to the style. It is what creates the characteristic flavor and texture of surface-ripened cheese. But it also transforms the cheese rind from a moderately hostile environment for Listeria into a near-optimal one. The organism grows well at neutral pH, tolerates the moisture levels of soft-ripened styles, and has no thermophilic requirement that would require warmer temperatures than aging caves provide.

The 2015 Joint FDA/Health Canada Quantitative Assessment of the Risk of Listeriosis from Soft-Ripened Cheese Consumption concluded that aging does not reliably mitigate Listeria contamination in raw milk soft-ripened cheese. The assessment found that L. monocytogenes populations in contaminated soft-ripened cheese can increase by multiple log units during ripening. The organism multiplies rather than declines during the aging period the 60-day standard is intended to address, and no reliable safety margin exists against it in these styles.

How Listeria Compares to Other Dairy Pathogens

The properties that make Listeria difficult to control in aged cheese are qualitatively different from those of the other pathogens in this cluster.

Property	L. monocytogenes	E. coli O157:H7	Campylobacter jejuni
Minimum growth temperature	-0.4°C	10°C (no ATR benefit on growth range)	30°C
Salt tolerance (max NaCl)	~10%	Moderate	Very low
Minimum growth pH	4.4	4.0 (ATR-adapted)	4.9
Minimum water activity	0.92	0.95	0.99
Biofilm formation	Yes, highly resistant	Limited	None documented
Primary risk in soft-ripened cheese	High, grows during ripening	Moderate	Negligible
Primary risk in hard aged cheese	Lower, suppressed by aw and pH	High, persists beyond 270 days	None

The comparison illustrates that no single property defines Listeria’s persistence in aged cheese. It is the combination of cold tolerance, salt resistance, low minimum water activity, and biofilm formation that makes the organism difficult to control through the standard hurdles that cheesemaking applies.

What the 60-Day Rule Does and Does Not Accomplish

The 60-day aging requirement codified in 21 CFR Part 133 was designed for Mycobacterium bovis and Brucella abortus, not for L. monocytogenes. The organism was first described in 1926 by Murray et al. and was not recognized as a significant foodborne pathogen in the United States until the 1980s. The 60-day standard does not address Listeria as a design principle; it predates the modern understanding of listeriosis as a foodborne disease.

In hard aged cheeses with consistent low moisture and pH, extended aging suppresses rather than eliminates Listeria, and the overall risk is lower than in soft-ripened styles. This is not because the 60-day rule addresses Listeria but because the combined physical conditions of hard cheese production are less favorable to its growth. In soft and surface-ripened styles, the FDA/Health Canada assessment found the opposite: aging conditions can actively support Listeria growth on the rind.

The properties of L. monocytogenes that drive its risk in aged cheese, cold tolerance, salt resistance, biofilm formation, and surface pH dynamics, are features of the aging environment rather than features of the milk alone. For this reason, facility sanitation, environmental monitoring, and surface hygiene in the aging room represent the primary control points for this organism in a way that is fundamentally different from pathogens whose risk is primarily driven by the initial microbial quality of the milk.