Food Safety

Dossier – Listeria monocytogenes in dairy production

May 26, 2025

Listeria monocytogenes is one of the most concerning pathogens for food safety in dairy production, as it causes listeriosis with a mortality rate of up to 24% amongst the infected (de Noordhout et al., 2014), despite a relatively low prevalence in foods. This analysis is based on a bulletin from the International Dairy Federation (Bourdichon et al., 2019), whose content remains current in light of ongoing alert notifications at European and national levels.

The severity of contamination has highlighted the inadequacy of existing regulatory approaches, particularly regarding the protection of vulnerable populations from raw milk products. The review addresses the ecology, growth, detection methods and control measures of L. monocytogenes, emphasising the urgency of introducing specific labelling to ensure the protection of at-risk consumers.

An effective prevention strategy requires understanding the pathogen’s ecology and adopting targeted measures, including appropriate warnings on labels.

Table of Contents

Listeria monocytogenes as a foodborne pathogen

L. monocytogenes was first described in 1926 as a zoonotic pathogen in laboratory rabbits but was recognised as a foodborne pathogen only in the 1980s following an outbreak linked to coleslaw in 1981 (Schlech et al., 1983). Since then, numerous outbreaks have been documented in various foods, with dairy products frequently implicated.

The IDF bulletin highlights that L. monocytogenes displays remarkable versatility in foodborne disease outbreaks and is associated with a wide variety of foods. Recent outbreaks in meat preparations, meat-based products and frozen vegetables and desserts in Europe, as well as many others (Bourdichon et al., 2019), all linked contamination of final products to the food-processing environment. Within dairy contexts, although pasteurisation has significantly reduced listeriosis occurrence, contamination of processed dairy products remains problematic.

Ecology and persistence in dairy environments

L. monocytogenes is well-known for its presence in raw milk and raw milk products, which have resulted in numerous foodborne disease outbreaks (Lovett et al., 1987; Paul et al., 2015). Additionally, Listeria spp., including L. monocytogenes, are recognised as persistent bacteria in food manufacturing environments, including pasteurisation process areas of dairy facilities (Carpentier & Cerf, 2011).

The research indicates that out of numerous species in the Listeria genus, the three most common species associated with dairy manufacturing environments are L. innocua, L. monocytogenes, and L. seeligeri (Barancelli et al., 2014; McIntyre et al., 2015; Rückerl et al., 2014). Other species occasionally isolated include L. ivanovii, L. welshimeri, and L. grayi (Alvarez-Ordóñez et al., 2015; Silva et al., 2003).

The presence of L. innocua in processing environments serves as an important indicator that applied hygiene regimens are not adequate to mitigate Listeria spp. in the plant, suggesting an increased likelihood of L. monocytogenes contamination (Ryser & Marth, 2007). This is because L. innocua and L. monocytogenes behave similarly in dairy products and manufacturing environments (Liu et al., 2009).

The presence of L. monocytogenes in dairy manufacturing plants is particularly significant in wet areas compared to dry areas (Redfern & Verran, 2017). In some cases, processing water itself can harbour L. monocytogenes, as demonstrated in a 2002 outbreak from cheese in Canada where processing water contaminated by birds carrying L. monocytogenes resulted in an outbreak (McIntyre et al., 2015).

Growth characteristics and survival

The IDF bulletin presents detailed information on the growth and survival characteristics of L. monocytogenes, highlighting its ability to survive in diverse conditions:

L. monocytogenes can grow at low temperatures (0.6 to 45°C) and in a wide pH range (4.4 to 9.4) (Lake et al., 2009). The pathogen has the ability to grow at refrigeration temperatures (4°C), survives under freezing conditions, and has been found in ice cream and various frozen dairy products (El-Kest & Marth, 1992);
while L. monocytogenes is rapidly inactivated at temperatures above 70°C (with a D-value at 72°C in milk estimated between 0.9-2.7s) (Sutherland & Porritt, 1997), its pH tolerance is significant. The optimum pH for growth is 7.0, with occasional strains showing potential for growth as low as pH 4.1 (Jay, 2005). For fermented dairy products, L. monocytogenes is unlikely to grow below pH 5.2 due to lactic acid inhibition (Aryani et al., 2016).

The minimum water activity (aw) permitting growth is considered as aw = 0.92 with 11.5% NaCl and 0.93 with 40% sucrose (Jay, 2005). L. monocytogenes has the ability to survive in dry foods (aw = 0.83) (Beuchat et al., 2011) and has been shown to survive in flour (Taylor et al., 2018), suggesting its potential to survive in low moisture dairy products.

Pathogenesis and public health impact

The bulletin describes two types of disease associated with L. monocytogenes:

non-invasive listeriosis (febrile listerial gastroenteritis) represents the milder form, with illness occurring within 24 hours of ingestion and includes diarrhoea, fever, headache and muscle pain. This form typically involves ingestion of high doses by otherwise healthy individuals and is usually self-limiting within 2 days (Ooi & Lorber, 2005);
invasive listeriosis affects high-risk individuals (young, old, pregnant women, and immunocompromised, YOPI) and can have an incubation period of between two weeks and three months (McLauchlin et al., 2004). Infection during pregnancy can occur at any stage but is most often reported during the third trimester, with the mother usually exhibiting mild flu-like symptoms while the unborn or newly born infant can develop severe systemic disease. This form of listeriosis has a high mortality rate, especially among vulnerable populations.

The infectious dose of L. monocytogenes varies based on strain virulence and host susceptibility. While theoretically one cell could cause listeriosis, the estimated dose causing illness in healthy individuals has traditionally been between 10^7-10^9 cells, and 10^5-10^7 cells for high-risk individuals (Farber et al., 1996; Dalton, 1997). However, recent outbreaks have occurred with much lower doses, in some cases fewer than 1,000 CFU (Pouillot et al., 2016).

Listeria monocytogenes occurrence in food safety alerts in Europe

According to the most recent data from the European Food Safety Authority and European Centre for Disease Prevention and Control (One Health Report, 2023), L. monocytogenes continues to be a significant concern in European food safety. In their latest zoonoses report, L. monocytogenes was the third most common cause of food-borne outbreaks in the European Union in 2022, responsible for 18 outbreaks affecting 151 people and causing 26 deaths, reflecting a case fatality rate of 15.2% (EFSA & ECDC, 2024). The report also highlights that ‘ready-to-eat (RTE) fish and fishery products’ and ‘meat and meat products’ were the food categories with the highest proportion of samples exceeding the food safety limit of 100 CFU/g. Notably, among the different types of ready-to-eat foods tested, soft and semi-soft cheeses also showed significant levels of non-compliance, clearly indicating the ongoing risk that L. monocytogenes poses to consumers of dairy products, especially those made from raw milk (EFSA & ECDC, 2024).

Need for reformed labelling requirements for raw milk products

Current EU regulations, particularly Regulation (EC) No 853/2004, inadequately protect vulnerable consumers from the risks associated with raw milk products. As already argued by the writer, there is a pressing need to reform these regulations to include specific warning labels on dairy products made from raw milk – excluding those subject to maturation for a period proven suitable for inactivating pathogens – to protect vulnerable groups (young, old, pregnant, immunocompromised – YOPI).

Children under ten, the elderly, pregnant women, and immunocompromised individuals are particularly susceptible to severe consequences from infections with pathogens like L. monocytogenes, STEC, and VTEC, which may be present in raw milk products.

The current regulatory framework allows these high-risk individuals to unknowingly consume potentially harmful products, as specific warning labels are not mandatory in the EU. The writer highlights the successful implementation of such warnings in countries like the United States and Australia, where mandatory warnings have been shown to effectively reduce foodborne illness cases among vulnerable populations, creating a compelling case for similar measures to be adopted throughout the European Union.

Control measures in dairy manufacturing

The bulletin emphasises that due to the ubiquitous nature of Listeria, complete elimination from processing environments is unrealistic. Instead, control should focus on several key approaches:

historically, heat treatment (pasteurisation of dairy products) is applied to raw materials to reduce the initial microbial contamination to acceptable levels. A recognised pasteurisation treatment of raw milk for 15s at 72°C would result in a greater than 6-log reduction of L. monocytogenes (Bourdichon et al., 2019); additional growth inhibition measures include:
pH less than or equal to 4.4 (EU 2073/2005, FDA, 2018);
water activity less than or equal to 0.92 (e.g., milk powder) or less than or equal to 0.94 in combination with a pH less than or equal to 5.0 (EU 2073/2005, FDA, 2018);
formulation containing inhibitory substances (e.g., biopreservation with food starters such as those used in hard cheese manufacturing) (Wemmenhove et al., 2018);
strict maintenance of cold chain;
minimising cross-contamination through good manufacturing practices and re-contamination through good hygiene practices.

Final product testing alone is not sufficient for ensuring food safety, as it provides little information in cases where L. monocytogenes occurs infrequently. Instead, the bulletin advocates for continuous and targeted sampling of the processing environment, focusing on areas with positive results for additional hygiene measures, including root cause analysis (Leong et al., 2016).

Process environment monitoring (PEM) should include testing for Listeria spp., as their presence, while not constituting a public health risk, serves as an indication of potential L. monocytogenes presence. Testing for Listeria spp. in PEM and treating positive results as if they were L. monocytogenes provides a more sensitive verification and control programme (Bourdichon et al., 2019).

For chemical disinfection, quaternary ammonium compounds (QACs) have been recommended as effective against L. monocytogenes, particularly in biofilms (FDA, 2008). However, some studies have shown L. monocytogenes can develop tolerance to QACs over time (Mereghetti et al., 2000; Olszewska et al., 2016). Therefore, sanitiser rotation (including peracetic acid/hydrogen peroxide) is recommended to minimise the development of tolerant populations (Kastbjerg & Gram, 2012).

Analytical methods

As early as 2019, the IDF bulletin outlined the analytical methods for the detection of Listeria, primarily based on ISO 11290 Parts 1 and 2 standards (ISO, 2017a, 2017b). Rapid alternative methods based on immunoenzymatic assays or molecular detection are also available and widely used in the dairy industry to reduce the time needed for pathogen results.

For monitoring and tracking contamination pathways, molecular typing methods are essential. While pulsed field gel electrophoresis (PFGE) has been the gold standard for identification and clustering of isolates for many years, whole genome sequencing (WGS) has emerged as a more powerful tool that provides higher resolution for strain relationships (Hendriksen et al., 2018).

WGS provides several key advantages, including the ability to differentiate sources of contamination even within the same outbreak, determine which ingredient was originally contaminated, identify unexpected vectors for food contamination, and provide information for root cause analysis (Kovac et al., 2017).

Interim conclusions

The present analysis highlights the dual challenge posed by L. monocytogenes in dairy production:

on one hand, the high ecological adaptability that enables its persistence in processing environments. And its recurrence in food alerts in the EU confirms its relevance for public health;
conversely, existing regulatory gaps continue to pose risks to the health of vulnerable populations, particularly through the consumption of raw milk products — except in the case of cheeses where the maturation process is demonstrably effective in inactivating pathogenic microorganisms.

An integrated approach is needed that includes advanced environmental monitoring, detection methodologies such as WGS and regulatory reforms, including the introduction of mandatory warning labels for at-risk products. Although complete eradication of L. monocytogenes is unlikely, effective control through adequate hygiene practices and appropriate consumer warnings can significantly reduce the public health burden of this important foodborne pathogen.

Dario Dongo

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