Listeria are bacteria. They are the causative agents of listeriosis, a rare, mainly food-borne disease.

Listeria is widely distributed in the environment, e.g. in sewage, in soil and on plants. Food of animal origin such as raw milk and raw milk products and raw meat, but also meat and fish products such as sliced, packaged sausage and smoked fish may contain listeria. Products made from pasteurised milk, such as smear or soft cheeses, can become contaminated during manufacture.

L. monocytogenes can often be found in the environment, soil and water. Animals can carry the pathogen without becoming ill, but miscarriages caused by Listeria occur in ruminants. Food processing plants can be a reservoir for these pathogens, resulting in contamination of (further) processed food. Due to their ability to grow even at low temperatures, Listeria even multiply in the refrigerator; therefore, contaminated food may contain high bacterial counts after storage in the refrigerator.

Listeria are mainly ingested through the consumption of contaminated animal and plant foods. In pregnant women, the pathogens can also be transmitted to the unborn child without any symptoms of maternal disease. Very rarely, further spread also occurs through person-to-person transmission (hospital infections of newborns) as well as through direct contact with infected animals (skin infections).

The incubation period varies depending on the disease manifestation: it is usually a few hours to six days for gastrointestinal symptoms, 1-12 days (median 2 days) for septicemic courses, and 1-14 days (median 9 days) for neuroinvasive manifestations. Longer incubation periods must be assumed for pregnancy-associated cases: 17-67 days (median 27.5 days).

In general, the human immune system protects sufficiently against severe courses of disease, and many infections pass virtually unnoticed and without consequences. In healthy adults, an infection usually runs its course without signs of illness or only with diarrhea. Severe diseases mainly affect immunocompromised persons (e.g. cancer patients, patients under high-dose cortisone therapy, etc.), elderly persons and pregnant women. When listeriosis is diagnosed, it is almost always invasive, meaning that the bacteria spread beyond the digestive tract. Invasive listeriosis is manifested by severe headache, high fever, nausea and vomiting. As a consequence, brain or meningitis or sepsis (blood poisoning) may occur, which is often fatal. The pathogens can also cause inflammatory processes in other parts of the body (e.g. pus accumulation in the spine or in joints), but these consequences are rarely observed. In pregnant women, there is a risk of infection of the unborn child with the risk of premature birth or stillbirth. Sepsis and meningitis may develop in the infected newborn.

In the case of invasive listeriosis, the administration of antibiotics is required. Nevertheless, despite targeted therapy, up to 30% of invasive listerioses are fatal.

General basic rules to minimize the risk of foodborne infections: Rinse fruits, berries, vegetables and pre-cut packaged leafy salads thoroughly with tap water before eating or processing, cook meat and fish dishes thoroughly, boil raw milk before eating, do not eat minced meat raw, always store possible risk foods such as soft cheese, smear cheese, sliced sausages or smoked fish separately from other foods. Immunocompromised people, pregnant women and the elderly should refrain from eating possible risk foods and should never eat them after the expiry date.

In 2023, 37 laboratory-confirmed cases of invasive listeriosis were reported to the Epidemiological Reporting System (EMS). The 28-day mortality rate (= overall mortality within 28 days of diagnosis) for invasive listeriosis was 8.11% (3 out of 37 cases). None of the cases were pregnancy-associated. 34 isolates were typed at the national reference centre for listeriosis. The following table contains figures reported in the EMS for the respective year.

In 2023, approx. 3,500 food samples were analysed for Listeria in the official tests according to the sample plan. Human pathogenic L. monocytogenes were found in 78 samples, with the value being below 100 CFU/g in 12 samples and above 100 CFU/g in four samples.

A selection of the food groups analysed (rounded):

• 1,200 samples of ready-to-eat foods
• 800 samples of milk and dairy products
• 760 samples of meat and meat preparations
• 200 samples of bakery products
• 140 samples of fish and fish products
• 160 samples of ice cream
• 150 samples of fruit and vegetables (products)
• 150 samples of packaged ready meals

In most cases, L. monocytogenes is not introduced into food via the animal but via the inanimate environment during processing. Monitoring of livestock for Listeria is therefore not considered appropriate. In the case of raw milk, contamination with faeces is considered to be the most frequent source of introduction; in isolated cases, direct introduction of the bacteria via mastitis has been documented.

Cultural pathogen detection from blood, cerebrospinal fluid, pus, punctates, or (in newborns) swabs from the umbilicus, ear, or meconium should be sought. Listeria can be detected by standardized qualitative, quantitative and molecular biological methods. PCR from CSF can be used if cultural pathogen detection is unsuccessful after antibiotic pretreatment. Serological tests are difficult to interpret because cross-reactions in healthy individuals and lack of antibody detection despite infection are common. Nearly 90% of ill people are associated with the three serovars 4b, 1/2a and 1/2b.

Our laboratory diagnostic services:

National Reference Laboratory for Listeriosis

National reference laboratory for Listeria moncytogenes

Obligation to report: According to § 1 of the Epidemic Act 1950, Listeria must be reported as a pathogen of bacterial food poisoning or as a pathogen of invasive bacterial diseases (sepsis, meningoencephalitis). For the reporting of pregnancy-associated cases of listeriosis, the following has also applied since June 2013: any miscarriage or stillbirth due to pregnancy-associated listeriosis in the mother must be reported. The listeriosis illness of the mother is to be considered a separate, reportable case.

Germ reservoir food

Provided there is no surface contamination or subsequent contamination after opening the packaging, some foods are largely free of Listeria: In untreated foods, e.g. carrots, tomatoes and acidic fruits such as apples and pears, the risk is extremely low, especially if any surface contamination has been removed by washing or peeling.

Contamination of food with Listeria can occur at various stages of production and processing. In particular, food of animal origin such as raw milk and raw meat can be contaminated during production, e.g. during milking or slaughter. In the case of cheese made from unpasteurised raw milk, contamination of the raw milk cannot be excluded as a cause for the presence of Listeria in the final product. In the case of cheese made from heat-treated milk, the Listeria are killed during pasteurisation. However, if hygiene is poor in the processing procedure, there is a possibility of renewed contamination of the product after heat treatment. In most cases, the contamination of cheese relevant for infection transmission only occurs during ripening via colonisation of the rind. In cheeses with a soft, greasy rind, Listeria can then multiply massively in the course of ripening. They are often not distributed evenly over the entire surface, but rather in micro-colonies at specific points.

The ability of Listeria to survive and multiply in food depends on the technological treatment or the production process. Cooking, frying, sterilizing and pasteurizing kills the bacteria. In foods that contain little water, a lot of salt or preservatives, or that are very acidic (e.g. sauerkraut, mixed pickles and yoghurt), reproduction is only possible with a delay or not at all. In comparison to competing germs, Listeria have good growth possibilities in the case of reduced oxygen supply (e.g. in vacuum packaging of cooked sausages, salmon and smoked fish) and long storage times of the food under refrigeration.

National Reference Laboratory for Listeria: Analysis of isolates from food and environmental samples.

The isolates received are typed by whole-genome sequencing (Van Walle et al. 2015). To clarify epidemiological questions such as the confirmation of a foodborne listeriosis outbreak, the sequences are evaluated using core genome MLST (cgMLST) analysis (Rupptisch et al. 2015). This in-house method was developed in 2015 in collaboration with Münster University Hospital and Ridom Bioinformatics and has since been used internationally (Van Walle et al. 2018). The cgMLST scheme is integrated in the Ridom SeqSphere+ software. For typing, 1701 defined target sequences are analyzed and aligned via a nomenclature server(https://www.cgmlst.org/ncs/schema/690488).

Publications:

Van Walle I, Torgny Björkman J, Cormican M, Dallman T, Mossong J, Moura M, Pietzka A, Ruppitsch W, Takkinen J, European Listeria WGS typing group. Retrospective validation of whole genome sequencing enhanced surveillance of listeriosis in Europe, 2010 to 2015. Euro Surveill. 2018;23(33):pii=1700798. https://doi.org/10.2807/1560-7917.ES.2018.23.33.1700798

Van Walle I, Pietzka A, Moller Nielsen E, Takkinen J, Damjanova I, Michelacci V, Mossong J, Eelco F, Van Pelt W, Wolkowitz T, Borges, V, Jernberg C, Fisher I, Peters T, Agren J, Rizzi V, Da Silva Felicio MT, Struelns M, Palm D. European Centre for Disease Prevention and Control. Expert Opinion on the introduction of next-generation typing methods for food- and waterborne diseases in the EU and EEA. Stockholm: ECDC; 2015. technical report - October 2015. ISBN 978-92-9193-723-3; doi 10.2900/453641; catalogue number TQ-02-15-849-EN-N.Ruppitsch W, Pietzka A, Prior K, Bletz B, Lasa Fernandez H, Allerberger F, Harmsen D, Mellmann A. Defining and evaluating a core genome MLST scheme for whole genome sequence-based typing of Listeria monocytogenes. J Clin Microbiol. 2015;53(9):2869-76. doi:10.1128/JCM.01193-15.