Controlling Listeria monocytogenes – a constant threat to food safety

Foodborne diseases cause of millions of illnesses each year. One of the top causes of fatalities is Listeria monocytogenes. With increasing consumer demands for safe and healthy food, and accelerated production by processing plants, efficient methods of pathogen control is crucial. What is the importance of controlling Listeria and how can companies face the challenge efficiently? We try to address these issues here.

Listeria monocytogenes: widely present and potentially lethal  

Listeria is an environmentally widespread bacterium that exists naturally in soil and water as well as in a variety of mammals, avian species, and fish. Its presence can affect a wide variety of the processed foods we consume from dairy and meat products to fish and vegetables.

Of six Listeria species, only Listeria monocytogenesis is infectious for humans. It is one of the most harmful food-borne pathogens, with 20% of clinical infections resulting in death [1]. For most healthy young and middle aged people, chances of severe illness are low, with typically mild, flu-like symptoms or gastroenteritis. But in vulnerable groups that include pregnant women, infants, the elderly and other people with compromised immune systems, the consequences can be very serious and sometimes fatal.

Major Listeria problems in food-processing environments and ready-to-eat foods

Listeria has a unique capacity to survive and grow in hardy conditions such as very low temperatures. It thrives in normal refrigeration conditions and can survive freezing, although it is eliminated through cooking at high temperatures. It also has the tendency to form highly resistant layers of bacteria known as biofilms which can be difficult to remove even through cleaning.

Because of its widespread presence and unique survival abilities, Listeria is a severe problem for food-processing environments that deal with the preparation of foods. In fact, the main method of Listeria transmission to humans is through foods contaminated during production and processing. Bacteria can develop on the surfaces of machines where Listeria-containing products, such as meats and fish, are processed or packaged. Transmission can occur from the raw processing sectors to the finished product area and contaminate the latter. Further, niches of bacteria can develop in hard-to-reach areas and survive normal sanitation procedures.

Foods that are sold as ready for consumption without the need for cooking or reheating are also known as ready-to-eat (RTE) foods. These foods are generally processed in some way before reaching the food retailer and have the highest risk of Listeria contamination. The highest prevalence of Listeria can be found in RTE foods such as unpasteurised cheeses, deli and cold cut meats and smoked fish. [2]

Listeria outbreaks are not a thing of the past

Although safety measures and regulations have increased over the past years, major outbreaks of Listeria continue to cause heavy damage. In late 2008, an outbreak of Listeria monocytogenes in a Maple Leaf Foods plant in Canada killed 22 people and caused 57 confirmed cases due to Listeria contamination in RTE meat products [3]. Even though testing and safety procedures were in place, the Listeria was not controlled, demonstrating its ability to spread.  

Listeria outbreaks have occurred on a frequent basis over recent years. Between 1991 and 2002, 18 outbreaks of invasive listeriosis were reported, many of which were linked to unpasteurized dairy products, soft cheeses, and processed meat products [4]. Today, outbreaks continue to occur in various regions across Europe.

Prevention better than cure

Contaminated products not only cause a heavy economic loss due to potential food recall,  plant closure for sanitization and inspections, and large delays, it can also have a large negative impact on the company's reputation and brand. In order to maintain standards and avoid painful economic consequences, measures to enforce proper safety protocols as well as constant vigilance can significantly reduce the risk of Listeria.

European regulations specify that RTE foods must remain below 100 colony forming units per gram (cfu/g) throughout the product's shelf life. High risk RTE foods shown to support the growth of Listeria have an additional criteria of testing negative for Listeria monocytogenes in 25 grams of sample before leaving the factory. RTE foods for infants and special medical purposes must remain negative for Listeria monocytogenes throughout the product's shelf-life. Environmental sampling for Listeria testing on surfaces is also a mandatory part of plant verification [5]. 

From the consumers' point of view, especially for those at high risk, it is important not to eat foods beyond the expiration date on the package and to cook those products that are of high risk before consumption.

High quality tools to ensure success

In an industry that is facing ever increasing pressures to produce and deliver high volumes of food in very short times, the chances of contamination problems are high. At the same time, costs of product storage and delays while waiting for test results through traditional methods can be very high. A key challenge for companies is to carry out an efficient control and safety program that includes monitoring and regular testing so they can take action rapidly and limit negative consequences.

Providing high accuracy and sensitivity along with fast time to results, the DuPont Qualicon BAX® System is a molecular method for microbial detection, offering new Listeria solutions for food and environmental testing as well as a broad range of tests for other pathogens, such as Salmonella, E. coli O157:H7 and Campylobacter.

In an easy to use design, the BAX® System PCR Assay for Listeria 24E delivers fast and reliable next-day results on food and environmental samples. For even quicker turnaround, the BAX® System Reverse-Transcriptase PCR Assay accurately detects even low levels of Listeria from environmental samples in just 8 hours, helping to ensure a safe processing environment. When integrated into a plant's workflow, its shorter time to result compared to traditional methods as well as its high accuracy allows for faster response to the presence of pathogens, along with reduced expenses related to storage and inventories.

Conclusion

The problem of Listeria will continue to be a major issue for many food-processing environments and for RTE foods. A properly implemented safety protocol for sample testing, sanitization and monitoring can significantly reduce the risk of outbreaks. New tools such as the BAX® System, integrated into the production workflow can help ensure that safe, high quality products are the only ones that reach the consumer.

References:

1. Clinical Infections Diseases 2008; 47: 867-874. DOI: 10.1086/591131
2. Food and Drug administration. 2007. Compliance policy guide for Listeria monocytogenes. www.cfsan.fda.gov
3. Listeria outbreak - Public health agency of Canada www.phac-aspc.gc.ca/alert-alerte/listeria/listeria_2009-eng.php
4. Food Safety Authority of Ireland. 2007. Listeria monocytogenes. www.fsai.ie/publications/factsheet/factsheet_listeria_monocytogenes%20.pdf
5. The European Regulation EC 2073/2005 Microbiological Criteria for Foodstuffs. Official journal of the European Union 12.2005.