Listeria monocytogenes is like an opponent you think you have punched out but keeps coming back for more. The food industry and government have long been aware of the persistence of Listeria in the environment and in processing plants, and they have introduced many policies and procedures to limit the contamination and growth of the pathogen. However, every few years there seems to be a major outbreak, often with high mortality, which catches industry, government and the public unaware and causes great angst. Cases appear to be rising, and recent trends in North America and Europe seem to show the growing elderly population most at risk. Other reasons given for the possible increase, apart from better surveillance and detection methodologies, are increased consumption of ready-to-eat (RTE) foods; increased storage time and temperature of RTE foods leading to growth of the pathogen; increased virulence of Listeria strains; and more persons at risk of infection. There are now cases documented in developing countries where listeriosis was not considered an issue previously.

Global Listeria Control
All these examples point to the need for better strategies for prevention and control of the pathogen worldwide. The Codex Alimentarius Commission (CAC), through its Committee on Food Hygiene (CCFH), has been looking at a standard that is universally accepted, because many of the implicated products are, in international trade, soft cheeses, delicatessen meats and smoked fish. The CCFH agreed that five key factors that contribute strongly to the risk of listeriosis associated with RTE foods are the following: amount and frequency of consumption of a food; frequency and extent of contamination of a food with Listeria; ability of the food to support the growth of Listeria; temperature of refrigerated/chilled food storage; and duration of refrigerated/chilled storage. In addition, it was agreed that multiple intervention steps were required rather than any one step to control the pathogen. Another issue is the variable susceptibility of individuals at risk for infection. Guidelines with three annexes (i.e., environmental monitoring in processing areas; criteria for Listeria in RTE foods; and microbiological testing for environmental monitoring and verification of process control) were finally accepted in July 2009 at the 32nd CAC session in Rome.

Another group with a concern for Listeria is the Global Harmonization Initiative (GHI), which was founded in 2004 as a joint activity of the Institute of Food Technologists International Division and the European Federation of Food Science and Technology (EFFoST). GHI’s objective is to achieve consensus on the science of food regulations and legislation to ensure the global availability of safe and wholesome food products for all consumers. It attempts to facilitate global discussions about the scientific issues that support decisions made by national governments and international regulatory bodies through the following: providing the foundation for sound, sensible, science-based regulations; creating a forum for scientists and technologists to interact with regulatory authorities globally; and providing industry, regulators and consumers an independent, authoritative information resource. For GHI, one of the outstanding food safety areas most affected by differing national policies and regulations is Listeria in RTE foods, ranging from no policy at all for control to zero tolerance for all foods. The GHI established a Working Group to consider the best science to recommend the most appropriate control measure for this pathogen, particularly in RTE food. Unlike CAC, the GHI does not work with governments but with individuals with interests in science.

Getting the Message Out
I was looking for a partner to put on a conference on “Controlling Listeria: Working Towards International Harmonization” through a U.S. Department of Agriculture National Institutes for Food and Agriculture grant, and it was logical to link with GHI, which had similar objectives. Thus, an ad hoc group of 35 scientists from Europe, North America and New Zealand, including GHI and EFFoST representatives, met in Amsterdam in May 2009. The main goals of the Workshop were to understand the forthcoming CAC decision on control of Listeria in RTE foods and to explore how specific prevention and control measures could be implemented for industry to use and for government to monitor, primarily addressing the issues in Annex 2. Following 10 presentations on surveillance of listeriosis, different government and industry approaches to control, challenge tests to determine growth or no growth of Listeria in RTE foods and setting risk-based criteria and food safety objectives for the pathogen, the group developed a set of recommendations.

Overall, the group felt there is a need for a risk-based policy with input from all the stakeholders at local and national levels. An important part of the background is to review the critical factors for control, including the unique growth, survival and virulence characteristics of the pathogen, identifying specific populations at risk and defining what RTE foods are. They also saw the need for Listeria food-source attribution through review of outbreak data, implementation of case-controlled studies, expert elicitations, microbial source tracking and development of risk assessment models. They also indicated that surveillance of both listeriosis and the non-invasive form of illness caused by the pathogen are important for public health agencies to establish or enhance. This would require coordination of laboratories through better communication and reporting for the analysis of clinical cases, foods and environmental sources. These laboratories should be accredited, with some being reference laboratories at national or regional levels. There was agreement on the microbiological criteria as specified in the CAC Guidelines, but it was recognized that there were challenges for industry to meet these criteria and for government agencies to assess compliance. Other issues to be considered are the development, validation and acceptance of quantitative methods capable of detecting the pathogen in food at low levels; determination of the food’s ability to support pathogen growth through challenge studies and risk assessment models; appropriate labeling of RTE foods; and a standardized approach to tracking products throughout the food chain. There is also a need for food worker education and training as well as consumer awareness and responsibility. Message mapping is one approach to instill essential food safety messages regarding listeriosis and the safety of RTE foods for employees and consumers.

The intent of these recommendations and the other papers to be published in Food Control[1] is to stimulate further discussion at the scientific level and encourage more data gathering and scientific research, as well as to see whether a broader group of those interested in controlling this pathogen in the food supply could take the information to the next stage and build on the CAC Guidelines and Workshop recommendations. The GHI is willing to do this with its L. monocytogenes in RTE Meals Working Group, but there is value for industry, government and academia to evaluate these suggestions and move forward with both the CAC Guidelines and the implementation of appropriate prevention and control strategies to reduce illnesses and facilitate trade. 

Ewen C. D. Todd, Ph.D. is a food safetyconsultant and former director of the National Food Safety & Toxicology Center and the Food Safety Policy Center at Michigan State University.

1. The Amsterdam Workshop papers and recommendations will be available in Food Control in September 2011.

More information on GHI can be found at and its Working Group on Listeria in RTE Meals at