There can be very few food industry professionals today who are not aware of the danger of Listeria in foods. In 2007, control of Listeria is a legal requirement in many countries and cleaning and hygiene procedures are designed around eliminating it from processing areas and products. Yet until comparatively recently, the bacterium was almost unknown, even to food microbiologists. How did this dramatic change come about? Microbiologists are still learning about this peculiar pathogen, its significance and what makes it such a nuisance to food producers. The emerging picture is a surprisingly complex one, but there are intriguing possibilities for more targeted and effective approaches to Listeria control.
The bacterial genus Listeria was first discovered about 100 years ago, and the best known and most dangerous species, Listeria monocytogenes, was soon identified as the occasional cause of disease in animals and humans. From then on, the study of Listeria and listeriosis remained a relatively obscure medical backwater until the early 1980s, when a startling change occurred. Numbers of human cases of listeriosis suddenly began to increase sharply at this time. For example, in France the number of people affected by the disease in the 1970s was a steady average of 15 per year. But by 1987, the number had risen to 687. Similarly, in the USA, figures for 1967-69 show that there were 255 cases of listeriosis, but in 1987 alone, 1,700 cases were recorded. Similar rises were seen in the UK and in many other countries worldwide. The first hint of a link with food came in Canada in 1981, when the very first documented outbreak of foodborne listeriosis was linked to coleslaw salad. At least 41 people were affected in this outbreak, but the real impact was in the seven deaths that occurred.
Since then, more than a dozen serious Listeria outbreaks have been recorded, often involving fatalities and linked to a wide variety of foods, including cheese, cooked meats and pâtés, chocolate milk, butter and smoked fish. Although the incidence of human listeriosis has fallen since the 1980s and in many countries is almost back to the levels of the 1970s, foodborne outbreaks still occur. For instance, in 2002, an outbreak in the USA linked to cooked poultry products affected at least 46 people, caused 11 deaths and resulted in the largest ever recall of meat products in US history. This illustrates that although the food industry has devoted significant resources to developing and implementing effective controls against Listeria since the 1980s, the problem has not gone away.
Listeria makes itself at home
So what is it about Listeria monocytogenes that makes it so dangerous, and how did it suddenly become a cause of foodborne disease? At least some of the answers can be found in the characteristics of the bacterium itself. Listeria monocytogenes is unlike most other foodborne pathogens in that it only rarely causes the typical symptoms of gastroenteritis. Infection is much more likely to result in more serious symptoms, and usually takes the form of meningitis or septicaemia. The elderly and those with weakened immune systems are especially vulnerable to infection, and this goes some way to explaining the high mortality rate of around 30%. The other group particularly at risk from listeriosis is pregnant women, who may suffer flu-like symptoms that lead to infection of the foetus and then to a miscarriage or stillbirth.
Relatively few healthy adults become infected by Listeria monocytogenes and the total number of cases worldwide is far lower than the number of people infected by Salmonella and other food poisoning bacteria. It is the potentially serious consequences of infection in vulnerable consumers that makes Listeria so very dangerous. Another factor is the very variable incubation time of listeriosis infection. It can be up to two months or more after exposure to the pathogen before symptoms appear, which can make it almost impossible to trace the source of the infection for individual cases and smaller outbreaks.
Listeria is also unusual among pathogenic bacteria in that it is able to grow at low temperatures (it is psychrotrophic). In fact it can grow at temperatures down to 0oC, albeit very slowly. This gives it the ability to multiply in refrigerated foods, given time, unlike other causes of food poisoning like Salmonella. Crucially, it is also significantly tougher than many other bacteria found in foods, being slightly more resistant to heat, drying, and even irradiation, than Salmonella, and more tolerant of varying environmental conditions. These attributes give Listeria a competitive advantage in certain types of food, especially chilled foods that are highly processed and have a long shelf life. This category includes many of the foods linked to past outbreaks of listeriosis, like soft cheeses and cooked meat products, where processing eliminates competing bacteria and the product is kept under conditions that allow Listeria to multiply.
It is still not entirely clear why the incidence of listeriosis took off in the 1980s, but there are several possible factors. It is likely that improved diagnosis of the infection had some impact on the figures, but this doesn’t explain such a significant rise in human cases. It is more likely that changes in food manufacturing methods that began in the 1970s had a major effect. The growth in mass-produced chilled processed foods with ever longer shelf lives effectively created an ecological niche that Listeria was able to exploit. While processors were aware of the measures needed to combat Salmonella and other food poisoning bacteria, the possibility of foodborne listeriosis was unknown and thus, not considered. It is also likely that the proliferation of cold, wet, food processing areas also created ideal conditions for Listeria to colonise many factories and so contaminate food products.
Of course, we now know about the risk of Listeria in chilled processed foods and some fairly effective measures have been developed to combat it. For example, the ‘listeria cook’ for meat products ensures that the bacteria do not survive into the finished product, and cleaning and hygiene schedules to control Listeria in processing environments help to minimise contamination. The legislators too have reacted to the problem. Most notably in the USA, where a ‘zero tolerance’ approach is taken for Listeria in food and the USDA requires processed meat producers to carry out regular monitoring of products and the production environment. The EU approach is different, and focuses on controlling the growth of Listeria in foods during shelf life, rather than eliminating it altogether. Most microbiologists regard this as a much more practical strategy that avoids the large number of probably unnecessary and costly recalls that have plagued the US food industry in recent years.
Despite all this, Listeria is still responsible for occasional foodborne outbreaks and product recalls around the world and many food processors face a constant battle to control contamination in their products and their factories. All the monitoring that has been undertaken since the 1980s has taught us that you can find Listeria pretty much anywhere if you look carefully enough. Cool wet environments suit the bacterium very well and it has proved very difficult to completely eliminate it from many food factories. But why is Listeria so hard to get rid of and do we really need to put so much effort into its control? Recent research is beginning to reveal that the current, relatively imprecise, approach may not be the best solution.
New techniques provide new insights
It is the rapid developments in molecular biology that have provided microbiologists with the tools to look more carefully at Listeria in food factories. It has long been known that Listeria monocytogenes can be divided into 13 different serotypes. All of these can cause disease in humans, but serotyping is not especially useful since most cases are caused by just three serotypes, and most recorded foodborne outbreaks are attributed to just one, serotype 4b. But in the last ten years, several much more discriminating methods for identifying particular strains of bacteria have been developed, such as ribotyping, pulsed field gel electrophoresis (PFGE) and random amplified polymorphic DNA (RAPD) analysis. These techniques allow a ‘genetic fingerprinting’ of individual strains, and their use, especially in combination, gives a very precise identification. Once specific strains can be identified, they can be studied in detail and tracked through the food chain from raw materials to finished products. Studies using these techniques have already revealed some important and valuable new insights into Listeria monocytogenes and how it contaminates food.
Much recent research has concentrated on identifying and tracing strains of Listeria monocytogenes that contaminate processing plants, especially those producing high-risk products, such as smoked fish. What emerged from these studies was that many processing plants have an in-house strain that persistently contaminates equipment and the environment, sometimes over a period of several years. Many of these ‘persistent’ strains are unique to one factory, but others have been found in several plants that have no apparent connection. Non-persistent strains can also be identified in most plants, but these are usually only isolated from a single sampling site over a brief period and then disappear. Oddly, the persistent strains are rarely isolated from raw materials, even though other strains are often found. This suggests that it is not continuous recontamination from raw materials that gives rise to persistent strains. The consensus is that persistent strains probably originate in raw materials, but are somehow able to colonise the factory environment much better than other strains.
How they do this has been the subject of much investigation, and answers to this question are also beginning to emerge. Persistent strains seem to be able to adhere to surfaces much more quickly and in greater numbers than non-persistent strains. This ability to form ‘biofilms’ probably helps the bacteria to survive cleaning and sanitation procedures, especially if these are poorly designed or executed. Many Listeria monocytogenes strains are also able to adapt rapidly to different sanitisers when they are present at low levels, and cross-adaptation has been observed too. This makes it very difficult to control contamination by rotating different biocides in the cleaning schedule. The combination of these two characteristics may allow persistent Listeria monocytogenes strains to gain a foothold in a processing plant, from which they are able to spread widely through the environment.
Factors that help this spread have been found to include poorly designed and inadequately cleaned processing equipment and insufficient separation between different steps in a processing line. For example, one study showed that a contaminated dicing machine was probably responsible for transferring a persistent strain from one factory to another when it was relocated. Ineffective hygiene barriers between areas handling raw and heat-processed products have also been shown to allow persistent strains of Listeria monocytogenes to colonise post-process areas of a plant.
New insights suggest new solutions
At first sight, these findings seem to raise more questions than they answer, but they also point the way forward for future research. It is clearly important to find out more about the characteristics that enable some strains to persist.
For example, knowing exactly how Listeria forms biofilms on surfaces may help the development of new materials and coatings that resist colonisation, and understanding the mechanism of adaptation to biocides could be useful in designing cleaning protocols. Furthermore, the ability to track individual Listeria monocytogenes strains precisely makes it possible to locate reservoirs of persistent contamination in a plant and focus on eliminating or controlling them. This is clearly more efficient than trying to decontaminate the entire plant – an almost impossible task. It is possible to eliminate Listeria from processing equipment, but it usually requires complete dismantling and a very thorough cleaning and sanitation regime. Knowing which equipment is most likely to be the source of contamination would help enormously.
In the future, it may also be possible to target strains of Listeria monocytogenes that are particularly dangerous to humans. Some strains are more virulent than others, but these may not be the same strains that become dominant in food factories. Not enough is yet known about the disease causing potential of individual strains to say that some are dangerous and some are not. The safe approach is to assume that they are all a serious risk to consumers. But as researchers learn more, it may be possible to estimate the food safety risk of individual strains and design appropriate controls. Under such circumstances a zero tolerance policy could become redundant.
Controlling Listeria – an alternative approach
Keeping Listeria monocytogenes out of food processing areas and out of finished products is a difficult task, and complete elimination from some foods is virtually impossible to achieve. While much can be done with effective HACCP plans and rigorous cleaning and sanitation regimes, where there is no ‘listericidal’ step in the process, or where persistent contamination is a problem, a different approach may be needed.
This may require accepting that Listeria will sometimes be present in the finished product and then developing ways of eliminating or controlling it there, rather than attempting to completely prevent contamination. Much attention has been focused on this approach in recent years and has led to the development of a number of ‘surface pasteurisation’ decontamination processes. These usually rely on the application of steam or hot water for short periods to raise the product’s surface temperature high enough to kill any Listeria cells present, but not enough to overcook the product. These processes can be applied either immediately before packaging, or to the product in its final pack in the case of vacuum packed products where there is no air space between product and pack surface. This approach has been tried with some success with meat products, such as sliced cooked chicken and frankfurters. Chemical decontamination methods such as ozone treatment have also been explored.
A very different way of achieving the same end is to make use of ‘natural’ controls. It has been observed that persistent stains of Listeria monocytogenes are much less likely to be a problem in processing lines producing fermented meat and fish products and this is thought to be due to competition from the starter cultures, usually comprised of lactic acid bacteria (LAB). The idea of inoculating foods with cultures of harmless LAB as a natural preservative to delay spoilage by out-competing other bacteria is not a new one and it has been successfully used in some fish products. However, the same effect could be employed to prevent Listeria from reaching dangerous levels during shelf life.
Another take on the natural control approach is to use bacteriophage cultures. Bacteriophages, or phages for short, are viruses that attack and destroy bacteria, and most species of bacteria are probably the natural hosts for at least one specific phage. Listeria monocytogenes is no exception, and there are phages that attack more than one strain of the pathogen. One of these has already been developed to the point where a product is commercially available. Listex P100 is a culture of a ‘broad spectrum’ phage that attacks many Listeria monocytogenes strains. Developed and marketed in the Netherlands by EBI Food Safety, Listex has recently been given GRAS (Generally Recognised as Safe) status for all food products in the USA. It can be used as a processing aid in Europe and so does not need to be declared on the product label, and has also been cleared for use in organic products. It is intended for application during the manufacturing process, where it is claimed to kill 100% of sensitive strains in a food matrix, or a food processing environment.
