The existence of Listeria monocytogenes in a food or beverage product can have serious consequences for consumers and businesses, as highlighted by recent listeriosis outbreaks in the US.
Dr. Paul Gibbs from Leatherhead Food Research considers where we should target our efforts in our battle to control the pathogen.
It is easy to understand why many countries have a zero tolerance to the pathogen, but categorizing all forms of Listeria monocytogenes as the same organism might be preventing us from actually understanding and eliminating the pathogen. New research shows that not all strains of Listeria monocytogenes are the same, with some types more likely to cause infection than others and certain types more likely to persist within food environments than others.
With a mortality rate of 20-30% (higher than the mortality rate for Salmonella and Campylobacter combined), the discovery of Listeria monocytogenes (L. monocytogenes) in a food or beverage product is considered a serious breach of food safety. Unlike Campylobacter which causes illness across all population groups, L. monocytogenes is particularly dangerous to those with reduced immunity. Those most at risk of infection are pregnant women, unborn and new-born babies, people aged over 60 years and people with medical conditions.
Impact on Frozen Food Recalls
Recent outbreaks of listerisos (the disease caused by L. monocytogenes) in the US demonstrate the devastating impact that L. monocytogenes can have on people’s lives, consumer confidence and a company’s reputation. In April 2015, an outbreak of listeriosis in the US was linked to ice-cream products manufactured by Blue Bell Creameries. Ten people were hospitalized and three died. Blue Bell Creameries issued a voluntary recall for all its products and shut down all of its facilities, resulting in huge job losses and a financial crisis that almost caused the company to close.
In March 2016, an ongoing outbreak of listeriosis in the US was linked to frozen vegetables produced by CRF Frozen Foods at its facility in Pasco, Washington. Four weeks’ later, the company recalled 11 frozen vegetable products that had the potential to be contaminated with L. monocytogenes and by 2 May 2016, the recall had expanded to 358 products sold under 42 separate brands in the US and Canada. In total, as many as 500 products sold at major retailers, including ready-made meals prepared using frozen vegetables supplied by CRF, were recalled. Analysis of fruits and vegetables used by the company indicated that onions from the Oregon Potato Co. may be a potential source of contamination, and this led to further downstream recalls.
Examination of the CRF Pasco factory have identified a number of risk factors which could increase the likelihood of L. monocytogenes survival, including damaged equipment that would be difficult to clean. The incident has been described as the largest recall in recent memory, resulting in eight reported cases of illness and two deaths. The factory has been closed and more than 300 employees have been laid off.
What Do We Know About L. Monocytogenes?
If you look for it, L. monocytogenes can actually be found everywhere. It can be very difficult to eliminate the pathogen because it is a good survivor, prospering in several natural environments, including waters, soils, plants and animals. The typical methods that manufacturers use to control microbial growth have been proven not to be successful in the control of L. monocytogenes. Physico-chemical means (e.g. temperature, pH and salt) are not successful because L. monocytogenes is capable of growth at temperatures as low as 2°C, and under high salt, low pH values. This means it is very difficult to design a safe food product that will inhibit growth of the pathogen, and still retain sensory characteristics acceptable to the consumer.
How Can We Eliminate L. Monocytogenes?
The key tool manufacturers have up their sleeves to control L. monocytogenes is heat treatment. The organism is not particularly heat resistant and can be eliminated by conventional pasteurization. The options for eliminating the pathogen from food and beverage products which cannot be pasteurized, however, can seem rather thin on the ground. It is no surprise then that recent outbreaks have been most commonly associated with raw and unpasteurized foods, including semi-preserved meats and fish (e.g. ready-to-eat and delicatessen products such as patés, frankfurters, cold-smoked fish), cheese, especially soft and mold-ripened cheeses, ice cream, raw vegetables and salad items.
What Is The Regulation Surrounding L. Monocytogenes?
In light of the seriousness of the disease, regulation around listeria has traditionally been stringent. Many countries, such as the USA and Russia, have a zero tolerance for the presence of L.monocytogenes i.e. if the organism is detected within a product, then it must be recalled. Current legislation in Europe does allow for the presence of some L.monocytogenes. For Ready to Eat (RTE) products that are able to support the growth of L.monocytogenes, there should be no detectable L.monocytogenes at the point of manufacture, but small traces of L. monocytogenes are acceptable in these kinds of products at the end of their shelf life, as long as the numbers do not exceed 100 cfu/g.
What Is The Future For The Control Of L. Monocytogenes?
While legislation is concerned with protecting the consumer, the current approach may actually be preventing us from understanding and eliminating the pathogen. Listeria legislation does not currently differentiate between different strains of L. monocytogenes, lumping all strains together as a single group of pathogenic organisms. The picture, however, is much more complex than this and attempting to address this complexity might assist efforts to eliminate the pathogen.
It is now recognized that not all strains of L. monocytogenes cause disease. Pathogenicity (the ability of an organism to cause disease) for humans is generally confined to certain strains of L. monocytogenes. One particular strain causes 40% of foodborne outbreaks. Current research is now focusing on understanding why certain strains of L. monocytogenes are more pathogenic than others. Research has revealed that certain genes are more commonly found in pathogenic strains of L. monocytogenes and are absent in less pathogenic strains. This may eventually allow the development of tests to further distinguish pathogenic and non-pathogenic strains. However, this is complicated since the severity of disease i.e. virulence, is linked to individual immunity and certain people are more likely to contract listeriosis than others.
Whole genome sequencing, which shows scientists a pathogen’s complete DNA makeup, will in time enable us to develop greater knowledge about the pathogenicity of different strains of L. monocytogenes. This, coupled with a greater understanding of why certain people are more susceptible to L. monocytogenes, will enable us to take a much more nuanced approach to dealing with L. monocytogenes, and ultimately limit the number of listeriosis outbreaks.