Listeria outbreaks: why they should be much rarer in ten years’ time
A listeria outbreak in the UK has infected nine people and killed five of them in June 2019. The bacteria was transmitted via sandwiches and salads eaten in seven hospitals across England, supplied by the Good Food Chain, a company based in the Midlands. The outbreak came to light after samples from its meat supplier tested positive for the bacteria, which microbiologists refer to as L. monocytogenes.
People who contract listeria risk developing an infection called listeriosis, which can be dangerous to the likes of pregnant women, immunocompromised people and the elderly. The riskiest foods include soft cheeses, unpasteurised milk, salads and certain types of processed meat and fish.
This is the third significant outbreak around the world in the past several years. South Africa experienced the largest ever incidence in 2017-18, when more than 200 people died after eating a contaminated ready-to-eat sausage called polony. The outbreak led to product recalls from some 15 countries in the region.
Between 2015 and 2018, an outbreak in Europe killed ten people and infected more than 50. It was caused after a vegetable processing facility in Hungary contaminated frozen products being shipped to numerous countries, including the UK, Finland and Austria. The bacteria persisted despite the manufacturers following all the regulations on sanitation.
Disasters such as these could probably have been avoided if samples of these factory food products had been pre-tested using the latest genome sequencing technology. This has yet to filter down to industry – because the costs are still too high – so food manufacturers are still relying on standard laboratory bacteria tests. The good news for the future is that this is soon likely to change.
How Listeria spreads
L. monocytogenes lives in soil, water courses and plants as an organism known as a saprophyte, which lives off dead and decaying matter. It also exists in the gut of animals – including humans – often causing no symptoms and then being passed out in faeces. Because fresh vegetables are grown in tight spaces, it is difficult to prevent them being contaminated from these sources – due to the likes of soil splash and interactions with wild animals in the field.
To become a problem for the food industry, there usually needs to be a contaminated “pinch point” in the manufacturing process that can spread bacteria to a large volume of products. In the Hungary outbreak, for instance, L. monocytogenes matching the culprit strain were traced to the floor drain of the processing factory and to spinach from a vegetable grinder.
One reason why listeria bacteria are so good at surviving in the food supply chain is because they form bacterial biofilms, in which the microorganisms stick together in a matrix of polymeric substances that they secrete from their bodies. These biofilms are resilient enough to survive a fair amount of antibacterial punishment, even on man-made surfaces such as stainless steel or polystyrene.
At the same time, recent evidence suggests that L. monocytogenes enter a protective state under stress, such as when a different detergent is used against them. In this state it is harder to eradicate them. They are also able to tolerate fridge temperatures remarkably well. In all, they are much hardier survivors than other bacteria such as E. coli or Salmonella and demand greater cleaning and sanitation in food factories to keep them at bay.
The testing regime
By law, food manufacturers in the UK and many other countries currently have to send food and environmental samples for microbial analysis. If they supply a major supermarket or high street restaurant chain they are probably required to exceed minimum requirements and send more samples to meet more stringent safety policies.
These tests will flag up bacterial contamination, but they are of limited value in relation to listeria. Because listeria is able to live almost anywhere, you can expect to find a certain low level of L. monocytogenes. It only becomes a cause for concern when the level increases over a short period of time.
Yet these standard microbial tests are a hit and miss method of spotting contaminations. The trouble is that it can often be difficult to see whether there is a problem without looking at the genetics of the bacteria: in “safe” food samples, the listeria will be genetically diverse, whereas in an outbreak they will be predominantly the same subtype. Standard tests only tell you whether you have L. monocytogenes or not – to get the genetic information, you need genome sequencing.
Standard lab tests also don’t give you any information about the source of the bacteria. For this reason, investigators have already started using genome sequencing during outbreaks including the ones I mentioned earlier. In a situation such as the pan-European outbreak of 2015-18, without genome sequencing the food suppliers may have had to shut down many more units for intensive cleaning before they could operate again, losing much more money in the process.
Unfortunately, genome sequencing tests can cost something like five times the current going rate for standard tests. This effectively prohibits its use in pretesting – all the more so for small suppliers with shallower pockets.
Ten years from now it could be a very different situation. We have already seen the way that the price to generate the raw data for a bacterial genome has fallen from US$50,000 to US$1 (£39,735 to £0.79) in the past decade. The technology has already become standard for the likes of researchers, which is usually a sign that it will reach industry a few years later. In the meantime, standards are rising anyway – operators in the supply chain are constantly reviewing their safety practices with a focus on consumer safety. These include increased microbial sampling and more sanitation and disinfection programmes.
So while listeria is a tough opponent for the food manufacturing industry, its days as a serious threat could well be numbered. Fairly soon, food manufacturers and their retailers are going to face a choice: pay for genome sequencing technology or save money and risk an outbreak, the result of which is a catastrophic effect on public health and the possible end of your company. In the end, they probably won’t need to think about it for very long.
Alva Smith does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.