It’s been in the news yet again, another incident of people becoming infected by Listeria monocytogenes and sadly succumbing to the infection (https://www.food.gov.uk/news-alerts/news/update-on-investigation-into-food-supply-chain-linked-to-listeria ). The source identified, as reported by the FSA, was sandwiches supplied to hospitals. In a way I think ‘nothing new there then’ as this has happened in the past; the epidemiology of listeriosis outbreaks in the UK seemingly intertwined with sandwiches (see a table produced in 2015 in https://www.researchgate.net/publication/289345195_Listeria_monocytogenes-Developments_in_epidemiology_and_laboratory_testing_by_MALDI-TOF ). Also remembering a Finnish listeriosis outbreak in which butter was the contaminated food, the scientist in you begins to ask whether it is terribly wise to supply sandwiches to people whose immune systems are not functioning properly? After all, a sandwich may contain all sorts of ingredients that could be contaminated because it is simply not possible to use an effective kill step that allows food, like lettuce, to retain its original eating qualities.
Although the FSA has said that the company was not the source of the recent outbreak the company has apparently ceased trading (https://metro.co.uk/2019/06/28/sandwich-company-wrongly-linked-listeria-scandal-left-5-dead-goes-bust-10087592/ ), another unwanted consequence of the outbreak. The question remains then, if the company was not the source of the outbreak then who was? Was it the sandwiches after all? Time will tell, and hopefully we will see the results of the investigation so that lessons can be learned.
Despite all this information about the risks associated with L. monocytogenes in sandwiches, the non-scientist in me needs to have a say. My wife had the misfortune to end up in a cardiac intensive care unit. It was all touch and go for a couple of days, but she survived (thank you Castle Hill hospital). As I visited, it became clear to me how much patients in the ward liked things to be as normal as possible; their own supply of Yorkshire tea bags on the bedside table for example. After all you’re marooned in an alien environment surrounded by equipment making strange noises, being examined, injected and tested on a regular basis. Is it too much to ask that your food can be as ‘normal’ as possible, such that you can have the odd sandwich if you want? OK, so this is not such a problem with the cardiothoracic ward as immunity is not the key issue, but what if you are in a cancer ward, possibly for long periods-should you be denied a little slice of normality?
This is not a straight scientific problem. How do you objectively weigh one side against the other?
The next question concerns how it might be possible to make sure that Listeria-contaminated sandwiches do not reach the hospital ward. My wife did have a sandwich for tea, but it was a fairly plain affair: bread, spread and cooked turkey slices. I’m not aware of bread being a source of any microbial disease. Butter has been, but it’s very rare. Cooked meat should be easy to make Listeria-free as its cooked and there are well defined time/temperature parameters for eliminating contamination, but most surveys of this food group tend to report low level contamination, probably resulting from recontamination during packaging etc. Listeria is a hardy bug and can colonise food processing plants, proving difficult to get rid of. If it is present on cooked meat it is likely then to be able to grow, increasing in numbers and presenting more of a challenge to anyone who eats it (dose response is an aspect too big to be dealt with here). There is no really good way of removing pathogens from fresh vegetables and, even then, some people are becoming concerned about the by-products of those treatments that do have some effect. New methods are clearly needed, and there are some candidates-my favourite being the use of bacteriophages that kill Listeria monocytogenes.
My last point concerns testing. Testing foods for the presence of pathogens is a very insensitive approach-you just can’t test enough samples to rely on the results (unless the food is badly contaminated). Some testing is needed to make sure that HACCP is operating correctly, but relying on it as a measure of safety is simply not an option. Positive results need to be acted upon-they are significant!