10 minute read
Why presumptive pathogen results occasionally don’t con rm.
The latest in a series of advisory articles from ALS, written by Andy Muirhead, ALS company microbiologist.
QUESTIONS TO BE ANSWERED
At ALS we fully understand the consequences to our clients when we issue a presumptive positive result for a pathogen such as Salmonella or Listeria monocytogenes.
Our clients will often have to execute a response which may involve quarantining the affected product, carrying out a deep clean of the affected lines and arranging additional environmental monitoring and a raw material investigation. In some cases, they may even have to contemplate a product recall.
We therefore understand that although there will be a huge amount of relief there is often understandable frustration when the presumptive result sometimes fails to confirm and the lab issues a result of “not detected”, meaning that all of the additional work was potentially unnecessary. We are often asked “what has gone wrong?” or “why can’t the test be more specific so that only genuine positives will trigger the presumptive reaction?” In this article I will try and explain that presumptive positive pathogen results which subsequently fail to confirm are a natural consequence of the selectivity of the test and that no-one has necessarily done anything wrong.
DETECTION PROCESS
A presumptive pathogen in a ready to eat product is every food manufacturer’s worst nightmare. Whilst such occurrences are almost certainly randomised, they do seem to happen late on a Friday evening on a Bank Holiday weekend when half of our clients’ senior managers and technical team are on holiday!
In order to understand what has happened in the test that has triggered the presumptive result, we need to acknowledge that the testing method has been designed to give ourselves the best chance of detecting the target pathogen, and there are two very important factors that we need to take into account.
Firstly, when we are trying to detect pathogens in food samples, our job is made difficult because the bacterial cells we are trying to isolate are often ‘stressed’. Compare this to clinical microbiology where the target organism has temporally overwhelmed the hosts immune system and is growing rapidly and vigorously and is therefore relatively easy to detect.
In food microbiology the pathogens we encounter have often been either heated, chilled, frozen, dried, been subjected to the actions of preservatives, been placed in acidic conditions or had salt and sugar added to lower their water activity. It’s little wonder therefore that these organisms are described as ‘stressed’ and require an initial resuscitation step to help them recover from the rigours of everything which is involved in food manufacturing. The second point to consider is that if pathogens such as Salmonella are present in any food matrix, they are often only there in relatively low numbers and are usually outnumbered by other organisms. This makes our job more difficult as they may outcompete and reduce our chances of
BACKGROUND
ALS Laboratories (UK) Ltd (www.als-testing.co.uk) is one of the UK’s leading providers of food and drink testing services. With six accredited laboratories located across the country, they offer a comprehensive range of high quality, analytical testing services, including microbiological, nutritional, vitamins and minerals, pesticides and contaminants, allergens and speciation. They also provide clients with a wide range of consultancy services and technical support on food safety, labelling requirements, allergens management and sensory testing.
fi nding the target pathogen. For this reason, the second step in the method is a selective enrichment step whereby (again using Salmonella as our example), we transfer our culture into a broth containing chemicals or antibiotics and incubate in conditions which we know Salmonella can tolerate but which may inhibit the growth of the other competing organisms. It is a fi ne line between establishing selective conditions which will inhibit all of the background organisms but still allow the ‘stressed’ target organism to grow and become detectable. Because of this, these other non-target organisms can sometimes survive and grow and potentially mimic the true target organism. If the initial screening test was therefore so specifi c that only a genuine Salmonella would trigger the presumptive reaction, we would run the risk of missing many of the ‘stressed’ organisms which we frequently encounter in food microbiology.
IS IT PATHOGENIC?
We are often asked, “if it’s not Salmonella, then what is the organism which has triggered the presumptive result and is it therefore pathogenic?” The answer to this is that in most cases it is a closely related fellow member of the Enterobacteriaceae group and just because the organism in question may share some of the antigenic or biochemical properties of Salmonella, it does not share any of its pathogenic properties. It’s a bit like when the police assemble an identity parade. All of the suspects could be the guilty person (or pathogen), but in reality, all bar one of them are innocent (or harmless), and the innocent ones just happen to resemble the appearance of the guilty party. We have used Salmonella as an example, but the same principle applies to all other pathogens where we have an initial presumptive detection stage followed by the identifi cation and confi rmation step.
To complete the police line-up analogy, we then look for other clues such as biochemistry, serology, molecular profi ling, mass spectrometry to enable us to fully confi rm the presumptive detection in the same way the police would look at fi ngerprints and DNA profi ling to match suspect to the crime, or in our case, the pathogen to the food. It is only when these confi rmatory tests are complete that we can issue a fully confi rmed result, and occasionally the confi rmatory tests reveal that the organism which has triggered the presumptive result is not the target pathogen after all, and this is when the fi nal result is released as “not detected”.
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