MICROBIOLOGY
Why presumptive pathogen results occasionally don’t confirm 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
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. 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
36 September 2021 www.sandwichandfoodtogonews.co.uk
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
