Last week saw yet another large recall of cooked meat products in the USA as a result of concerns over Listeria contamination. On this occasion Gusto Packing, based in Illinois, took the decision to recall more than 67,000 pounds of “spiral sliced ham” products after routine sampling turned up positive results for Listeria monocytogenes. Fortunately, no related illnesses have been reported and there is no suggestion that the recalled ham could be linked to an outbreak of listeriosis. This event is not unusual – similar recalls are a regular occurrence in the USA – but it does serve to highlight the different regulatory approaches to Listeria contamination in Europe and North America.
In the EU, the rules state that Listeria monocytogenes cannot be present in a ready-to-eat product at levels above 100 cfu/g at the end of shelf life, whereas in the US a ‘zero-tolerance’ policy does not permit any amount of Listeria to be present in ready-to-eat foods at any point. There has been much inconclusive debate over which approach is the more effective, but what can be said is that the regulatory differences do influence how manufacturers meet the challenge of Listeria. In Europe, the onus is on food businesses to understand the microbiology of their products and set shelf lives that do not allow the pathogen to multiply to dangerous levels (greater than 1,000 cfu/g) before the food is consumed. That, plus effective control of contamination in processing areas, is considered an effective way to protect consumers, and there have been relatively few listeriosis outbreaks associated with processed foods in Europe over the last 10 years. In the USA by contrast, manufacturers must do everything possible to prevent contamination during processing, whether or not the product supports the growth of Listeria. In an ideal world this would be the safest approach, but there are drawbacks. The need to be sure that the processing areas and products are free from contamination demands lots of sampling and testing, and any positive result requires action, hence the large number of product recalls. Furthermore, in reality a ‘zero tolerance’ policy dictates that Listeria be eliminated from processing areas to prevent contamination occurring. But food microbiologists will tell you that such a goal is almost impossible to achieve. Once Listeria is established in a food factory, it can be controlled, but probably not completely eliminated.
Part of the reason for this is the formation of resistant biofilms, complex microbial communities that develop on surfaces and protect their inhabitants from the hostile environment around them. Last week saw the publication of new research showing how much more resistant Salmonella cells are to common biocides when they are present in a biofilm, even a newly established one. Some strains of Listeria also form biofilms very readily and they are very difficult to remove once present. The authors of the Salmonella study suggest that biocides should be evaluated on their ability to kill bacteria in biofilms as well as suspended in liquid. That comment applies even more to Listeria in my view. More effective decontamination methods that take account of the microbial ecology of food processing plants would give food manufacturers a better chance of making the ‘zero tolerance’ approach to Listeria work in the real world.
If you would like to know more about biofilms, see our Food Safety Watch feature article on the subject