Until the early 1980s Listeria was an obscure bacterial genus few people had heard of. Back then it was of interest only to a handful of medical microbiologists and seemed to have no relevance to food safety whatsoever. A lot has changed in the last 30 years. Listeria is now a familiar name and a constant concern for food microbiologists. There are two main reasons for the change. The first is the potentially serious consequences of foodborne listeriosis – outbreaks often result in mortality rates approaching 30%. The second reason is the ability of certain strains of Listeria monocytogenes to survive and grow in chilled foods with a long shelf life. These foods, like soft cheeses, pâté and other ready-to-eat meat products, were once thought to be low risk from a food poisoning point of view. But the combination of low temperature, the presence of salt and the elimination of competing bacteria by mild cooking processes create an environment that Listeria is able to exploit – with disastrous consequences.
Since Listeria monocytogenes was first recognised as a foodborne pathogen it has become one of the most studied of bacterial species. A very recent publication by a team of researchers in Ireland reports the identification of a single point mutation that helps Listeria cells to withstand stresses caused by cold and raised salt concentrations. The same study also discovered that cells adopt a previously unknown change in shape in response to stress, becoming helical rather than rod-shaped. The fact that a single mutation confers an enhanced ability to grow in preserved chilled foods on individual Listeria strains is clearly important and could open up a new area of study for food microbiologists. It has long been known that strains of Listeria monocytogenes can behave very differently. Some are virulent pathogens, while others seem not to cause infection. A few specific strains are able to colonise food processing environments and persist – defying all attempts to eradicate them – while others die out quickly. Only a limited number of strains seem able to grow to high levels in foods, and the Irish research is helping to show why that may be.
What this illustrates is the importance of continuing to study foodborne pathogens. For example, we still know relatively little about Campylobacter, despite it now being the number one cause of food poisoning in much of the developed world. Scientific research is the lifeblood of designing effective food safety controls. It’s as important as intelligence gathering in time of war, and does much the same job.