Listeria monocytogenes
Listeria monocytogenes is hardy; it is salt-tolerant and not only can survive in temperatures below 1°C, but also grow in these conditions, unlike many other pathogens. It is also notable for its persistence in food-manufacturing environments. The bacterium is ubiquitous in the environment and can be found in moist environments, soil, and decaying vegetation.
Listeria monocytogenes is most commonly associated with soft, mould-ripened cheese (e.g., brie and camembert) and blue-veined cheese (e.g., blue stilton). It can be present even in cheeses that are pasteurised and they can grow below 4°C!
Bacterial growth is not altogether halted by holding food below 5°C, and while reheating thoroughly until piping hot throughout (not just melted) would kill the bacteria, these foods are often eaten chilled or at room temperature.
Good food rotation practices and careful notation of 'Use by' dates are especially important with foods such as this.
The source of bacteria are dust, soil, silage, sewage, poultry and meat. Listeria is believed to be present in the intestines of up to 70% of humans, even though such carriers show no symptoms. It is transmitted by a wide range of food including milk, milk products, cooked meat, meat products, raw vegetables and seafood. It grows faster in foods which have a high fat content e.g., full-cream milk and full-fat cheeses.
Gastroenteritis caused by Listeria monocytogenes has a relatively short incubation period, from a few hours to 2 or 3 days. The severe, invasive form of the illness can have a very long incubation period, estimated to vary from 3 days to 3 months.
Duration and type of illness: Acute listeriosis is relatively rare in the developed world. It can occur in the very young, the elderly, or those already ill or on immuno-suppressant drugs, such as steroids. In acute cases, recovery is very slow, and the fatality rate is estimated to be between 30% to 50%.
Symptoms: In mild cases the symptoms are similar to mild flu. In acute cases symptoms can resemble those of meningitis or septicaemia (blood poisoning). Infected lesions may also appear on the skin or in a number of sites within the body. It can also cause pregnant women to miscarry.
Control: Good hygiene practices. Ensure that the shelf-life of chilled ready-to-eat products is appropriate for the product. Prevent cross-contamination between raw and ready-to-eat foods. Ensure food contact surfaces and utensils are cleaned and disinfected.