Listeria monocytogenes was found in 25 g of the following food samples examined in 2013: in four of 1,058 samples of milk, dairy products and cheese; in seven of 222 samples (3.2 %) of fish and fish products. A total of 21 of 855 samples taken from meats of different animal species (both raw and cooked, ready-to-eat and not ready-to-eat) contained Listeria monocytogenes, although the share of listeria-positive samples in the various meat samples (beef, pork, mixed meats and meat products) was distributed equally, with the exception of pâté products (3 out of 24 samples). L. monocytogenes was only detected in 1 % of other food samples (7 out of 497) taken from baked goods and pastries, ready-to-eat products, vegetables, salads and sauces.
More than 100 colony forming units of L. monocytogenes per gram (cfu/g) were found in one sample of goat cheese, one sample each of ready-to-eat food and meat contained 10-100 cfu/g of L. monocytogenes. L. monocytogenes was found in quantities of fewer than 10 cfu/g in all other food samples that tested positive for listeria (detectable in 25 g).
Food as a Reservoir for Pathogens
Some foods are mainly listeria-free as long as there has been no prior surface contamination or contamination after the opening of the package. Untreated food, such as carrots, tomatoes and acidic fruit (e.g. apples, pears) have very low contamination risks, in particular after washing or peeling when a possible surface contamination could be removed.
Food can be contaminated with listeria during a number of production and processing stages. Animal based foods, such as unpasteurised milk and raw meat, may already be contaminated during production -- e.g. milking or slaughtering. In the case of cheese made from unpasteurised milk, the contamination of the original milk cannot be excluded as the cause of listeria in the finished product. However, there are additional sources of contamination after the heat treatment of products, if hygiene standards during the manufacturing process are neglected. In most cases, the type of contamination relevant to infections occurs during the maturing process when listeria can settle on the rind. The listeria can proliferate considerably in cheeses with soft, spreadable rinds during maturing. Often, they do not spread evenly over the entire surface, but in patches of microcolonies.
The ability of listeria to survive and multiply in food depends on technological treatments and manufacturing process. Boiling, roasting, sterilising and pasteurising kill off the bacteria. Foods containing little water, lots of salt or preservatives, or foods that are very acidic (e.g. sauerkraut, mixed pickles and yoghurt) delay or even completely inhibit bacterial proliferation. Unlike other, competing bacteria, listeria has the ability to grow in anaerobic environments (e.g. in vacuum packages of sausages, salmon and smoked fish) and in foods that are refrigerated for longer periods.