Listeria monocytogenes



  • A bacterium causing illness which grows in soft ripened cheeses (unpasteurized milk has been wrongly implicated), pâtés, shop-prepared salads, etc. It will grow in these foods at less than 4°C with a doubling time of 18 hours. Samples cultured at 37°C normally have a doubling time of 7.4 hours but after subjecting them to cold shock at 4°C the doubling time drops to 2.5 hours, at the same time refrigeration appears to select for more virulent strains. The incubation period is up to 4 weeks and the resulting illness can range from a general feeling of malaise to meningitis and septicaemia. It is more likely to cause stillbirths and miscarriages, and for this reason pregnant women are recommended to refrain from the food items mentioned unless home cooked. It does not grow in raw milk farmhouse cheeses due to the low pH (less than 5), and better hygienic practices. Subsequent pasteurization cannot be used as a let-out, and there is more rapid transfer of the milk from cow to cheese making.