Listeria monocytogenes

The genus name honours Joseph Lister, the surgeon who pioneered antiseptic surgery; the species name, monocytogenes, records that the infection produces a rise in monocytes in the blood. Everitt Murray, working in Cambridge, isolated it around 1924 from laboratory rabbits and described it in 1926 as Bacterium monocytogenes. Harvey Pirie later proposed the genus Listeria in Lister's honour, and the name Listeria monocytogenes was settled by 1940. It was recognised as a food-borne disease only in 1981, after an outbreak traced to coleslaw.

It is a gram-positive, non-spore-forming, facultatively anaerobic rod, motile at cool temperatures but not at body heat. For the curesmith the important feature is its hardiness in the conditions where cured products are kept. It grows at refrigeration temperature, down to about minus 0.4 degrees C, across a pH range of about 4.4 to 9.5, at a water activity down to about 0.92, and it tolerates salt to ten per cent and beyond. The practical conclusion is direct: cold is not a control, and salt is not a control. Listeria can grow slowly in the cold over the shelf life of a moist product.

**How it endangers cured meat.** It rarely survives a genuine lethality step. Its route into a finished product is recontamination after processing, from the environment, onto the surface of a ready-to-eat food that is then stored cold and eaten without further cooking. The usual vehicles are the curesmith's ready-to-eat lines: sliced cooked and cured meats, pate, and cold-smoked fish. This makes it a matter of Hygiene Control. In the United States the FSIS applies a zero-tolerance standard for L. monocytogenes in ready-to-eat meats, and the practical defence is thorough environmental hygiene to prevent recontamination after any cook or cure, supported by formulation limits and realistic shelf lives. For the maker of pate or sliced product, the risk is in the handling after the heat step, not in the cure.

The reason for this care is the nature of the illness. In a healthy adult it is usually a mild gastroenteritis or causes no symptoms. In pregnancy, in the elderly and in the immunocompromised it can become invasive, reaching the bloodstream and the brain, and is then often fatal, with case-fatality figures of about twenty to thirty per cent. In pregnancy it can cross to the foetus and cause miscarriage, stillbirth or serious illness in the newborn. It is an uncommon infection but one of the leading causes of death from food-borne illness, which is why the standard applied to it is zero.