Salmonella

The genus is named after Daniel Elmer Salmon, the American veterinary pathologist who led the United States Department of Agriculture's animal-disease research in the 1880s, though much of the founding work was done by his assistant, Theobald Smith. In 1885 Smith isolated from pigs a bacterium then thought to cause hog cholera, later shown to be unrelated to it, which became the first described member of the genus. The typhoid organism had been seen earlier, by Eberth in 1880 and cultured by Gaffky in 1884. The genus name Salmonella was applied around 1900. It is a gram-negative, motile, non-spore-forming, facultatively anaerobic rod of the family Enterobacteriaceae, with more than two and a half thousand serotypes classified by their surface antigens.

The serotypes fall into two groups. The typhoidal Salmonellae, chiefly Salmonella Typhi, are restricted to humans and cause typhoid and paratyphoid fever. The non-typhoidal Salmonellae are the ones relevant to curing: they are zoonotic, carried in the gut of poultry, pigs and cattle and shed onto meat, and they cause the acute gastroenteritis usually meant by food poisoning. The organism invades the cells lining the gut by injecting proteins through a needle-like secretion system.

**How it endangers cured meat.** Salmonella is present in the raw meat, so the process must remove it rather than only hold it in check. Refrigeration stops it multiplying but does not kill it, and no single hurdle in a fermented or dried product can be relied on to eliminate it. The standard used for shelf-stable ready-to-eat fermented and dried products, set by the US FSIS, is a five-log reduction of Salmonella across the whole process, a hundred-thousand-fold reduction, achieved by the combined effect of the fermentation acidity, the fall in water activity through drying, the time held, and heat where the process uses it. The important word is validated: the maker must show that a given recipe and schedule achieve that reduction, rather than assume that a dry, acidic sausage is safe. This is worth distinguishing from the degree-hours control of the ferment, which governs the warm window for Staphylococcus aureus specifically and does not on its own address Salmonella, the reason the five-log reduction is validated across the whole process rather than read off the fermentation clock.

The illness, salmonellosis, is a self-limiting gastroenteritis in healthy people and a more serious, sometimes invasive threat to the very young, the old and the immunocompromised. The organism is readily killed by heat, at about 57 to 60 degrees C given time, so a cooked product is straightforward and a raw fermented one is the harder case, the one the five-log rule addresses.