Escherichia coli O157:H7

Escherichia coli takes its genus name from Theodor Escherich, who first described the gut bacterium in 1885, and the designation O157:H7 is its antigenic type, the O antigen 157 and the H antigen 7. Most E. coli are harmless gut residents; this one is not. It belongs to the Shiga toxin-producing E. coli, or STEC, also called verotoxigenic, and when it causes bloody illness, enterohaemorrhagic. Its main virulence factor, Shiga toxin, was acquired from a bacteriophage that inserted the toxin gene into an otherwise ordinary coliform. In the laboratory the strain is recognised partly by its failure to ferment sorbitol, which most E. coli ferment.

It was first recognised as a human pathogen in 1982, when two outbreaks of haemorrhagic colitis in Oregon and Michigan were traced to undercooked hamburgers. Through the 1980s and early 1990s it became known as a hazard of ground beef. Then, in December 1994, an outbreak with direct relevance to curing occurred: a commercially distributed dry-cured salami in Washington and California caused about two dozen illnesses, around one in eight of them developing haemolytic uraemic syndrome. Fermented sausages had until then been considered inherently safe because of their low pH and low water activity. This organism showed otherwise: it has an inducible acid-tolerance response and survives the acidity that was assumed to control it, as it does in apple cider and mayonnaise. In response the US FSIS required, from the mid-1990s, that makers of fermented sausages validate a five-log reduction of E. coli O157:H7 in their process, and no further outbreak from United States salami has been reported since.

A small outbreak changed the rules because the organism is unusually dangerous. Its infective dose is very low, on the order of tens of cells, so ordinary contamination is enough. Shiga toxin damages the lining of small blood vessels, and beyond the initial bloody colitis it can cause haemolytic uraemic syndrome, a destruction of red blood cells and acute kidney failure that is the leading cause of kidney failure in young children, and, less often, thrombotic thrombocytopenic purpura.

**How it endangers cured meat.** It is central to the safety of fermented sausage, and its lesson is the same as for Salmonella but more demanding: a low pH is not by itself a barrier, and a dry, acidic salami cannot be assumed safe. Because the organism survives acid and drying better than expected, safety depends on a validated process that achieves the five-log reduction through the combined effect of fermentation, drying, time and, where used, heat, demonstrated for the actual recipe rather than assumed from the finished product. It is the reason a fermented sausage is no longer treated as safe without a validated process.