Food Poisoning Bulletin: E. coli O157:H7 evolved to become deadly 30 years ago, according to a new study by researchers at the University of Edinburgh and Public Health England. The study explains why deadly outbreaks began in the 1980s and demonstrates why the bacteria should be monitored closely, researchers say.

E.coli bacteria have been living in the intestines of people and animals for at least 175 years. Most strains are harmless. But some, such as of E. coli O157:H7, produce shiga-toxins, which cause serious infections in humans. Of the two types of these shiga-toxins they identified, stx1 and stx2a, researchers day stx2a causes the most serious illnesses.

Although they show no signs of the disease, cows are the main source of E. coli O157. Animals infected with strains producing the stx2a shiga-toxin excrete higher levels of dangerous bacteria in their manure, increasing the odds of spreading disease.

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