A study led by researchers at the University of Edinburgh and Public Health England has discovered that strains of E. coli O157:H7 became dangerous human pathogens about 30 years ago when they acquired the ability to produce the stx2a shiga-toxin.
The researchers studied sequenced genomes obtained from over a thousand E. coli O157 cultures isolated from both humans and animals over the last 30 years. They applied a technique called phylogenomics to the data to try and understand how highly pathogenic strains of the bacteria developed.
The results showed that a common ancestor of known E. coli O157 strains emerged more than 175 years ago, but carried only the stx1 type of shiga-toxin. Some strains acquired the ability to produce stx2a toxin about 60 years ago and those associated with serious human illness then acquired the same gene in the 1980s when foodborne outbreaks began to appear.
More recently, E. coli O157 strains that produce only stx2a toxin have been isolated from human infections. It is feared that these may be more dangerous still.
A report of the study is published in the journal Microbial Genomics and can be found in full here.