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02-08-2011 | Genetics | Article

Origins of highly virulent German E. coli strain uncovered

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MedWire News: Study results suggest that horizontal genetic exchange led to the evolution of the highly virulent, Shiga-toxin-producing, enteroaggregative Escherichia coli strain (O104:H4) that caused the recent German outbreak.

"This research gives us insights into the reasons why this particular strain of E. coli is so virulent, allows us to hypothesize about the evolution of this bacterium and provides clinically relevant information about the treatment of this infection," said Matthew Waldor from Brigham and Women's Hospital in Boston, Massachusetts, USA.

The team used third-generation, single-molecule, real-time DNA sequencing to completely sequence the strain of E. coli implicated in the German outbreak that began in May 2011 and has since caused 50 deaths.

They also sequenced seven diarrhea-associated enteroaggregative E. coli O104:H4 serotype strains from Africa and four enteroaggregative E. coli strains from other serotypes for comparison purposes. The sequence results were also compared with 40 previously sequenced strains of E. coli.

Eric Schadt (Pacific Biosciences, Menlo Park, California, USA) and colleagues explain that this strain is unusual, as it closely resembles an enteroaggregative E. coli strain, but appears to have acquired a Shiga-toxin-encoding prophage, which is normally a feature of the Shiga-toxin-producing E. coli group. They say that horizontal gene transfer is likely to account for the unusual grouping of virulence and antibiotic-resistance factors seen in this strain.

"Based on our understanding of the genetic profile of this E. coli strain, we would suggest caution in the use of certain antibiotics to treat these infections," Waldor said.

He explained that this is because certain antibiotics, including ciprofloxacin, trigger expression of the gene encoding the Shiga toxin leading to increased production of the toxin.

"This analysis also emphasizes the importance of the exchange of DNA between bacteria in the emergence of new pathogens," Waldor commented.

"There is evidence that the outbreak strain acquired many genes by horizontal genetic exchange, which means that bacteria gave DNA not only to the bacteria that they reproduce, but also to neighboring bacteria."

The results of this study are published in the NEJM.

By Helen Albert

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