By Marian Turner of Nature magazine
Women, beansprouts, cucumbers, bacteria, cows: the cast of the current European Escherichia coli outbreak is already a crowd. Enter the phage. Bacteriophages are viruses that infect bacteria, and they are star players in the chain of events that led to this outbreak.
Bacterial infections often originate from contaminated food, but it is now about six weeks since the start of this outbreak and the trail is going cold. It's hard to be sure of the culprit--but this simply serves to highlight the importance of understanding how infectious bacteria get into the food chain in the first place.
Case-control studies of patients in the German outbreak pointed to salad vegetables, and both cucumbers and beansprouts have been suspects. It is possible that the vegetables were contaminated with bacteria originally carried in soil or water; but the more likely source of the bacteria is animals. Pathogenic E. coli are typically passed to humans from ruminant animals (cows or sheep) via fecal contamination in the food chain or through consumption of raw milk or meat products.
But how do pathogenic E. coli arise in the first place? This is where bacteriophage come in. The bacterium in this outbreak, currently recognized as strain O104:H4, makes Shiga toxin, which is responsible for the severe diarrhea and kidney damage in patients whose E. coli infections develop into hemolytic uremic syndrome (HUS). The genes for the Shiga toxin are not actually bacterial genes, but phage genes being expressed by infected bacteria. So when an E. coli bacterium gets infected with a Shiga-toxin-producing phage, it becomes pathogenic to humans.
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http://www.scientificamerican.com/article.cfm?id=phage-key-to-european-deadly-ecoli-outbreak
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