E. Coli, Bacteriophages, and Antibiotics

The German E. coli outbreak is a warning about the unpredictable effects of our interventions in the microbial world. Nature News reports:

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 faecal 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 [viruses that infects bacteria] come in. The bacterium in this outbreak, currently recognised as strain O104:H4, makes Shiga toxin, which is responsible for the severe diarrhoea and kidney damage in some patients. 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.

Our use of antibiotics may be helping those viral genes to spread. If bacteria are exposed to some types of antibiotics they undergo what is called the SOS response, which induces the phage to start replicating. Active replication of the phage causes the bacterial cells to burst open, which releases the phage. It also releases the toxin, which is why antibiotics are not usually used to treat E. coli infections.

Strain O104:H4 is resistant to many antibiotics, so probably it was exposed to them in livestock or environmentally. In laboratory experiments, exposing bacteria to sub-lethal doses of antibiotics causes them to exchange phage genes. It has not been confirmed that this happens in the wild, but if it does, it is another really good reason to stop giving antibiotics to domestic animals.

And yes, that really is an electron micrograph of viruses attacking a bacterium. Sometimes the power of our scientific instruments astonishes me.