I'd be willing to bet the ultimate source of the contamination is migratory "guest workers" sh*tting in the fields where the crops are harvested, apparently in Spain. Another reason I'm glad I don't eat salad, that's for rabbits!
That would explain ordinary (treatable) e.coli
I am not implying terrorism nor a conspiracy with this post. I am noting that yet another untreatable variant of an otherwise common infection (staph would be another example) has popped into existence.
There are two factors at work here. One is nature. Infectious diseases continue to mutate into new forms that we do not have treatments for. The other is the artificially slow rate at which new antibiotics* are developed and released. The government bureaucracy and its over regulation of everything is like a large weight around the legs of the biotech industry, preventing them from keeping up with nature.
Fortunately neither staph nor e.coli are a threat to become a pandemic as both of them (currently) are not airborne infections like, say, the flu. But it is only a matter of time before we encounter another pandemic situation. History is full of them and we exist in the modern world with a false sense of security as to medical technology's ability to deal with just about anything.
*Antibiotics, of course, have no effect on viruses. Viral infections on a pandemic scale are the ultimate nightmare. The Spanish flu of 1918 killed 50 million people worldwide, over a half million in the US. No treatment can kill a virus. If your body's defense system can't handle the infection, you die. We have learned a lot about how diseases are transmitted since then so we are able to prevent a lot of these big outbreaks from occurring. However, we also have much denser population centers now which can hinder some of these preventive measures. Being stuck on an airliner, for example, with an infectious person could be potentially disastrous because each person could unknowingly get on a connecting flight and spread the disease further than anything imaginable at the turn of the last century. There are various factors involved such as the incubation period of the particular strain and whether or not someone is contagious during that time. We haven't had a really big pandemic in a long time and as they say, nature abhors a vacuum.