Colistin is the antibiotic that doctors use as a last resort to wipe out dangerous bacteria.

"It's really been kept as the last drug in the locker when all else has failed," says Dr. Jim Spencer, a senior lecturer in microbiology at the University of Bristol in the United Kingdom.

But now Spencer reports that E. coli bacteria, which can cause kidney failure as well as urinary tract and other infections, have changed. In an article published in the journal Lancet Infectious Diseases, Spencer and his co-authors tell how researchers in China have found that the bacteria not only are increasingly resistant to colistin, but have developed a mechanism to transfer resistance to neighboring bacteria. And those bacteria don't even have to be the same strain as those that originally developed the resistance. So bacteria that cause other health problems could be affected.

The prospect of colistin resistance spreading on a large scale is worrying, says Spencer, because there aren't any good alternatives.

Spencer and his colleagues found the resistant E. coli bacteria in pork, pigs and people in China.

"We found colistin resistance over a relatively large part of the south of the country," Spencer says.

And he suspects it's not happening just in China: "The ease with which we've been able to see this resistance move between bacteria and the high incidence [of resistance] that we saw in this study would suggest to me that it's very likely that once people start looking for this outside of China, they'll find it very quickly. I don't think this is a problem that's local to China at all."

While this transfer of drug resistance has been seen before, Spencer says this is the first time it's been seen for colistin.

David Plunkett at the Center for Science in the Public Interest has been involved for years with the issue of the overuse of antibiotics in livestock production.

Plunkett calls this new report out of China really frightening.

"You're looking at the last line of defense against antibiotic resistance falling," he says. "And the potential for it now to spread not only in China but around the world — you're looking at the potential for untreatable epidemics."

Indeed, public health officials have been warning about the impending "post-antibiotic era" for years. The Centers for Disease Control and Prevention says that annually 2 million Americans are infected with bacteria that are resistant to antibiotics and more than 20,000 die from those infections. Each year the world's arsenal of antibiotics loses a bit more of its strength as bacteria mutate and develop resistance.

The growing resistance stems from the overuse of antibiotics by humans, but also in livestock operations. To keep their herd healthy, farmers often preemptively give antibiotics to their animals even if none are sick.

The Food and Drug Administration has been warning about the problem for decades. The World Health Organization has been lobbying to put restrictions on access to antibiotics globally. Earlier this year President Obama launched an action plan.

Spencer says the world isn't going to lose access to antibiotics in the next few years, but still finds the rise of this resistance troubling. "People reach for colistin because that's the last resort," he says. "When colistin goes, it's very hard to see what the alternative would be."

"Were this to happen on a large scale," he observes, "we'd have to ask whether some surgical procedures would simply be too dangerous because of the risk of infections that we are unable to combat effectively."

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