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Infections Now Resistant to Old Antibiotic

Polymyxin B Joining List of Antibiotics That Are Facing Resistance From Infections
WebMD Health News
Reviewed by Louise Chang, MD

Oct. 30, 2009 (Philadelphia) -- An old antibiotic, largely abandoned after causing kidney ailments some 50 years ago, has become the treatment of last resort for some drug-resistant infections.

But now there are signs that bacteria are developing resistance to this antibiotic, called polymyxin B, as well, says Jason Kessler, MD, of Columbia University in New York.

Polymyxin B is active against a variety of bacteria that cause respiratory and urinary tract infections, Kessler says.

It acts like a detergent, destroying cell membranes and killing the bacteria, he says. But that proved "quite toxic," particularly to the kidney, he says.

As a result, many doctors stopped using it shortly after it was introduced in 1960s.

But the fact that it has not been widely used has an upside: Many bacteria, such as Pseudomonas aeruginosa and Klebsiella pneumonia, which are resistant to other classes of antibiotics, have not developed resistance to polymyxin B, Kessler says.

For the study, Kessler and colleagues looked at blood samples and other patient specimens tested in their microbiology lab between 2005 and 2008.

"Over 30% had resistance to at least five classes of antibiotics, meaning they could probably only be treated with polymyxin B," he says. While the portion of specimens that showed resistance to polymyxin B in 2008 was still low, at 6%, that's a 50% increase from 2006, Keller reports.

He presented the findings at the annual meeting of the Infectious Diseases Society of America.

“Our current experience has been that the drug is relatively well-tolerated,” Kessler adds.

That could be because lower doses are used or because doctors can now better manage kidney problems, says Neil Fishman, MD, of the University of Pennsylvania. Fishman moderated a news conference at which the findings were discussed.

Fishman says that when he went to medical school in the early 1980s, polymyxin B was never mentioned, except for this warning: "They’re toxic and you’ll never have to use them."

"Then, about two and one-half years ago, I did have to use them,” he tells WebMD.

"Because they are old antibiotics, there aren’t a lot of companies making them and their use in increasing," Fishman adds. "It's getting increasingly difficult to get them."

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