Bacteria resist new class of antibiotic

The ongoing struggle against bacterial infection received another setback this month when it was revealed that MRSA, the hospital superbug, has shown resistance to a new class of antibiotic launched less than two years ago.

Staphylococcus aureus, otherwise known simply as staph, are bacteria carried harmlessly on the skin by many people but can cause potentially fatal infections in frail or elderly people. Once it was regularly routed by penicillin, but it has rapidly evolved to overcome every antibiotic drug placed in its way.

The so-called superbug has mutated to resist methicillin, a powerful cousin of penicillin. There are now an estimated 80 000 to 100 000 patients infected with MRSA (methicillin-resistant staphylococcus aureus) in British hospitals every year. They are usually treated with the even more powerful vancomycin, but the bug has now shown its ability to evolve past the reach of that antibiotic too.

At a time when few new antibiotics are being developed, linezolid, sold by Pharmacia as Zyvox, was welcomed when it was introduced to the United Kingdom in January last year. It has been used in only about 10 000 patients this year in the UK because doctors want to preserve it against resistance.

Last week, however, the public health laboratory service (PHLS) revealed that linezolid has failed in a case of MRSA developed by a 52-year-old man who had been operated on for suspected lung cancer at a hospital in London. It is the first case of resistance to linezolid in Europe and the second in the world — one was reported in the United States last year.

David Livermore, director of antibiotic resistance monitoring at the PHLS, said it was worrying, even though the man was later successfully treated with an older antibiotic. “One is always concerned when any new resistance emerges,” he said. “The experience tells us that every antibiotic ultimately does select some resistance for itself. It is how evolution operates.’’

But the two failures of linezolid are more alarming because of the rarity of new antibiotics and the uphill struggle in hospitals against infection. If doctors and nurses washed their hands after every contact with a patient, the spiralling number of infections in the wards could be cut by a third, said Peter Wilson, consultant microbiologist at University College hospital, where the resistance was identified.

Livermore said the problem had to be tackled on three fronts: better control of infection in hospitals and better use of the antibiotics. Doctors should not hand them out for coughs and colds, which are probably caused by a virus and not bacteria. They should also be careful to prescribe the right dose for the right duration. But — at a time when all the money and interest are in diseases like cancer — there should be more attention to developing new antibiotic drugs.

“The development of new antibiotics is essential because, however well we use antibiotics and however good at stopping the spread of infection, I don’t honestly believe we can beat evolution,’’ he said.

But the big drug companies have been dropping out of research because it is not a very profitable field, given the chances of resistance making the drug useless within a relatively small number of years. Livermore said: “Wyeth in the US a fortnight ago laid off 90% of their research staff on anti-infectives. There is an increasing number of mergers.’’

At the moment, according to Pharmacia, linezolid earns about 15% of all the money spent on drugs for serious MRSA in the UK. It is expensive, at $104 a day, although it can be taken orally by a patient who needs to get home from hospital, and will thus save a lot of money.

The man who developed a linezolid-resistant form of MRSA took the drug for three weeks and at first it seemed to clear up the infection in his chest. But when the infection reappeared, tests showed that he had resistant strains of MRSA in his body.

Luckily, the resistant bacteria did not spread to anybody else. That is the scenario that most worries doctors.

“This has very much repeatedly been the story of staphylococcus aureus — that it has developed resistance and then, through breakdown of infection control, it has spread between patients,’’ said Livermore. — Â

 

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