^BC-HEALTH-SUPERBUGS
^Superbugs may mutate into stubborn forms -study
WASHINGTON (Reuters) - New antibiotic-resistant strains of bacteria can mutate rapidly into even more hardy forms, Swedish researchers have reported.
The so-called superbugs are hard to wipe out, but are not particularly virulent and do not usually cause severe illness.
They are usually found in hospital patients with depressed immune systems who have received many drugs.
But Johanna Bjorkman and colleagues at the Swedish Institute for Infectious Disease Control found that some mutant strains could evolve into more virulent forms.
Writing in the latest issue of the Proceedings of the National Academy of Sciences, they said salmonella bacteria that they grew mutated into dangerous forms after only 18 to 36 generations -- a very quick time for bacteria.
They used streptomycin, rifampicin and nalidixic acid against bacteria infecting the livers and spleens of laboratory mice. The drug-resistant salmonella bacteria they used were usually harmless in mice, they wrote.
But the drug-resistant forms quickly mutated into forms that were just as virulent as ``wild-type'' bacteria.
``Over the last decade, there has been an alarming increase in the appearance of antibiotic-resistant bacteria as a result of an increased use of antibiotics combined with the exceptional ability of bacteria to develop resistance,'' they wrote.
Experts say the drug-resistant superbugs evolved because antibiotics have been used too widely. When bacteria are exposed to enough antibiotic to kill some but not all of them, the ones that survive develop resistance to the drug.
One strategy would be to use fewer antibiotics, but such an approach assumes that the resistant bacteria are less fit than more common strains. That might not be so, Bjorkman's team found.
``We infer from these results that a reduction in the use of antibiotics might not result in the disappearance of the resistant bacteria already present in human and environmental reservoirs,'' they wrote.