Glasgow - Medical experts have long assumed that misuse of antibiotics breeds drug-resistant superbugs, but a recent study has finally provided smoking-gun proof as to how this occurs.
Bacterial diseases that were once easy to treat, such as tuberculosis, gonorrhoea, typhus and pneumonia, are becoming ever-tougher challenges as germs evolve into strains that outstrip science's ability to keep up.
The new research provides hard evidence that antibiotics which are mismatched to the type of bacteria being targeted can hike this worsening resistance problem.
In the experiment, Surbhi Malhotra-Kumar and a team of microbiologists in Belgium and the Netherlands gave a partial course of two macrolide-class antibiotics - clarithromycin and azithromycin - that are commonly prescribed for bronchial infections to two randomly-selected groups of 74 people, one drug for each group.
They also gave placebos to a control group, which was to provide a yardstick for any changes observed.
The drug-taking subjects were not sick. The point was to monitor the drugs' effect on streptococci bacteria that were normal and harmless components of the flora in the mouth and throat.
The verdict was ironclad. Azithromycin swiftly created large numbers of bacteria that were resistant to macrolides, while clarithromycin encouraged the emergence of a highly resistant form.
This form was not only more resistant to macrolides, it also showed greater resistance to lincosamide, streptogramin B and tetracycline antibiotics.
The study, which is published by The Lancet, found that antibiotics had an enduring effect on this oral bacteria, of more than 180 days.
In other words, the harmless bacteria became a potential reservoir of drug-resistant DNA for pathogenic ones.
Penicillin is usually the weapon of choice for tackling harmful forms streptococci, but doctors often resort to macrolides if a patient is allergic to that drug.
In an assessment of the work, Stephanie Dancer, a microbiologist at Southern General Hospital in Glasgow, Scotland, said the finding provided crucial evidence to back a commonly-observed problem about misprescribed or misused antibiotics.
"All of us could see what was happening, but now we have proof," she said in an interview.
The stakes could hardly be higher, said Dancer.
"It is an immense problem... that is affecting every antimicrobial agent that man has been able to find," she said. "We are squandering a precious resource."
She pointed the finger in particular at purchases of antibiotics over the counter or the Internet, and urged the launch of a public awareness campaign on the lines of "No drugs for bugs - unless needed." - Sapa-AFP