Repeated antibiotic treatment not only can increase bacteria's resistance to drugs but also help them reproduce faster than before, warns a new study.

The finding that growth of bacteria can be stimulated by antibiotics underlines the importance of using the right antibiotic on patients as soon as possible. For the study, the researchers exposed E.coli bacteria to eight rounds of antibiotic treatment over four days and found the bug -- which can cause severe stomach pain, diarrhoea and kidney failure in humans -- had increased antibiotic resistance with each treatment.

This had been expected, but researchers were surprised to find mutated E.coli reproduced faster than before encountering the drugs and formed populations that were three times larger because of the mutations.

This was only seen in bacteria exposed to antibiotics -- and when researchers took the drug away, the evolutionary changes were not undone and the new-found abilities remained, said the study published in the journal Nature Ecology & Evolution.

"Our research suggests there could be added benefits for E.coli bacteria when they evolve resistance to clinical levels of antibiotics," said lead author Robert Beardmore, Professor at University of Exeter in Britain.

"It's often said that Darwinian evolution is slow, but nothing could be further from the truth, particularly when bacteria are exposed to antibiotics," Beardmore said.

"Bacteria have a remarkable ability to rearrange their DNA and this can stop drugs working, sometimes in a matter of days," Beardmore explained. The researchers tested the effects of the antibiotic doxycycline on E.coli as part of a study of DNA changes brought about by antibiotics. "It is said by some that drug resistance evolution doesn't take place at high dosages but our paper shows that it can and that bacteria can change in ways that would not be beneficial for the treatment of certain types of infection," Mark Hewlett, also of the University of Exeter, pointed out.

"This shows it's important to use the right antibiotic on patients as soon as possible so we don't see adaptations like these in the clinic," Hewlett noted.