How antibiotic resistant superbugs win out in the wild

13 September 2006

Scientists have discovered why superbugs like MRSA are dominating our hospitals, when accepted wisdom says that the cost for bacteria of competing against non-resistant strains should be too great in most circumstances, researchers heard today (Wednesday 13 September 2006) at the Society for General Microbiology’s 159th meeting, in York.

Burns victims and cystic fibrosis sufferers are frequently attacked by antibiotic resistant bacteria called Pseudomonas aeruginosa causing serious infections. Scientists from the National University of Ireland, Galway, developed a highly toxic and resistant strain of the bacteria and used it to investigate how it could spread through hospitals and into the wider environment where it could end up in drinking water and other places.

“Normally the cost of making the extra proteins needed by bacteria to fight off antibiotics slows down the cells, making them weaker and less able to compete against their ordinary cousins”, say Dr Ger Fleming and his team leader Paul Mc Cay from the National University of Ireland, Galway. “When there aren’t any antibiotic residues about, the ordinary, susceptible strains become dominant, crowding out the superbugs”.

The university team discovered that when low levels of antibiotics were introduced – as little as 15% of the level normally needed to kill the ordinary bacteria – the resistant strain became dominant after just a few divisions.

“In places like hospital corridors or wards, or on farms where antibiotics are routinely fed to cows in feedstock to limit infections, there is a constant low level exposure to antibiotics which encourages the resistant strains to succeed, especially if there are already a few resistant bacteria about”, says Dr Fleming and Mr Mc Cay. “Our results suggest that hospitals need to re-evaluate the minimum levels of treatments they think will stop infections, and include data on the way bacteria are selected and survive as well”.

Administering too low a level of antibiotics can cause a much more dangerous and long lasting infection by encouraging a resistant strain of bacteria to emerge. It is also important that the patient finishes their course of antibiotics so that infectious bugs are completely eliminated from the body.  Disinfectant resistant bacteria are also emerging with increased use, and the scientists think this may be contributing to the problem through cross resistance.

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