Scientists found that exposing infectious bacteria to increasing amounts of disinfectant turned the bugs into hardy survivors.
Not only did they become immune to the cleansing chemicals, but they developed resistance to a commonly prescribed antibiotic, ciprofloxacin.
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The findings could have important implications for controlling the spread of hospital infections, the researchers believe.
Scientists in the Republic of Ireland carried out the tests on Pseudomonas aeruginosa, a bacterium that causes a wide range of infections in people with weak immune systems.
Sufferers of diseases such as cystic fibrosis and diabetes are also vulnerable to the bug, which is responsible for many hospital-acquired indfections.
The scientists, led by Dr Gerard Fleming from the National University of Ireland in Galway, found that the bacteria adapted to disinfectant exposure by improving their ability to pump antimicrobial agents out of their cells.
They also developed a DNA mutation that helped them resist ciprofloxacin-type antibiotics.
Disinfectants are commonly used in homes, workplaces and hospitals to kill bacteria on surfaces, while antibiotics are administered to infected patients.
Bacteria that can resist both these control points could be a serious threat in hospitals, said the scientists.
The study, reported in the January issue of the journal Microbiology, showed that exposure to small non-lethal amounts of disinfectant encouraged the survival of resistant bacteria.
Dr Fleming said: “In principle this means that residue from incorrectly diluted disinfectants left on hospital surfaces could promote the growth of antibiotic-resistant bacteria. What is more worrying is that bacteria seem to be able to adapt to resist antibiotics without even being exposed to them.”
He stressed the importance of studying environmental factors that might promote antibiotic resistance.
“We need to investigate the effects of using more than one type of disinfectant on promoting antibiotic-resistant strains,” he said. “This will increase the effectiveness of both our first and second lines of defence against hospital-acquired infections.”