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The common use of disinfectants in homes, workplaces and hospitals could be helping to fuel the emergence of hard-to-kill microorganisms that pose a threat to human health, a new study suggests.

Microbiologists at the National University of Ireland in Galway added increasing amounts of a disinfectant to laboratory cultures of pseudomonas aeruginosa, a bacterium that cause infections in people with weakened immune systems, cystic fibrosis or severe burns.

They found that even small amounts of the disinfectant, benzalkonium chloride, spurred the growth of bacteria that could resist the chemical. But what's worse, the bacteria also became resistant to the frequently prescribed antibiotic ciprofloxacin - without even being exposed to it.

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The bacteria essentially developed a mechanism that can rapidly eject the disinfectant as well as ciprofloxacin. "It's almost like trying to pump a bicycle tire with a puncture. The air is being released as quickly as you are pumping it in," explained Gerard Fleming, the lead author of the study published in the journal Microbiology.

Dr. Fleming said the study doesn't mean people should stop using disinfectants: They are still the first line of defence, especially in hospitals. But disinfectants need to be used appropriately, and in some cases other means of combatting microbes should be considered.

"Washing with soap water is one of the best ways of reducing the number of bacteria on the surface - especially on the hands," Dr, Fleming said.

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