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Hospitals Shouldn't Get Stingy with Disinfectants, Says Study

Evan Sweeney, for HealthLeaders Media, February 23, 2010

Over the past several years, MDROs have become a leading concern for medical facilities attempting to reduce their hospital's healthcare-associated infection (HAI) rate.

A study published in the January Microbiology found that certain organisms can actually become less susceptible to both disinfectants and antibiotics. Researchers grew the bacteria Pseudomonas aeruginosa in the presence of a common disinfectant—benzalkonium chloride—and found that the bacteria eventually became 12 times less susceptible to the disinfectant and, perhaps more importantly, 256 times more resistant to the drug ciprofloxacin.

Previous studies of antimicrobial resistance haven't easily translated to real world situations, says Gerard T.A. Fleming, researcher for the Department of Microbiology at the School of Natural Sciences at the National University of Ireland in Galway and lead researcher for the study. This study used a system called chemostat, or continuous culture, which allows culture parameters to remain constant, similar to the way organisms grow in their natural environment.

"The thing about chemostat is it's a wonderful selection machine," Fleming says. "It's a wonderful evolution tool because being in an environment where you are competing for nutrients, those mutants that are not fit are those that have not become resistant to the disinfectant and are lost in the system. So that means we are evolving the strains through natural selection—which happens out there in the environment—toward resistance."

In addition, researchers used bacteria and a disinfectant that had real world applications. Pseudomonas aeruginosa, which is present in hospitals; is heavily associated with people who suffer from cystic fibrosis, pneumonia, and other HAIs; and has significant implications for burn victims, says Fleming. Benzalkonium chloride is one of the most common disinfectants or component of disinfectants used in the hospital environment, says Fleming.

After a month of testing, the researchers found that the bacteria was tolerating 12 times more disinfectant than it was at the start, meaning it had become hyper-resistant to the benzalkonium chloride.

However, the experiment didn't just yield frightening results on the susceptibility of bacteria. For the first time, this study showed that bacteria could remain resistant to disinfectant for an extended period of time, even when it was tested in the chemostat with no disinfectant whatsoever.

"For example, if it was 12 times more resistant, 200 hours later it remained 12 times more resistant in the absence of disinfectant," Fleming says.

Further, the study tested the original strain of bacteria against the resistant strain to see which one survived. In the absence of disinfectant, the original, nonresistant strain won out, but in the presence of very low residual levels of disinfectant—levels often found in the hospital environment—the resistant strain survived the original bacteria, indicating that in these residual levels, the resistant bacteria could actually grow.

Even more concerning, the bacteria that was 12 times more resistant to benzalkonium chloride was also found to be 256 times more resistant to ciprofloxacin, a drug commonly used to treat GI tract and pulmonary infections, even though the bacteria had never been exposed to that antibiotic.

"We then took the strain again and grew it in the presence of the antibiotic and it became three-fold more resistant to the disinfectant than the sensitive strain," Fleming says. "That's the balancing experiment there. That kind of concludes that the two are linked. If A produces B, then B must be associated with A, and we showed that."

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