Preservative-resistant bacteria spread through hospital air, early research suggests

Common disinfectants used to clean the skin of hospitalized patients can remain on surfaces for hours, creating a breeding ground for bacteria to become resistant to or become resistant to chemicals that would normally kill them.

When bacteria develops “resistance,” they can survive certain concentrations of chemicals more easily than other bacteria, but can usually die depending on the dose of disinfectant used for cleaning. Disinfectants include chemicals such as alcohol, iodine, and hydrogen peroxide that are used to disinfect surfaces and skin. “Resistance” is of greater concern because it allows bacteria to grow even when exposed to concentrations of disinfectants that would normally kill them.

Bacteria may be exchanging pieces of DNA with each other as they learn to tolerate small doses of disinfectants, according to a study published April 2 in the journal Environmental Science & Technology. That same DNA could also help evade drugs designed to treat bacterial infections: antibiotics.

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This study adds to a growing body of work mapping bacterial resistance genes and the hidden environmental stressors that drive their evolution.

“Antimicrobial resistance comes from a lot of places,” said lead author Erica Hartman, a professor of civil and environmental engineering at Northwestern University’s McCormick School of Engineering. “To truly address this problem, we need antimicrobial stewardship and responsible use in agriculture, and we also need to think about the responsible use of chemicals in other environments,” she told Live Science.

Stewardship practices mean sparing the use of antimicrobial agents, such as antibiotics and disinfectants, to prevent bacterial resistance and the development of resistance.

Resistant bacteria travel through the air

In this study, Hartman and colleagues tracked bacteria that are resistant to chlorhexidine, a commonly used chemical applied to patients’ skin before surgery or catheter insertion. They searched for these bacteria in the intensive care unit of an Illinois medical center.

In 2018, researchers wiped 219 samples from bed rails, nurse call buttons, door thresholds, keyboards, light switches and sink drains in six areas around the ICU. Although the room was fairly clean, they were able to isolate approximately 1,400 bacteria and found that 36% showed some degree of resistance to chlorhexidine.

In the lab, the researchers applied chlorhexidine to common materials such as plastic, metal, and laminate and tracked how long the disinfectant remained on the surfaces, including after cleaning the materials with water or other chemical cleaners. They found that even after cleaning, traces of disinfectant remain on surfaces for at least 24 hours.

These residual traces were not strong enough to kill bacteria. But this kind of microenvironment, where bacteria are exposed to non-lethal doses of chemicals that would normally kill them, is always alarming.

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Bacteria that thrive in these environments are those that have genes that help them survive the effects of chemicals. These resistant bacteria multiply more because they compete with bacteria that do not have the resistance gene. The worst-case scenario is that bacteria become so used to fighting chemicals that they become good at fighting them off and resistant to their effects.

The team found chlorhexidine-resistant bacteria throughout the hospital room, even though the disinfectant was only applied to the patient’s skin. Sinks have emerged as hotspots for these bacteria.

In recent decades, hospital sinks have become a focus of attention for those studying antimicrobial resistance. Bacteria love the humid, warm U-shaped bend in your sink and will do everything they can to stay there, even when exposed to chemicals diluted by water flushed down the drain. This creates the perfect environment for tolerance and resistance to emerge.

Sinks also generate aerosols, tiny particles that become airborne and can spread germs. When water comes out of a faucet, hits standing water, or bounces down a drain, these particles can become airborne. Researchers’ swab tests showed that resistant strains can be found on door sills, suggesting they traveled through the air and settled there.

Preservatives still work well

Some preservative-resistant bacteria have plasmids (small DNA loops that can be transferred between bacteria) that not only help the bacteria resist chlorhexidine, but also antibiotics such as carbapenems. This type of gene transfer is a well-known way for bacteria to acquire resistance to antibiotics and can occur between completely different species of bacteria.

This is “very important,” said Dana Gifford, a lecturer in antimicrobial resistance at the University of Manchester, UK, who was not involved in the study. This finding suggests that antibiotic resistance can be promoted by exposure to disinfectants alone, “even without the use of antibiotics,” she said.

But let’s be clear: chlorhexidine is still very effective at killing bacteria. The bacteria observed in the study could only survive in very low concentrations of the chemical, far lower than the amounts used to clean patients’ skin.

“I don’t think this really supports a conservative approach” to the use of chlorhexidine, Professor Gifford said, adding that restricting the use of disinfectants in high-risk settings like ICUs without proper clinical evidence could put vulnerable patients at risk of infection. But this study, like other recent studies, still raises the question of whether we need to be more cautious about using disinfectants, Hartman and Gifford agreed.

To better understand how these disinfectant residues affect bacteria, further research should investigate whether these effects are also seen in other environments, such as homes and veterinary clinics, the study authors wrote.

Gifford said whether people should have disinfectant on hand for “high-risk situations” is “probably worth further investigation.” Home cleaning may be an environment where you can reduce the use of disinfectants, as “plain soap and water is often more than sufficient for cleaning and hygiene,” Hartman said.

In the meantime, “we’re running out of effective antibiotics,” she says. “We’re not quite there yet, but if we don’t intervene with what we’re doing now, we’ll find ourselves in a situation in the future where we won’t be able to do simple things like treat dental infections or do surgeries because we won’t be able to give patients antibiotics after treatment.”