UW researchers tested the efficacy of several common disinfectants against antibiotic resistance genes in bacteria. Shown here is lead author Huan He preparing agar plates for measuring inactivation of bacteria in disinfection experiments.Mark Stone/University of Washington
Antimicrobial resistance is a lurking threat in hospitals around the world. As more strains of bacteria and other microbes evolve defenses against available drugs, more patients run the risk of contracting infections that defy treatment.
Now, University of Washington researchers offer new insights into measures currently used to control the spread of antibiotic resistant bacteria and other infectious agents in health care facilities.
In a recent paper published in Environmental Science & Technology, the team studied the efficacy of nine common disinfectants used in health care facilities or households - such as ethanol, hydrogen peroxide, benzalkonium chloride and UV light - against three well-known strains of antibiotic-resistant bacteria. The researchers first evaluated how successfully each disinfectant killed (or more accurately "inactivated") the bacteria.
Then the team went a step further. It also assessed the damage the disinfectants did to the root cause of the resistance: the bacterial genome itself. And while all the cleaners did a great job of stopping the spread of bacteria, the picture was very different when the team zeroed in on DNA.
"What we're learning is that it's not just the bacteria that we need to deal with in hospitals and elsewhere. It's also the behavior of their DNA in these environments," said lead author Huan He, who completed this research as a UW doctoral student in the civil and environmental engineering department and is now an assistant professor at Tongji University.
Lead author Huan He examines growth of bacteria on agar plates before and after disinfection.Mark Stone/University of Washington
Within bacterial cells, the source of antibiotic resistance is specific genes - individual portions of DNA - that instruct a cell to protect itself against certain antibiotics. Modern disinfectants do an impressive job of stopping bacterial cells in their tracks, but a bacterium's genes may survive even the death of the cell. And, thanks to a trick called "horizontal gene transfer," genes from one bacterium - even if that bacterium has been killed - can sometimes find their way into a new living bacterium, thus passing on antibiotic resistance.
In short, stopping the bacteria themselves isn't always enough to prevent the creep of resistance.
"Increasingly, environmental engineers are thinking about and treating resistance genes as an emerging contaminant," said He. "In public health settings such as hospitals, we might disinfect and sterilize an operating room to remove any bacterial contamination, but what if resistance genes survive? They could potentially reach other bacteria and contribute to more dangerous antibiotic resistant hospital-acquired infections.
"Our previous work has demonstrated resistance genes can stay active in horizontal transfer after water and wastewater disinfections, which led us to wonder whether similar things could occur in health care and personal-care disinfection practices."
The experiment pitted the nine disinfectants against three kinds of antibiotic-resistant bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), the microbe responsible for life-threatening staph infections. Researchers placed samples of the bacteria in different environments, mostly as dried drops on stainless steel and nonstick surfaces that are common in hospitals and at home. They then applied the disinfectants and measured the effects on both the bacterial cells and the genes in question.
As expected, the disinfectants did a great job of stopping the bacteria. However, most had a negligible impact on the resistance-conferring genes. The DNA survived largely intact, and it was free to find its way into new bacteria.
There were some positive and negative standouts, though frequently not the cleaners the team expected.
"Chlorine, under the conditions we tested, seemed to be less effective against DNA than we originally anticipated, whereas another common cleaner called phenol, which we didn't think would be effective, actually ended up working relatively well in some cases," said senior author Michael Dodd, a UW associate professor in the civil and environmental engineering department.
The winner in many of the experiments was UV light, which did significant damage to the offending genes - though ultimately less damage than the team anticipated.
"UV irradiation seems to be one of the more effective approaches to both inactivating bacteria and degrading their DNA," Dodd said. "We know that UV light directly damages DNA, so we weren't necessarily surprised to see it perform well here. But it was a welcome result nonetheless."
The researchers were quick to point out that existing disinfection regimens in hospitals are still effective and critical for preventing the spread of disease.
Senior author Michael Dodd with Huan He.Mark Stone/University of Washington
This work can help researchers home in on the tools that offer the best one-two punch against problematic bacteria and their genes. Moving forward, the team wants to learn more about how best to optimize these cleaners' effects, especially when new factors like ambient temperature, humidity and density of bacterial cells are taken into account. But, the results from this paper could already help hospitals refine their disinfection protocols.
"If you know you have a patient in a hospital or other health care facility who's infected with an antibiotic-resistant pathogen, I think we do have enough evidence at this stage to suggest trying certain disinfectants over others when cleaning surfaces or instruments that the patient may have been in contact with," said Dodd. "For example, UV light could be a good choice, whereas benzalkonium chloride might not be."
Additional co-authors on this paper are Sin-Yi Liou, a former UW graduate researcher who is now a postdoctoral researcher at Gwangju Institute of Science and Technology; Kyle K. Shimabuku, a former UW research assistant who is now an associate professor at Gonzaga University; Peiran Zhou, a former UW graduate researcher who is now a medical resident at the UW School of Medicine; Yegyun Choi, a former UW guest researcher who is now a research professor at Gwangju Institute of Science and Technology; John S. Meschke, a UW professor in the environmental and occupational health sciences department; Marilyn C. Roberts, UW professor emeritus in the environmental and occupational health sciences department; and Yunho Lee, a professor at the Gwangju Institute of Science and Technology.
This research was funded by the National Science Foundation, the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities and the Allen & Inger Osberg Endowed Professorship.
