Zapping microbes with lasers and enzymes

Probing the power of tears
Lysozyme, found in tears, saliva and egg whites, may not pack the same antimicrobial punch of indocyanine green. But by locking it into place with the aid of ultra-strong carbon nanotubes, Virginia Davis and colleagues at Auburn University in Auburn, Ala., have exploited its bug-fighting powers to create durable new coatings that eventually could cover gym lockers, sporting goods and other commonly touched surfaces.

“Certainly multiple approaches are needed to prevent bacterial infections and the spread of disease,” said Davis, an assistant professor of chemical engineering.

In a study published this month in the journal Nano Letters, Davis and her collaborators wrapped multiple copies of lysozyme around individual carbon nanotubes no bigger than one-fiftieth the width of a human hair.

“Carbon nanotubes are perfect cylinders of carbon and the strongest known material,” she said. “The system structure is basically like a shell of lysozyme surrounding a carbon nanotube core; the antimicrobial properties come from the natural action of the lysozyme while the strength comes from the carbon nanotube core.”

Compared to demand in 2005, Davis said, the U.S. market for antimicrobial coatings could triple, to more than $550 million, by 2012. Growing alarm over hospital-acquired infections and the spread of resistant pathogens via contaminated surfaces are both fueling demand. Robust antimicrobial surfaces, she said, can help reduce the risk of infections spread by contact and could eventually find their way onto a variety of consumer products and commonly touched surfaces on airplanes and trains.

For such coatings, long-term stability and durability are important considerations. Davis’ preliminary results suggest that lysozyme bound within the nanotube network, which attacks a primary component of bacterial cell walls, meets those criteria and works efficiently while retaining its antimicrobial properties over time.

Nanoparticles such as nanosilver already have found widespread use as microbicides in a range of consumer products, though recent concerns over the potential environmental effects of nanosilver leeching from products and ending up in wastewater have prompted some researchers and watchdog groups to petition the Environmental Protection Agency to strenthen its oversight.

More research is needed
Although Davis and her colleagues do not conduct their own safety testing on carbon nanotubes, she said they support multidisciplinary efforts to investigate the health, safety and environmental risks and benefits in the very early stages of nanotechnology and agree that more such research is needed.

“This proactive approach is a definite improvement to the historic approach of not investigating the potential risks until many years after large scale product use,” she said. Her group’s future studies, she said, will include experiments simulating repeated use and cleaning of products coated with the lysozyme-nanotube combination to determine whether the nanotubes are released.

Although MRSA is generally resistant to lysozyme, the enzyme works well in targeting other pathogenic microbes. Davis cautioned that the long-term effects of the lysozyme-nanotube network still need be investigated, though she noted that lysozyme is not on the front lines in combating infections once they’ve been acquired. “Therefore its use should not change our ability to cure bacterial infections in people,” she said. “The coating should simply reduce the spread of disease.”

DeLeo said he’s less enthusiastic about such prophylactic approaches to the problem of bacterial infections, pointing to the difficulty of producing stable surface coatings. Nevertheless, he said sanitizing surfaces is obviously a good idea and a strategy like the one proposed by Davis’ team, if proven effective, could be used to develop a similar coating containing the microbe-derived enzyme lysostaphin, which is effective against MRSA.

“Ultimately, one could lead to another and you have to start somewhere,” he said. “I think it’s all worthwhile.”

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