What Protects Against the Sun and Superbugs?

Each year medical device implants are associated with about a million superbug infections. Over the past decade, researchers have explored various ways to combat this problem using zinc oxide nanoparticles.

Zinc oxide is an inorganic compound used as an additive in a variety of materials and products including sunscreen, diaper rash cream, and other ointments. In 2015, University of Michigain (U-M) researchers found that a coating of zinc oxide nanopyramids can disrupt the growth of methicillin-resistant Staphylococcus aureus (MRSA), reducing the film on treated materials by over 95%. Their results were published in the journal Nanomedicine.

J. Scott VanEpps, MD, PhD, a clinical lecturer and research fellow at the university medical school's emergency medicine department, led the biological study of the coating. The researchers discovered that if the nanoparticles are shaped like a pyramid with a hexagon-shaped base, they are quite effective at preventing an enzyme called beta-galactosidase from breaking down lactose into glucose and galactose, which the bacteria use for fuel.

They noted that shape is important because the enzyme needs to be able to twist to cut the lactose into the smaller sugars. Two amino acids, or protein building blocks, sit opposite one another across a groove in the enzyme. The lactose fits into the groove, and the amino acids come together to catalyze the breakup into glucose and galactose.

Nicholas Kotov, a chemical engineering professor at U-M, whose group made the nanoparticles, said he believed that zinc oxide nanopyramids interfere with that twisting motion.

The team’s research suggests that part of the nanoparticle—an edge or the point—inserts itself into the groove. By clogging up just one of the four grooves, the nanoparticles can shut down the whole enzyme by preventing the twisting action.

Kotov’s group covered some pegs with the nanopyramids and then VanEpps’ team stuck them into a substance that would allow bacteria to grow. They evaluated four species of bacteria on coated and uncoated pegs — two staphylococcal species (including MRSA), and one species that causes pneumonia and E. coli.

After 24 hours of growth, the number of viable staphylococcal cells recovered from the coated pegs was 95% less than those from the uncoated pegs. The pneumonia and E. coli species were less susceptible to the nanoparticles though.

“While the coating was unable to completely eradicate all staphylococcal cells, this dramatic reduction could likely enable antibiotic treatments to succeed or simply allow the human immune system to take over without the need for antibiotics,” VanEpps said.

The researchers explained that staph, including MRSA, is particularly vulnerable to the nanopyramids because its cell wall is a matrix of proteins and sugars. They suspected that as the MRSA tried to colonize the pegs, the nanopyramids bound to the enzymes that build the cell wall. Since the enzymes couldn’t maintain the cell wall, the cells broke down.

"We envision reduction in the incidence of infection for almost any conceivable device," VanEpps told MD+DI in a 2015 interview. We feel that this technology could be applied to almost any material even after fabrication of the device."