Continuously killing bacteria on coated stainless steel—add bleach to recharge

Stainless steel is the gold standard for kitchen appliances and cookware, described as modern and sleek. But bacteria can grow on stainless steel surfaces, contaminating food. Current coatings available on the market are pricey and potentially harmful, so scientists have now developed an affordable specialized polymer coating for such surfaces that they can recharge with bleach treatments.

The researchers are presenting their results at the 255th National Meeting & Exposition of the American Chemical Society (ACS).

"I noticed that stainless steel is frequently used in the health care industry and for food processing, but not a lot of research has been done to prevent bacteria from growing on these surfaces," Buket Demir, Ph.D. says.

Current efforts to avoid such bacterial growth incorporate silver or copper ions, which can be pricey. "In addition, silver and copper are usually alloyed in the metal, and they have been tucked away so they are not very effective," Demir notes. "There is also a health concern with using silver." Silver could leach from the stainless steel and into foods that are later consumed, she says.

Scientists have previously added molecules called N-halamines to textiles for their antimicrobial properties because it is easy to attach these substances to cotton. N-halamines are a group of compounds that have one or more nitrogen-halogen covalent bonds. These compounds are affordable, chemically stable and nontoxic to humans. Demir, a postdoctoral researcher in the lab of Dave Worley, Ph.D., at Auburn University, wants to expand the use of N-halamines into the medical and food industries.

Other groups have linked N-halamine polymers to stainless steel surfaces with the aid of a binder, but Worley's team has now shown, for the first time, that N-halamine can be attached to stainless steel directly. To do this, the researchers first roughed up the surface with hydrogen peroxide and sulfuric acid. This creates negatively charged oxygen groups on the surface to which the N-halamine polymer can attach through a chemical reaction. When the researchers put bacteria on the coated surface, the microbes did not grow. Demir tested many types of bacteria, including E. coli and Staphylococcus aureus, which are common culprits implicated in many foodborne bacteria outbreaks at food processing facilities. All of the bacteria tested were killed within 15 minutes of coming into contact with the treated surface.

Early testing showed that the N-halamine polymer coating was effective for five rounds of killing bacteria before it lost some of that activity. Further, Demir says, "we could regenerate the antibacterial activity of the coated surface by simply wiping it down with a diluted bleach solution."

"Bleach contains chlorine, a halogen, which by itself is extremely unstable; however, when attached to the N-halamine polymer, it becomes stable," Demir notes. "When bacteria come into contact with the treated surface, the chlorine is released, killing the bacteria." She says that the project is still in the early stages of development, but she hopes that the coating could someday be used in kitchens around the world.

Future plans include manipulating the surface of stainless steel in a more user-friendly way. "The greatest challenge is roughing the surface of the stainless steel. Although this is easily done in the laboratory, it would not be easily done in a manufacturing plant," Worley states. "We need a better way of attaching N-halamines to the surface, and we have some ideas as to how we can do that." The researchers also need to perform toxicity testing on samples to investigate the safety of the coated surfaces and examine any biofilms that may develop. "An antimicrobial surface in a food processing plant is badly needed, and it could revolutionize the industry," Worley says.