Australian and Chinese researchers have built a device that uses a 12-volt battery to create a plasma that could kill one of the most persistent bacterial structures, biofilm. A biofilm is a bacterial group that encases its entire colony with a slimy material. Normally, we encounter biofilms as plaque when we brush our teeth, but they can also grow and thrive in wounds, holding up the healing process. Using a quick blast of plasma to break up biofilms would make it easier for doctors and dentists to deal with infection. The study is published in the Journal of Physics D: Applied Physics.
Plasma is essentially ionized gas, a free-flowing state of matter that is electrically charged, David Graves says. Graves, a professor of chemical and biomolecular engineering at UC Berkeley who has been studying plasma for more than 30 years (but was not involved in this study), says plasma is everywhere: from stars to lightning to those buzzing florescent lights in the office. "The basic idea is very, very simple. If you pass electricity through a gas by applying a high voltage, you get a plasma," he says.
When an electrical current passes through air, Graves says, the resulting ionization creates a variety of components, including a faint electric charge, UV light, and an assortment of charged gases (called species). This charged cocktail had been shown to have antimicrobial properties, so the researchers in this study set out to see if they could harness those talents. For their test case, they chose Enterococcus faecalis, bacteria that commonly infect root canals, living communally in layers called a biofilm.
The plasma flashlight that the scientists created produces a faint light, but enough plasma to be effective against bacteria. The flashlight is about the size of a rolling pin and has a tapered point at one end where the plasma jet emerges. To test it against E. faecalis, the researchers first grew the bacteria into a biofilm, which they exposed to the plasma jet. After 5 minutes, the bacteria were dead.
E. faecalis. Credit: Wikimedia Commons
Biofilms that form on wounds or infection sites can form nearly impenetrable barriers, preventing treatments like antibiotics from seeping down any deeper that the first few layers of the structure. "Biofilms are notoriously difficult to get rid of," Graves says. But after just 5 minutes under this new handheld device, the plasma managed to penetrate and kill off the entire biofilm structure.
Just why the plasma flashlight is so successful, however, remains an open question: Scientists are not exactly sure which component deals the death blow to microbes, according to Ken Ostrikov, a co-author of the new study and a plasma researcher with Australia's national science agency, CSIRO (Commonwealth Scientific and Industrial Research Organization). Some researchers say that the UV light or the electrical charge does the damage, but Graves says a growing number of scientists think that it might be those charged species of oxygen and nitrogen that rip holes in the cell membrane to destroy the biofilm. "They basically degrade the wall of the bacteria and kill it," he says.
Graves says it's so difficult to prove the case for charged gas over the other suspects "because when you use the plasma, you get the whole package." However, Ostrikov says that his team's plasma flashlight probably relied more on reactive species of gas than on UV to kill bacteria because there wasn't much UV light present in this plasma.
Even though scientists are still puzzling over plasma's lethal mechanism, more and more people have begun exploring the biomedical applications. Just last fall, Graves put out a study on his attempts to purify water with plasma by exposing water to the plasma for 10- to 15-minute blasts. The experiment worked as a test case, and the water remained bacteria-free a week after being exposed to the plasma. Other researchers around the world are trying to use plasma to treat skin conditions, sanitize hands and food, and even fight cancer.
It will take decades for some of these treatments to make their way to humans. "[But] in principle, anything is possible," Ostrikov says. His group hopes that someday the tool could become invaluable to medical professionals in the field, where the ability to sanitize wounds and surfaces quickly would be invaluable.