Light spells doom for bacteria

Caren B. Les, caren.les@laurin.com

Those sterile-looking fluorescent lights in hospitals and other public places may someday empower the paint on walls to clean up germs. New nanotechnology paints activated by common fluorescent lights could be used on hospital walls, ceilings and surfaces to kill potentially dangerous “superbugs,” including the food-poisoning bacterium E. coli.

The antimicrobial paints could be the interior décor of choice for cleanrooms, child-care centers, restaurant kitchens, and public and domestic bathrooms. They contain nanoparticles of titanium dioxide, a white compound used as a brightener in commercial paints and as the powder for the bright white lines on some tennis courts. Nanoparticles of this substance kill bacteria and destroy dirt when they absorb UV light energy from the sun. They also produce active molecules that clean up the painted surfaces.

Led by Lucia Caballero, researchers at Manchester Metropolitan University in the UK conducted a study on the survival of E. coli on a variety of acrylic paint formulas containing titanium nanoparticles under various types and intensities of light. They determined bacterial growth/survival by exposing a suspension of E. coli to paint-coated film for up to 96 hours under constant fluorescent light illumination. The same amount of UV light present in the common fluorescent light was used in the testing, according to Caballero.

The investigators found that paints with higher concentrations of the nanoparticles were more effective at killing the bacteria, and that paints with additives such as calcium carbonate, silica or talc decreased the antibacterial efficiency of the paint. The presence of calcium carbonate was found to reduce the kill rate by 80 percent.

Caballero said that the next step is to enhance the efficacy of the paint by trying various additives and concentrations to determine the best formula for achieving the highest bacterial kill rate.

The germ-killing paint most likely would be effective for three to five years, depending on the amount of weathering it undergoes, according to Caballero. Nanotitanium is relatively nontoxic and as safe as other ingredients found in paint, she said, but obviously should not be eaten.

Another approach

A process involving a harmless green dye and near-infrared light could be a cheap alternative for treating infected wounds and ulcers that do not respond to conventional antibiotics, according to Dr. Ghada S.M. Omar, who led research on it at University College London. Infection of postsurgical wounds is a serious problem faced by thousands of hospital patients annually.

This near-infrared laser was used in a study to determine the effectiveness of photodynamic therapy in eradicating bacteria from wounds. Photo courtesy of Ghada S.M. Omar.

The investigators’ laboratory study showed that 99 percent of the potentially dangerous methicillin-resistant Staphylococcus aureus bacteria in postsurgical infected wounds could be killed with indocyanine green dye. When triggered by near-infrared light, the dye gives off toxic molecules that rapidly kill the bacteria.

The team irradiated bacterial suspensions with a light dose of 98 J/cm² under aerobic and anaerobic conditions. Lethal photosensitization under aerobic conditions resulted in 99.56 percent kills of S. aureus and 99.96 percent of Streptococcus pyogenes. Under anaerobic conditions, the kills were 96.77 percent and 71.62 percent for S. aureus and S. pyogenes, respectively.

The team also showed that, even with very low oxygen levels in the damaged tissues, the light-activated dyes still can kill the most dangerous bacteria, including 70 percent of S. pyogenes and S. aureus, which is one of the most drug-resistant bacteria in hospitals.

Omar explained that, because of the growing resistance of many organisms to conventional antibiotics, this approach may be the only one available for use against organisms that are resistant to all known antibiotics. The process has a number of advantages over conventional antibiotics, she said. The chemicals produced when the dye is activated harm the bacteria in so many ways that it is unlikely that the bacteria could develop resistance to the treatment. Also, there are no adverse system effects because both the dye and the light are applied topically – and the FDA-approved dye is safe for use in humans.

Although the process has yet to undergo clinical trials for the treatment of infected wounds, it has been tested for the treatment of Acne vulgaris in humans. Omar said the group now plans to study animal models of wound infections.

Reports on both studies were presented at the Society for General Microbiology’s Autumn 2008 meeting in Dublin, Ireland.