Light and polymer germicide show promise

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

Polymer surfaces working in tandem with fluorescent lighting could reduce the spread of dangerous bacteria in hospitals and other clinical settings. Hospital staff could simply turn on regular fluorescent lighting to activate the antimicrobial properties of such surfaces, reducing the need for more labor-intensive methods of disinfection. And scientists have now taken a step forward in developing this light/polymer combination technique.

The Centers for Disease Control and Prevention in Atlanta estimates 1.7 million hospital-associated infections and 99,000 associated deaths each year in the US from bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and, particularly, Gram-negative bacteria, which tend to be harder to kill than Gram-positive bacteria.

Researchers are experimenting with surfaces that may kill harmful bacteria in hospitals with the help of fluorescent lighting. The colorized scanning electron micrograph depicts clumps of the MRSA bacteria, magnified 2390x. Courtesy of the Centers for Disease Control and Prevention.

David G. Whitten at the Center for Biomedical Engineering at the University of New Mexico in Albuquerque and Kirk S. Schanze at the University of Florida in Gainesville have developed a number of conjugated polyelectrolytes (CPEs) that have potential as biocides toward bacteria, said Heather E. Canavan, assistant professor in the department of chemical and nuclear engineering at the University of New Mexico and at the Center for Biomedical Engineering.

They have shown that while certain polymers demonstrate biocidal activity toward certain bacteria, they do not appear to be toxic to mammalian cells that have been exposed to polymers in both light and dark conditions. Results of their study, which is at the in vitro stage, have indicated that the technique may be safe for humans and animals.

Specifically, the researchers first demonstrated that certain CPEs with arylene ethynylene repeat-unit structure exhibit dark- and light-activated antimicrobial activity. For example, certain light- activated CPEs exhibit little activity toward bacteria in the absence of light but display significantly greater bacteria-killing activity with the addition of light. In experiments, the polymer/lighting combination has been effective at killing Gram-negative bacteria, specifically the Pseudomonas aeruginosa strain PAO1 and Cobetia marina.

The researchers are now experimenting to determine whether the CPEs, in amounts needed for disinfection applications, would be toxic to humans and animals. “We’re finding that mammalian cells such as bovine aortic endothelial cells are capable of tolerating those polymers at much higher doses (an order of magnitude or more),” Canavan said. “It’s the first step to developing commercial applications from these materials.”

One of the first challenges that must be overcome is ensuring that, while the CPEs are suitably toxic to prokaryotic cells (bacteria), they are nontoxic to mammalian cells, especially human cells, when they come into normal contact with them. “If we can demonstrate that, then the next step would be to perform simple initial in vivo tests,” Canavan said.

In the experiment, the researchers cultured mammalian cells, specifically bovine aortic endothelial cells, on 48-well plates. They replaced the cells’ regular growth medium with a serum-free medium that had been treated with the CPE at concentrations varying from 1 to 100 µg/ml. The cell culture plates, loosely covered in foil, were returned to the incubator for 24 hours. Then the cells were washed with Dulbecco’s phosphate-buffered saline, and the cells in the negative control wells were treated with a 70 percent (vol.) methanol solution for 30 min. To assess cytotoxicity, the researchers used a live/dead fluorescence assay and obtained images on an inverted microscope with an epifluorescence attachment.

The underlying mechanism of how the CPE material kills bacteria is still under investigation, Canavan said. The researchers see differences in the extent and efficiency of killing of various CPEs in light and dark situations, which may indicate that the light activation includes release of reactive oxygen species such as singlet oxygen.

“There are many applications where a material may come into contact with both eukaryotic (mammalian, plant) and prokaryotic (bacteria) cells, and you’d want to minimize the transfer of bacteria to mammalian cells,” Canavan said. “For instance, in addition to the fixed surfaces in hospitals, there are a lot of consumable materials, such as catheters and wound dressings that come into direct contact with patients. All of these carry the potential to transfer bacteria to the patient, resulting in health care-associated infection.”

The overall interdisciplinary project involves students and faculty in different locations and research groups working together to develop new polymers, test their biocidal activity toward bacteria, test their biocompatibility with mammalian cells, and model how the polymers interact with cells.