Material gives bacterial films the heave-ho.

Daily brushing may get rid of plaque, that sticky bacterial film on teeth, but unfortunately for many people, the bacteria that build up on medical implants are not so easily displaced. In a new strategy against the potentially deadly infections that often develop around devices such as catheters, artificial heart valves, and replacement joints, scientists are devising materials that actively prevent bacteria from clinging.

James D. Bryers, codirector of the Center for Biofilm Engineering at Montana State University in Bozeman, described his group's work on such materials at a recent meeting in San Francisco of the American Chemical Society.

Doctors commonly try to thwart infections with high doses of antibiotics. The treatment works well against free-floating bacteria but is often useless against the thick films of bacteria on implants, Bryers says. The antibiotics "may kill the top ones, but they never penetrate down to the depths." Moreover, he has some evidence that the bacteria change genetically when they hit a surface, swapping genes for antibiotic resistance.

To target bacterial films, the researchers added the antibiotic ciprofloxacin to a plastic called polyether urethane (PEU), a staple of medical devices. The treated PEU releases a potent concentration of ciprofloxacin at its surface, killing bacteria that come in contact with it but not providing a high dose to the rest of the body. The problem is that dead cells pile up on the material.

A better strategy, Bryers says, would be to prevent the bacteria from sticking to PEU in the first place. "We'd like to stay away from the whole issue of killing and really orchestrate adhesion," he says. He and his colleagues synthesized a protein that binds to receptors on the bacterium Staphylococcus aureus, which often contaminates implants, and then loaded it into PEU.

As it leaches from the plastic, the protein blocks the bacterium's receptors, preventing them from binding tightly to the PEU. Not only did fewer bacterial cells adhere to this treated PEU, but those that did came off the material more readily than they did from the ciprofloxacin-treated PEU. In other words, fewer bacteria stayed to form a film.

Though effective against Staphylococcus, this strategy may not impede other bacteria, such as Pseudomonas aeruginosa, which is responsible for most urinary catheter infections. "Pseudomonas will be a difficult organism to fight in this way because it has so many different types of receptors," Bryers says.

Although PEU releases compounds continuously, materials could also be designed to respond to changes in acidity or other conditions in the body that accompany infection (SN: 9/7/96, p. 159). PEU can release antibiotics for up to a year, says Thomas A. Horbett of the University of Washington in Seattle, one of Bryers' colleagues.

The researchers would also like to engineer materials to promote adhesion of the body's own cells, Bryers says. That way, an implant would "take up residence in your body and not be treated as a foreign object."