When bacteria get organized, look out. Some species of bacteria form durable coatings called biofilms on solid surfaces, like your teeth. Once formed, the films resist antibiotics and can be impossible to remove. Now researchers have overturned the conventional wisdom that sugars produced by the bacteria are the glue that hold biofilms together. Instead, they say, DNA tethers the tough matrices.
One nasty biofilm-forming bacteria, Pseudomonas aeruginosa forms a life-threatening coating in the lungs of cystic fibrosis patients. It's nearly impossible to eradicate and aggravates inflammation in the lungs. Researchers tried for years to isolate the molecule--thought to be a long sugar chain--that stitches together the biofilms. But DNA contamination seemed to confound their efforts. They chemically removed the DNA from their samples to prevent contamination, but they kept coming up empty handed. Certain that sugars held the key, they tried doggedly time and again to rid their samples of DNA without losing the sugar--never considering the DNA might be more than a contaminant.
John Mattick and his colleagues at the University of Queensland in Brisbane, Australia, suspected that DNA itself might hold the biofilms together, rather than a sugar. They knew from previous reports that Pseudomonas secretes DNA through membrane vesicles, and they wondered if the bacteria were using that DNA to form biofilms. To test their hypothesis, the researchers exposed the bacteria to an enzyme called DNase I, which breaks down DNA. The bacteria continued to grow and divide in the test tube, but the cells couldn't form a matrix, the team reports in the 22 February issue of Science. When they exposed existing biofilms of various ages to DNase I, all but the oldest samples (84 hours old) dissolved. Perhaps, the researchers suggest, another structural component supplements the DNA in older biofilms, strengthening the matrix.
"The idea that DNA could have a structural function in biofilms is ... something that dogma would make us tend to resist," says biofilm researcher Pete Greenberg of the University of Iowa, Iowa City. Still, he says, the new data don't rule out the participation of sugar chains, and polymers besides DNA will probably help hold biofilms together.