Perhaps you notice it after a visit to the dentist.You pass your tongue across the front of your teeth and they feel slick and squeaky-clean. Four hours later, although you might not yet be able to tell the difference, the beginning of a rough fuzz is growing. These are streptococci, the first bacterial settlers in the film that saliva deposits on your teeth. Another four hours and the bridge germs, the fusobacteria, have climbed on board. They are the ones that make it possible for the really bad actors, like Porphyromonas gingivalis, to grab on and start building colonies.
By the next morning, if you still have not brushed your teeth, a definite fuzzy scum is starting to form. If you could look at that fuzz under a microscope without disturbing its structure, you would see towers or entire communities of bacteria, each building upon others. Some of those microbes are dangerous indeed. P. gingivalis not only grows in the pockets of your gums, helping to loosen your teeth from your jaws, but also causes the release of inflammatory chemicals that get into your circulation, complicating diabetes treatment and possibly increasing the risk of heart disease. Traces of the germ have also been found in arterial plaque.
If you have ever been admitted to a hospital, it is very likely you have experienced another, related kind of scary bacterial growth—and in this case you almost certainly did not notice it. Hospitalized patients are routinely hooked up to urinary catheters that enable doctors and nurses to measure urine flow (not incidentally, the catheters also liberate health-care workers from having to take patients to the bathroom). Swiftly coated by a conditioning film made of proteins in the urine, the catheters are then inexorably covered by layers of interacting bacteria, which alter the chemistry of their surface and can cause crystals to form. Within a week, an infection is growing on the catheters of 10 percent to 50 percent of catheterized patients. Within a month the infection has reached virtually everyone.
These slimy bacterial colonies, known as biofilms, add a remarkable new dimension to our understanding of the microbial world. Ever since Louis Pasteur first grew bacteria in flasks, biologists have pictured bacteria as individual invaders floating or swimming in a liquid sea, moving through our blood and lymph like a school of piranhas down the Amazon. But in recent years, scientists have come to understand that much, and perhaps most, of bacterial life is collective: 99 percent of bacteria live in biofilms. They vary widely in behavior. Sometimes these collectives are fixed, like a cluster of barnacles on a ship’s hull; other times they move, or swarm, like miniature slime molds. Bacteria may segregate into single-species biofilms, or they may, as in the case of dental bacteria, join together in groups that function like miniature ecological communities, competing and cooperating with each other.
