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Format: 04/16/2014
Format: 04/16/2014


Discover magazine: Slime City: Where Germs Talk to Each Other and Exercise Precise Attacks

Tuesday, July 21st, 2009

By Wendy Orent

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.

The unifying factor in all these biofilms—the thing that makes them so strange and wonderful and dangerous—is that their cooperation is, in a sense, verbal. Using streams of chemicals that they pump outside their cell walls and membranes, they “talk” incessantly, among their own clones and species and even to unrelated bacteria dwelling nearby. Understanding that chatter could be vital for gaining the upper hand in the endless battle against infectious disease.

Biofilms were first discovered in 1978 in the clear waters of a frigid mountain stream in British Columbia. Microbiologist William Costerton, now of Allegheny General Hospital in Pittsburgh, and his team of scientists wondered why there were so few bacteria in the water, while billions upon billions of the germs nestled in the crevices of the streambed’s rocks. “We were finding 9 bacteria per milliliter in the water, but there must have been 100 million in a square centimeter when we took a rock out of a stream and brought it down to the lab,” Costerton says.

The bacteria were not just sitting idly on the rocks, he found. They were forming complicated structures, cities of germs encased in a slippery substance the bacteria exude called an exopolysaccharide matrix. This slime protects them from grazing amoebas and provides them with food that is excreted by bacteria within the biofilm or even bits of DNA released when other germs die.

When Costerton published his results, he coined the term biofilm and introduced a whole new understanding of how bacteria behave. “We reasoned one stubborn fact,” he recalls. “Bacteria have no idea of where they are. They are just programmed to do their thing.” In other words, they are always going to form biofilms —whether they are living on a rock or in the human body.

Two years later Tom Marrie, a young doctor working in Halifax, Nova Scotia, examined a feverish homeless man who had wandered off the street and into his emergency room. The man had a raging staph infection and, on his chest, a lump the size and shape of a cigarette pack. It was an infected pacemaker, Marrie reasoned. For three weeks the man was given huge doses of antibiotics but did not get better, so Marrie and his team decided to operate. They invited Costerton to sit in. “If there were ever going to be a biofilm infection in a human being, it was going to be on the end of that pacemaker,” Costerton says. “We took out the pacemaker and there was our first medical biofilm. It was a great big thick layer of bacteria and slime, just caked on.”

Biofilms on implants are now recognized as a serious and growing health problem. Bacterial infections hit 2 percent to 4 percent of all implants. Of the 2 million hip and knee replacements performed worldwide each year, 40,000 become infected. More than a third of these infections lead to amputation, and not with very successful results: Most of those people die. “Implant operations have a 98 percent success rate, so people don’t want to talk about the infections,” Costerton says. “They’re a bit of a disgrace, really.”

Biofilm infections are not limited to implants. They can be found in the bodies of the young and the healthy. Many children suffer from undiagnosed biofilm infections in their ears, which require months of oral antibiotic therapy while the underlying infection smolders untouched. Millions of others live with chronic biofilms: urinary tract infections in women that last for years; prostatitis that no antibiotics permanently cure; bone infections (osteomyelitis) that cripple and immobilize people for the rest of their lives. Each year roughly 500,000 people in the United States die of biofilm-associated infections, nearly as many as those who die of cancer.

To read more, visit Discover's web site.


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