Eviction from Scum Home Makes Disease-Causing Microbe Vulnerable
April 10, 1998
A team of microbiologists has found a chink in the armor of an opportunistic microbe that is a top cause of infections in hospitals. By genetically altering the bacteria Pseudomonas aeruginosa, the team was able to break up the colonies that normally make the germ resistant to detergents and antibiotics. The work relies on mucking up the chemical communication system that the bacteria use to "talk" to each other when releasing toxins and gaining a foothold in humans, animals, and even plants.
The work, by scientists at Montana State University, the University of Iowa, and the University of Rochester, is reported in the April 10 issue of Science.
Pseudomonas is a very common bacteria on our food, in dirt, and throughout our houses. The tiny rod-shaped germ targets people whose defenses are already weakened: burn victims, people who have had surgery, cancer patients on chemotherapy, or patients on ventilators or with IV tubes or catheters. Altogether it's the single most common cause of infections acquired by patients during their stay in the hospital. The bacteria are also a big killer of patients with cystic fibrosis: Nearly all children with the disease are infected by Pseudomonas, which makes patients' lung mucus much thicker and compounds their breathing problems.
More so than many bacteria, Pseudomonas can be extraordinarily resistant to antibiotics, largely because the bacteria form themselves into sticky clusters called biofilms. A biofilm is like a slime home base where billions of bacteria stick together, becoming hundreds of times more resistant to detergents and antibiotics than when they're circulating alone.
"Bacteria love to stick, and once they settle down and make a biofilm, it's notoriously difficult to get rid of them," says Barbara Iglewski, professor of microbiology and immunology at Rochester and one of the authors. "We'll take a culture from a patient and send it to a clinical lab, and the lab will say the strain is sensitive to a certain drug, yet the infection continues even when you give the drug in large doses."
In this study the team pinpointed a gene that lies at the root of the bacteria's ability to form a biofilm. The gene, lasI, works as a master switch that spurs other genes to action and cause disease. The team showed that by knocking out that gene, the bacteria form only wimpy biofilms that are vulnerable to attack by detergents. Instead of aligning themselves in fancy mushroom- and pillar-shaped structures as healthy bacteria do, the bacteria without lasI simply pile up on top of each other. When the scientists subjected the germ to a detergent that it normally weathers easily, the mutant bacteria began falling off within 30 seconds, and the biofilm was completely destroyed in just five minutes.
The lead author on the study is David Davies of the Montana Biofilm Institute at Montana State University (MSU). Also working on the project were William Costerton of MSU and Matthew Parsek and Pete Greenberg of the University of Iowa, and graduate student James Pearson and Iglewski at Rochester. Funding for the project came from the National Institutes of Health, the National Science Foundation, the U.S. Navy, and the Cystic Fibrosis Foundation.
"This is basic research, but it leads us in two research directions," Greenberg says. "It may help us find a way to dislodge the biofilm, or it could lead to ways in which we could impair a biofilm, making it more sensitive to antibiotics."
In previous work, Iglewski's team has shown that Pseudomonas controls several genes, including the lasI gene, through a mechanism known as quorum sensing, where bacteria release a chemical signal known as an autoinducer into shared surroundings and monitor its level. When the signal hits a certain threshold, bacterial behavior suddenly changes. Scientists first discovered the phenomenon about 20 years ago among bacteria responsible for the light that flashlight fish give off.
In Pseudomonas, the scientists found that only if enough bacteria were releasing the signal molecule ultimately coded by the lasI gene did the bacteria form a biocide-resistant biofilm.
"Only in the company of other bacteria will a cell turn on its genes and devote itself to making these energy-expensive products like toxins," says Pearson. Pearson is the one who actually made the vulnerable mutant form, using molecular biology techniques like restriction enzymes or "molecular scissors" to snip the gene from the bacteria's genetic code.
It was in Greenberg's laboratory at Iowa that Pearson, as a master's degree student four years ago, first solved the chemical structure of the Pseudomonas autoinducer. Later, as a student in Iglewski's laboratory, he learned how to manipulate the gene controlling that signal. With the mutant bacteria in hand, the Rochester and Iowa groups teamed up with the MSU laboratory, a renowned center for biofilm research, to check the gene's possible role in biofilm formation.
"We could get quite subtle in our approach to bacteria," says Costerton, who runs the MSU biofilm center. "We're used to killing them. Now perhaps we could just disrupt or manipulate them."
The scientists are now looking for easier ways to knock out the signal, checking to see how effectively antibiotics fight the mutant Pseudomonas, and hoping to find genes that control the formation of biofilms formed by other bacteria as well. Physicians aren't the only ones who'd like to wipe out biofilms; in industry such bacterial colonies clog pipes in all kinds of settings, including oil rigs and beer-making equipment.
"It's only recently that people have begun to appreciate that these aren't just random masses of organisms," says Pearson. "It's not just scum."