St. Louis, Nov. 19, 1998 -- Scientists have discovered how the bladder responds to bacteria that cause cystitis. They also have found that the bacteria can hide out within the bladder lining, where they could promote further bouts of infection.
"We discovered that cells that line the bladder have a built-in defense mechanism that kicks in when bacteria attach to them - they commit suicide and slough off," says Scott J. Hultgren, Ph.D., who directed the research. "But we also found that some of the bacteria avoid being removed from the bladder by invading underlying cells."
Hultgren is an associate professor of molecular microbiology at Washington University School of Medicine in St. Louis. One of his postdoctoral fellows, Matthew A. Mulvey, Ph.D., is first author of the paper, which appears in the Nov. 20 issue of Science.
Cystitis, or bladder infection, sends 7 million women to the doctor each year. Intense pain, burning and frequent urination are among its symptoms.
Hultgren's group previously determined that cystitis-causing E. coli attach to the bladder lining with hair-like projections called type I pili. The pili are tipped with a protein that locks into receptors on the bladder lining like velcro, enabling the bacteria to cling.
Mulvey wanted to know how the bladder responds to bacterial infection, so he enlisted the help of electron microscopists John Heuser, Ph.D., professor of cell biology and physiology, and G. Michael Veith, senior microscopist in biology.
Images of mouse bladders two hours after infection showed E. coli adhering to the lining. These are the first high-resolution snapshots revealing how bacteria can cling to cells.
Covering the bladder lining were hexagonal tiles of proteins called uroplakins. The pili were interacting directly with the uroplakin layer, and they were much shorter than when E. coli grows in broth. "Either pilus retraction or hindrance of pilus growth could result in the pile up of unassembled pilus components in the bacteria, and this is known to switch on certain genes," Hultgren says. "So this could be how E. coli senses that it has attached to the bladder lining."
After the bacteria adhered, many of the cells lining the bladder sloughed off, carrying the attached bacteria with them. Six hours after infection, about 90 percent of the bacteria were lost, and underlying bladder cells were exposed. "This process of bladder cell elimination is thought to be a natural defense mechanism of the urinary tract," Hultgren says.
It also may be an example of altruistic suicide by animal cells in response to infection. Mulvey showed that the bladder cells activated protein-destroying enzymes and cut up their DNA before they sloughed off. "It's fantastic first line of defense to have a group of cells purposefully killing themselves in order to protect the rest of the tissue," he says.
Twelve hours after infection, mouse bladders treated with a cell suicide-preventing drug contained 85 percent more bacteria than untreated bladders. "This supports the idea that suicide of infected bladder cells helps clear the bacteria," Mulvey says.
The researchers were surprised to find that a laboratory strain of E. coli with type I pili was as effective at provoking the suicide response as the clinical strain of E. coli. But the same two strains lacking FimH - the adhesive protein at the tips of type I pili - were ignored by bladder cells. "These experiments show that bacterial attachment by FimH is a critical step in triggering the suicide response of bladder cells," Mulvey says.
Despite the bladder's vigorous response to E. coli, significant numbers of bacteria remained 48 hours after infection. Most weren't on the surface, however. Electron micrographs taken two hours after infection showed the bacteria invading the bladder lining, which seemed to be enveloping them. Using biochemical techniques, Mulvey determined that most of the bacteria persisting within the bladder after two days appeared to be hiding out within bladder cells.
"This suggests that the bacteria can resist the bladder's built-in defense mechanism by invading into deeper tissue," Hultgren says. "That may explain why many patients have recurrent bladder infections despite antibiotic treatment, which may not efficiently kill bacteria protected within the bladder cells."
Hultgren's work eventually may help women avoid bladder infections altogether. His group has developed FimH into a vaccine that proved effective against cystitis in mice and should be tested in humans within the next two years. "A vaccine could prevent all of this from occurring in the first place," Hultgren says.
Mulvey MA, Lopez-Boado YS, Wilson CL, Roth R, Parks WC, Heuser J, Hultgren SJ. Induction and evasion of host defenses by type I-piliated uropathogenic Escherichia coli. Science, Nov. 13, 1998.
A grant and fellowship from the National Institutes of Health and a Lucille P. Markey Special Emphasis Pathway in Human Pathobiology fellowship supported this research.
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