Scott Hultgren is that rare biologist who has tracked the problem of uropathogenic Escherichia coli (UPEC)—the bacteria responsible for the vast majority of urinary tract infections—from the atomic level to the clinic. In a quarter century of study, he and his collaborators have attacked the problem by crystallizing protein structures, analyzing binding pockets, mutating genes, tinkering with cell adhesion, and rigorously testing their ideas in a mouse model of urinary tract infections (UTIs).
Now, with that bevy of knowledge behind them, Hultgren and his colleagues are poised to begin clinical testing of new therapies aimed at shutting down UTIs before they even get started. Their latest findings published this week in mBio show that their therapeutic approach is on the right track and might be even more effective than previously thought.
“One million women each year are suffering over and over again from recurrent UTIs, which greatly reduces their quality of life,” explains Hultgren, the Helen L. Stoever professor of molecular microbiology at Washington University School of Medicine in St. Louis. “At the same time, we are facing a tipping point when it comes to antibiotic resistance.”
Those UTIs that become chronically recurrent infections are often treated with the last line of defense, broad-spectrum antibiotics. In addition, Hultgren notes, some multi-drug resistant UTI strains are turning up in the clinic.
Hultgren’s group has been focused on laying out a molecular blueprint of the mechanisms that UPEC use to colonize the urinary tract and cause disease. Then, like electricians studying the wiring of a house, the researchers can pinpoint the weak point in the system that will short-circuit the bacteria’s ability to cause infection. “Ideally, we’d like to develop an antibiotic-sparing therapeutic to lessen the rise of multi-drug resistance,” says Hultgren, who is also the director of the Center for Women’s Infectious Disease Research.
One of the very first, key steps to infection is that UPEC attach to the bladder epithelial cells so they are not washed away by the urine flow. Hultgren’s group has previously shown, using a mouse model, that the bacteria use their hundreds of hair-like type 1 pili “like Velcro” to attach to the bladder. Specifically, the FimH adhesin molecule at the tip of the pili grabs onto mannose sugars on proteins that decorate the outer surface of bladder cells.
In the last several years, Hultgren’s lab has tested approaches that disrupt pilus assembly and FimH attachment. They observed that bacteria growing in urine often lacked the pili structures and wondered if something in urine prevents pili formation.
In this new study, the team tested UPEC grown in human urine, pooled from several healthy individuals, and found that, indeed, factors in the urine shut down pilus production. The urine was acting upon the operon that controls the expression of all the type 1 pilus building blocks and machinery, switching its promoter to the phase OFF state and keeping it there. However, when the group tested UPEC grown on a layer of bladder cells, they discovered that this urine effect disappeared when the bacterial cells were already attached—the pilus assembly operon remained phase ON.
Next, they wanted to look at urine’s effect on the FimH molecule itself and here they got a bit of a surprise. Unknown factors in urine disrupted FimH’s ability to attach. What’s more, that disruption of FimH’s function also resulted in the pilus assembly operon switching to phase OFF. In fact, any disruption of FimH’s sticking function—either by genetic mutation or by chemical inhibition—caused pilus assembly to turn phase OFF.
“It’s as if the bacterium senses it has a non-functional adhesin and switches everything off,” says Hultgren. The finding that blocking FimH’s action also throws the genetic switch to the phase OFF state and stops pilus production means that drugs targeting FimH may be very effective at preventing the start of infection.
“It looks like this genetic switch is really hot-wired,” says Hultgren. “If you just look at the pilus sideways, it’s going to turn off assembly.” The findings exposed FimH attachment as the bacteria’s weakest link to attack.
Working with Washington University medicinal chemist James Janetka, the team has developed oral drugs called mannosides that block FimH attachment to cells with over one million-fold higher potency than D-mannose, and prevent further adhesion. “Now we know that these drugs also translate to blocking the expression of pilus assembly. That’s a two-pronged approach, which will increase efficacy and potency,” says Hultgren.
And these drugs are not antibiotics that kill the bacteria, he points out. They simply act to prevent bacterial attachment so that the body can flush the bacteria away. “We’d be targeting the Achilles’ heel of the bacteria,” he notes.
Soon, the team will begin safety testing of drug candidates in preparation for first-in-human clinical trials. The challenge will be to determine if their results from laboratory models translate to human patients. If successful, mannosides could become one of the first small molecule anti-virulence drugs to treat bacterial infections. Hultgren is cautiously optimistic: “We have an opportunity to translate all of this basic science into an efficacious therapeutic that is badly needed.”