Clostridium difficile is a dangerous bug. Infections with this bacterium can cause life-threatening diarrhea, and they are most likely to affect the elderly or people with health problems who spend a lot of time in hospitals (where C. difficile flourishes). The Centers for Disease Control and Prevention estimates that in 2011 alone, hundreds of thousands of people were infected and 29,000 died from C. difficile infections or CDIs.
Antibiotics won't likely help the situation. In fact, those drugs are part of the problem: Broad-spectrum antibiotics wipe out colonies of helpful bacteria in patients, leaving the colon vulnerable to widespread colonization by C. difficile, which ordinarily live as one of many harmless species in the gut. Doctors may prescribe powerful antibiotics for CDIs, but they don't always work. Many strains of C. difficile are resistant to many antibiotics. Researchers estimate that CDIs recur in 25 to 30 percent of cases.
“Developing new antibiotics is not the most plausible route to go” says Charles Darkoh, a molecular microbiologist at the University of Texas Health Science Center, School of Public Health in Houston. “If they develop resistance to the few antibiotics currently used, then we've run out of options.”
How then to fight a bacterial infection without antibiotics? Some physicians have found success with fecal transplant, which involve repopulating a person's colon with healthy bacteria from a donor's stool. That procedure carries a significant “ick” factor that may deter patients, and Darkoh points out that researchers don't completely understand how it works, or what risks it brings.
He thinks there's a better, smarter way to fight C. difficile infections, and he's getting closer to it. He's spent the last nine years analyzing the molecular machinery behind C. difficile's malice, and he recently identified a weakness in the germ's armor.
C. difficile produces two particularly virulent toxins, called Toxin A and Toxin B, that cause disease. In 2015, Darkoh led a study published in mBio that traced synthesis of those toxins back to the accessory gene regulator (Agr) quorumsignaling system, which activates the toxin genes in response to signals from the bacterial population. Now, in new findings also reported in mBio that combine genetic analysis with mouse experiments, Darkoh and his collaborators have further narrowed their search for the genetic culprit.
Genetic analysis of C. difficile showed that the agr1 gene locus, associated with the Agr system, is responsible for producing those toxins. Mice infected with genetically modified C. difficile strains that lacked agr1 did not get sick, and the bacteria didn't produce toxins. Researchers did not previously know the role of agr1 in C. difficile infections.
Since it's unwieldy and likely impossible to change the C. difficile genes as a therapeutic approach, Darkoh's going for a more direct route. His newest findings suggest that if researchers can cripple the pathways controlled by the agr1 genes, they could halt production of the toxins.
“Since we know the toxins are the main cause of disease, if we can stop the toxins, we can stop the disease,” he says. “We know that strains that cannot produce toxins cannot cause disease.”
He was recently awarded an R01 grant from the NIH to develop compounds that can block the pathways regulated by the agr1 genes, which could be the non-antibiotic therapy he's looking for.
Think of it as a superdrug – for the superbug.