New Hope in the War Against Clostridium Difficile: Missile Defense

August 21, 2011

A collaboration of researchers has made a discovery in the battle against Clostridium difficile that may offer an alternative to the typical treatment of antibiotic therapy.

Clostridium difficile, also called C. diff, or C. difficile, is a bacterium that may develop after a prolonged use of antibiotics.  Clostridium difficile infections cause diarrhea and possibly more serious intestinal conditions like colitis.  Infections are typically treated by discontinuing any antibiotic to which the patient was previously exposed, and then treating the infection with an appropriate course of antibiotics such as metronidazole or vancomycin.

According to the Centers for Disease Control and Prevention, C. difficile is a spore-forming, gram-positive anaerobic bacillus.  It produces two exotoxins:  toxin A and toxin B.

Researchers from various academic institutions — the University of Texas Medical Branch at Galveston (UTMB), University of California, Los Angeles (UCLA), Case Western Reserve University, Tufts University, and the Commonwealth Medical College — were in the process of investigating the Clostridium difficile toxins when they discovered a key mechanism used by intestinal cells to defend themselves against the antibiotic resistant bacteria.

In a press release issued by the University of Texas Medical Branch at Galveston, Professor Tor Savidge, the lead author of an article about the discovery published online in Nature Medicine August 21, likened the discovery to missile defense:

“About one percent of all hospital patients develop a C. difficile infection — they’re treated with antibiotics to the point that benign gut bacteria are knocked out, and because C. difficile is resistant to antibiotics it’s able to proliferate[.]” … “Then it releases these toxins that trigger colonic disease.”

The toxins wreak havoc on cell structural proteins and biochemical communications networks, eventually killing the cell. But in order to do this damage, the toxins first have to get into the cell, and that means passing through the protective membrane that surrounds it.

It’s there that Savidge and his collaborators … may have found a way to stop them.

On the molecular scale, C. difficile toxin proteins are quite large — big enough that they have to “cleave” so that a smaller piece can slip through the membrane and into the cell. This cleavage is accomplished by a built-in molecular guillotine called a cysteine protease, which activates when the toxin encounters a molecule called InsP6 that is present at much higher levels inside the cell than outside.

“It’s sort of like a sensor mechanism that detects when it’s in a cell — the toxins say, InsP6 is here, it’s time to cleave,” Savidge said. “But we’ve identified a previously unknown protective response that activates after the toxins have induced gut inflammation, in which the host uses a process called nitrosylation to shut down the cysteine protease and prevent cleavage.”

A toxin that’s unable to cleave stays stuck in the cell membrane, incapable of attacking the cell.

The researchers used test-tube, cell culture, patient specimens and animal model experiments, along with computer simulations of molecular interactions, to thoroughly explore this response — and to successfully devise a way to mimic it for therapeutic purposes.

“Think of these toxins as missiles that the bacteria is producing to go off and detonate inside the cell,” Savidge said. “One way to defend against missiles is to send out signals that trick them into either disarming their sensory mechanisms or get them to prematurely detonate.”

The team conducted cell culture and mouse experiments that indicated that a combination of the nitrosylating agent and the “disarming” part of Savidge’s analogy and InsP6 (the “premature detonation” part) did in fact work to prevent cell damage from C. difficile.

The multi-disciplinary team is now moving forward to test the discovery in a clinical trial to be sponsored by UTMB’s Institute for Translational Sciences.

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