Thursday, 24 August 2017

Calcium key to fighting a dangerous hospital infection


The discovery, made in research laboratories at the University of Michigan Medical School and the U.S. Food and Drug Administration, is published in the online journal PLoS Pathogens. It helps solve a key mystery about C. diff: What triggers it to germinate, or break its dormancy, from its hard spore form when it reaches the gut.

Though the findings were made in mice, not humans, the researchers say the crucial role of calcium may help explain another mystery: Why some hospital patients and nursing home residents have a much higher risk of contracting C. diff infections and the resulting diarrhea that carries its spores out of the body.


That group includes people whose guts are flooded with extra calcium because they're taking certain medications or supplements, have low levels of Vitamin D in their blood or have gut diseases that keep them from absorbing calcium.


The new discovery shows that C. diff can recognize this extra calcium, along with a substance called bile salt produced in the liver, to trigger its awakening and the breaking of its shell.

Previous research had suggested it couldn't do this without another key component, an amino acid called glycine. But the new findings show calcium and the bile salt called taurochlorate alone are enough. Mouse gut contents that were depleted of gut calcium had a 90 percent lower rate of C. diff spore germination.

"These spores are like armored seeds, and they can pass through the gut's acidic environment intact," says Philip Hanna, Ph.D., senior author of the new paper and a professor of microbiology and immunology at U-M. "Much of the spore's own weight is made of calcium, but we've shown that calcium from the gut can work with bile salts to trigger the enzyme needed to activate the spore and start the germination process."

Ironically, the researchers say, one way to use this new knowledge in human patients might be to add even more calcium to the system.

That could awaken all the dormant C. diff spores in a patient's gut at once, and make them vulnerable to antibiotics that can only kill the germinated form. That could also prevent the transmission of more spores through diarrhea to the patient's room. That could slow or stop the cycle of transmission that could threaten them or other patients in the future.


Hanna's graduate student, Travis Kochan, made a key observation that led to the discovery. He noted that the fluid "growth medium" that the researchers typically grow C. diff in for their studies had calcium in it. He realized this could artificially alter the results of their experiments about what caused C. diff spores to germinate.

So, he used a chemical to remove the calcium while leaving all the other nutrients that keep C. diff growing. The result: no new spore germination happened in the calcium-free growth medium.

FDA's Center for Biologics Evaluation and Research conducted further research in laboratory dishes and in the guts of mice. FDA's Paul Carlson, Ph.D., a former U-M research fellow, and his laboratory found that C. diff spores that were mutated so that glycine couldn't act on them could still germinate and colonize mice. This suggested that calcium, and not glycine, was critical for this process.

Both mutant and regular forms of the bacteria could still activate an enzyme inside the C. diff spore that led the bacteria to start dissolving their hard shell. This released the store of calcium that the spore had been harboring inside itself, and increases the local level of the nutrient even further.

"These spores don't want to germinate in the wrong place," says Kochan, whose grandfather suffered from a severe C. diff infection which ultimately led to his death. "C. diff spores have specialized to germinate in the gut environment, especially in the environment of the small intestine, where calcium and the bile salt injection from the liver comes in."

Hanna notes that the bile salt connection to C. diff spore germination was first discovered at U-M in 1982 by a team led by Ken Wilson, M.D.

See:

Travis J. Kochan, Madeline J. Somers, Alyssa M. Kaiser, Michelle S. Shoshiev, Ada K. Hagan, Jessica L. Hastie, Nicole P. Giordano, Ashley D. Smith, Alyxandria M. Schubert, Paul E. Carlson, Philip C. Hanna. Intestinal calcium and bile salts facilitate germination of Clostridium difficile sporesPLOS Pathogens, 2017; 13 (7): e1006443 DOI: 10.1371/journal.ppat.1006443

Posted by Dr. Tim Sandle