New, noncommittal mechanism of drug resistance discovered

Microorganisms like bacteria and fungi can evade treatment by acquiring mutations in the genes targeted by antibiotics or antifungal drugs. These permanent mutations were once thought to be the only way for drug-resistant strains to evolve. 

Now a new study has shown that microorganisms can use a temporary silencing of drug targets -- known as epimutations -- to gain the benefits of drug resistance without the commitment. Though the new mechanism was discovered in a fungus called Mucor circinelloides, it is likely to be employed by other fungi as well as bacteria, viruses and other organisms to withstand treatment with various drugs. The finding appears July 27, 2014, in Nature.

"This mechanism gives the organism more flexibility," said Joseph Heitman, M.D., Ph.D., senior study author and professor and chair of molecular genetics and microbiology at Duke University School of Medicine. "A classic, Mendelian mutation is a more permanent binding decision, like a traditional marriage. These epimutations are reversible, more akin to moving in together. If conditions change, it is easier to revert to the way things were."

The epimutations are so transient, in fact, that the researchers almost disregarded them. Cecelia Wall, a graduate student in Drs. Heitman and Maria Cardenas' labs, had been looking for mutations that would make the human fungal pathogen M. circinelloides resistant to the antifungal drug FK506 (also known as tacrolimus). This pathogen causes the rare but lethal fungal infection mucormycosis, an emerging infectious disease that predominantly affects individuals with weakened immune systems.