Two species of fungi isolated from an abandoned mining pit in Montana, when cultured together, produce a compound that kills four antibiotic-resistant strains of Staphylococcus aureus bacteria (J. Nat. Prod. 2017, DOI: 10.1021/acs.jnatprod.7b00133). Although its structure resembles a known class of antibiotics, the compound appears to kill bacteria in a new way, the researchers say.
After companies suspended copper mining at the Berkeley Pit outside of Butte, Mont., in 1983, runoff and groundwater collected in the pit and created a lake. Oxidized rock exposed by mining acidified the water to pH 2.5, and heavy metals including iron, copper, arsenic, and cadmium leached into the water. The water is so toxic that last winter, several thousand snow geese died after waiting out a snowstorm in the pit during their annual migration.
Microbial life, however, finds the pit more hospitable. In extreme environments, such as acidic pH and high metal concentrations, microbes often produce molecules with interesting biological activity. For almost 20 years, Andrea A. Stierle and Donald B. Stierle, both natural products chemists at the University of Montana, have been studying compounds produced by individual fungi isolated from the water and sediment at the Berkeley Pit. They have found molecules that slow inflammation, cell death, and cancer metastasis. But until now, the researchers had not found antibiotics.
The Stierles wondered whether culturing together two different species of Penicillium fungus, originally isolated from the same sample, would produce new bioactive compounds that neither strain makes when grown by itself. They started growing one fungus in a liquid broth and introduced the other species a day later. After the two fungi grew together for six days, the researchers collected the organic molecules the cells produced, extracting them from the broth with chloroform. They used a protein inhibition test the team had developed previously to identify molecules with potential biological activity and then determined the structures of those molecules using various spectroscopic methods. The researchers found a variety of 16-membered macrolides—molecules with a lactone ring—that were not produced when either fungus was grown alone.
Because these structures resembled known antibiotics such as erythromycin, the Stierles and their colleagues tested several of the isolated compounds for antibiotic properties. One compound, Berkeleylactone A, was active against four strains of methicillin-resistant S. aureus.
Structural differences and biochemical tests indicate that this molecule could work differently than similar macrolide antibiotics, Andrea Stierle says. Berkeleylactone A lacks sugars or a double bond, two structural features thought to be important to the antibiotic properties of other 16-member macrolides isolated from bacteria or fungi. And although its exact method of action is unknown, it does not inhibit protein synthesis or stall ribosome activity like other macrolide antibiotics do.
Lesley-Ann Giddings, a natural products chemist at Middlebury College, thinks it’s interesting to co-culture two fungi, because most researchers who have used this strategy to identify new molecules have grown a bacterium and a fungus simultaneously. Co-culturing microorganisms establishes a competition for resources that can trigger one microbe to express otherwise inactive genes and produce compounds that kill the other.