Three's company: symbiosis, drug discovery, and biofuel development

Dr. Cameron Currie and others from Harvard Medical School in Boston recently published in Nature Chemical Biology (D.-C. Oh et al. Nature Chem. Bio. doi: 10.1038/nchembio.159; 2009) their discovery of dentigerumycin: the third dimension of the symbiotic relationship mentioned above.

For years, it has been known that some species of ant have developed systems similar to agricultural civilization. Leaf-cutting ants bring little pieces of plant leaves back to the colony; the ants use these fragments to grow fungal colonies for their own sustenance. However, the relationship has an added layer of complexity: fungus-growing ants engage in mutualistic associations with not only the fungus that they cultivate for food, but also actinobacteria that produce selective antibiotics to defend that fungus from specialized fungal parasites. Without the mutualistic cooperation of these actinobacteria, fungal parasites invade and consume the ants' fungal garden.

Importantly, research on the bacteria hitching a ride on leaf-cutting ants could yield new antibiotics. This recent discovery could quicken the pace of antibiotic and biofuel development. The antibiotic produced by these bacteria to protect ants' fungal  crops from associated parasitic fungi was studied in great detail by Currie's group. Amazingly, this compound slowed the growth of a fungus that causes yeast infections in people. The researchers involved in this study hope that they will determine how to make better antibiotics by studying how the bacteria have adapted to fight the parasite.

This fungus could also facilitate the development of biofuels: a current obstacle is the fact that researchers have not found a way to break down cellulose (the fibrous material in plants) in the lab. Currently, the raw materials most commonly used for biofuel production are sugary crops like corn. Remarkably, ants from a single colony harvest up to 400 kg of leaves annually. Researchers seek to decipher the mechanism underlying this breakdown because currently, the key factor is missing: fungus samples cannot digest the cellulose in plant leaves when in the Petri dish.

The scientists who authored the study used a metagenomics approach to isolate the fungal enzymes responsible. They sequenced small bits of DNA from bacteria and other organisms living in fungal colonies, then compared these DNA samples to the database to determine which species were living in the fungal gardens studied. Ironically, they found that many species in the fungal garden can digest cellulose. Therefore, there must be something important in the natural environment that allows the reaction to occur, while this element is missing from systems studied in the lab. Currie's group have mapped out the genetic structure of these fungal enzymes with the ability to break down cellulose: the next step is to mimic nature's genius as only humans can.

For more on metegenomics, see:

http://www.nature.com/scitable/topicpage/Genomes-of-Other-Organisms-DNA-Barcoding-and-662

http://www.nature.com/scitable/content/Metagenomics-and-industrial-applications-56052

http://www.nature.com/scitable/content/Metagenomics-Exploring-unseen-communities-16767