Recently, researchers at Washington University in St. Louis and Harvard University collaborated to take a closer look at Dictyostelium discoideum, which is a single-celled organism discovered in 2011 and called a world’s smallest “farmer.” This social amoeba D. discoideum carries two strains of bacteria Pseudomonas fluorescens. to new locations and harvests them like crops: one is the “seed corn” for a crop of edible bacteria, and the other is a weapon that produces defensive chemicals.
They found that the edible bacteria evolved from a toxic one. The two strains differ by many mutations but a single key mutation in the non-food strain alters expression of 10% of its genome. This key mutation (an activator that regulates the pyrrolnitrin pathway) had the surprising effect of making the bacterium edible by changing its chemical profile.
The discovery is reported in the July 29 issue of the Proceedings of the National Academy of Sciences.
D. discoideum was found by Debra Brock, a graduate student in the laboratory run by David Queller and Joan Strassmann at Rice University in Houston, Texas. When she cultured a champion D. discoideum, there were two strains of bacteria looked different, which were identified genetically both as P. fluorescens.
Jon Clardy of the Harvard Medical School in Boston, studies the chemistry of mutualism, contacted the Queller/Strassmann lab to suggest the two labs collaborate to unravel the interactions among the newly discovered threesome.
“Ultimately Pierre figured out that the nonfood strain was producing two chemicals: chromene and pyrrolnitrin. And excitingly, chromene is a new compound. We determined chromene increases spore production in the farmer strain and suppresses spore formation in the nonfarmer strain,” Strassmann said. “We saw the same increases in the farmer and decreases in the non-farmer with pyrrolnitrin. A known antibiotic and antifungal, pyrrolnitrin probably also suppresses other organisms in the soil that might compete with the farmer strain.”
Photo credit: Scott solomon, http://news.sciencemag.org.