In 2011, scientists reported a startling announcement in Nature Letters. They had been collecting single-celled amoebae of the species Dictyostelium discoideum from the soil in Virginia and Minnesota.
While the laboratory strain of “Dicty” grazes contentedly on bacteria provided by its keepers, roughly a third of the wild strains turned out to be primitive farmers. When food was short, they gathered up bacteria, carried them to new sites, and seeded the soil with them, according to the Queller-Strassman lab, then at Rice University.
The discovery of the “world’s smallest farmer” garnered lots of attention.
Who’s in charge?
At the time most people assumed the amoebae were somehow in charge in this relationship. They were, after all, bigger, their spores sometimes contained bacteria, and they ate the bacteria.
Perhaps the farming amoebae had different genes than the non-farming amoebae.
In the Proceedings of the National Academy of Sciences, Professors David Queller and Joan Strassman, now at Washington University in St. Louis, reveal that things are a bit more complicated than first thought.
Bacteria, not amoebae, may be in charge—but not the bacteria the amoebae are farming. There is a third member of this symbiotic relationship.
Surveying the bacteria found in association with their stable farmer clones, the researchers identified both edible and inedible bacterial species, but the assemblage always included bacteria of the Burkholderia genus. This was intriguing because amoeba raised on a lawn of Burkholderia die; this is not a genus of bacteria they find edible.
Investigating further, the scientists soon learned that when they infected nonfarmer amoebae with Burkholderia, they were transformed into farmers. Once infected, they began to pick up and carry bacterial passengers, such as the food bacteria Klebsiella pneumoniae.
And if farmer strains of amoebae were treated with antibiotics that killed the Burkholderia bacteria, they reverted to the non-farming type and no longer picked up or carried food bacteria.
The scientists concluded that Burkholderia have both pathogenic and beneficial properties; pathogenic ones that facilitate infection and beneficial ones that promote the maintenance of a relationship once established.
Symbiosis apparently benefits all three partners. Dicty that carry edible bacteria are better able to survive starving times; and bacteria that hitchhike on Dicty are dispersed more widely. Dicty sometimes eat the edible bacteria, but the Burkholderia sometimes eat the Dicty.
“Now we know that Burkholderia are the drivers,” says coauthor Susanne DiSalvo, “likely to benefit by exploiting new terrain and sometimes harming their vehicle in the process.”