The amoeba Dictyostelium discoideum (aka Dicty) feeds on a variety of bacteria. But a curious thing happens when Dicty become infected by inedible bacteria: The amoebas turn into tiny farmers. They gather up their edible bacteria, carry them to a new spot, and then seed the soil with them. The findings were published in Proceedings of the National Academy of Sciences this week.
About a third of wild Dicty strains farm bacteria when their food supply becomes limited. This relationship was discovered in 2011 and, at the time, researchers thought that the amoeba farmers were in charge and the bacteria were their crops. But as it turns out, there’s a third player: bacteria from the genus Burkholderia. Dicty don’t eat these bacteria. In fact, when the amoebas are raised on a lawn of Burkholderia, they die.
To investigate the role of the inedible bacteria in this system, a team of researchers from Washington University in St. Louis allowed Burkholderia bacteria to colonize non-farming Dicty. Once infected, the amoebas began picking up and carrying around the bacteria they use for food, such as Klebsiella pneumoniae. When farming amoebas were treated with antibiotics that wiped out Burkholderia, the Dicty reverted back to a non-farming lifestyle.
All three players seem to benefit from this “farming symbiosis” relationship. Dicty that farm edible bacteria are able to survive better when food is scarce, while all the bacteria carried by Dicty (edible or not) get dispersed more widely. Burkholderia gets the added benefit of infecting and colonizing Dicty. "Now we know that Burkholderia are the drivers," Washington University’s Susanne DiSalvo said in a statement. By inducing an amoeba to move, the Burkholderia are able exploit new terrain, although sometimes they end up harming their vehicle in the process.
It’s not entirely clear how Burkholderia transform their amoeba hosts. “I think the Burkholderia are infecting Dicty and disrupting some process whereby it digests its bacterial food,” DiSalvo told National Geographic. Dicty ends up having to carry around bacteria that it would normally digest. By protecting itself from being digested, Burkholderia make it possible for the amoebas to rear their own food bacteria, rather than just feed on them immediately. Since they’ve endowed Dicty with farmer-like characteristics, Burkholderia can be either beneficial or pathogenic, depending on how you look at it.