that I'm studying the bacterium Sinorhizobium meliloti  and how it interacts with its host plant, in the genus Medicago (which includes alfalfa). One of the things that I observed was that there were some strains of Sinorhizobium meliloti that were incompatible with a particular host, e.g. Medicago truncatula, but that rarely they would mutate and become compatible. Upon investigation we discovered that this mutation was the loss of a plasmid. A plasmid is a circular piece of DNA that isn't necessary to the bacterium. In fact, it's often parasitic. And in our case, it was preventing full symbiosis with the plant host. We also determined that this plasmid, despite disrupting the symbiosis, also made the bacterium more competitive at initiating the symbiosis. We've sequenced one such plasmid (we've found four different ones, so far) and are in the process of determining which gene(s) is/are responsible for disrupting symbiosis.
 See my post Matt Gets Published! I.
 You can see the abstract here (to read the full paper you have to have a subscription to Molecular Plant–Microbe Interactions or pay a one-time $27 fee to download the article).
 You can see it, along with a blurb about the article, here.
 See my post What Is It That Matt Does, Anyway?
 You can see photographs of Sinorhizobium meliloti as well as other species of rhizobia growing in Petri dishes at my post The End of Procrastination III.
Medicago truncatula is by Ninjatacoshell, available at http://commons.wikimedia.org/wiki/File:Medicago truncatula A17 plant.JPG.