Researchers at MIT had recently tricked a bacteria, Rhodococcus, into producing two variations of a previously unknown antibiotic by making it fight for survival against another bacteria, Streptomyces. The original paper can be found in the Journal of the American Chemical Society. The researchers believe the genes encoding the enzymes necessary to produce these new antibiotics are on or related to a megaplasmid (a sequence of DNA) that Rhodococcus acquired from Streptomyces.
When I originally saw this, I thought they had coaxed a bacteria into evolving a new antibiotic rather than getting it to produce one it already had the genes for. It would still be an interesting approach, though to develop new antibiotics or modifications to existing ones.