Via Small Things Considered - Click "source" for more.
"Rattling around inside my head for some time has been the reported discovery that there are bacteria that live within other bacteria. To me, this is an honest-to-goodness gee-whiz piece of microbial lore. Made me wonder why the story had not been followed up on. Could be because you have to go to mealybugs to find these eccentrics. But now I'm happy to report that someone has. A recent paper by the discoverer of this phenomenon, entomologist Carol von Dohlen, and the endosymbiont expert John McCutcheon has the tantalizing title of An Interdependent Metabolic Patchwork in the Nested Symbiosis of Mealybugs."