Bacteria live inside bacteria in mealybug.

The ability of life forms to co-opt each other has taken a novel twist: Scientists have provided the first proof that a bacterium takes up long-term residence inside another bacterium. Both the host and the tenant, in turn, dwell inside a mealybug, which passes the bacteria on through its own eggs.

Many types of insects, including mealybugs and aphids, play host to bacteria. In return, the bacteria often provide nutrients to the insect.

Since the early 1900s, scientists have known that there was something particularly bizarre about mealybugs. When viewed with a microscope, the bacteria inside the abdominal cells of some mealybug species appear to be packaged in capsules.

Now, a group at Utah State University in Logan reports that each of these capsules is itself a bacterium. Carol D. von Dohlen, William R. McManus, and their colleagues started out by identifying the DNA for two bacterial species in the citrus mealybug, Planococcus citri. Then, using fluorescently labeled DNA, they pinpointed the location of the two sets of genetic material. That's when the researchers saw that one bacterial species actually encapsulates another, they report in the July 26 NATURE.

The new observations gel with decades-old electron microscope images showing details of the capsules. The researchers can now clearly interpret those details as bacterial structures. The Utah biologists observed that the bacterial capsule invades citrus mealybug eggs before hatching. Such behavior is common among insect-dwelling bacteria. It ensures that the bacteria will pass into successive generations of insects.

Lynn Margulis of the University of Massachusetts in Amherst says that the Utah group's data appear to be the first thorough molecular documentation of bacteria living inside bacteria. She notes, however, that other microscopists have "excellent" images of such phenomena.

John Stolz of Duquesne University in Pittsburgh agrees, but he rates much of the prior evidence as "circumstantial" and falling short of confirming long-term bacteria-in-bacteria associations. The new data clearly document such persistence, Stolz says.

Finding bacteria living inside each other has implications for the evolution of all cells with nuclei. Such eukaryotic cells form the building blocks of animals, plants, and many microbes. Bacteria lack a nucleus, but Margulis theorizes that the nucleus arose when one type of bacterium moved inside another.

"The discovery is a really fundamental one," says Paul Baumann at the University of California, Davis. However, it's not clear what one bacterium does for the other in the mealybug, he notes.