It has long been known that some insects house bacteria. But researchers peering inside one of these bugs have found that, like a Russian doll, the bacterium can have yet another bug living inside it. The finding, reported in the 26 July issue of Nature, is the first known case of one bacterium living symbiotically inside another.
Insects that feed on sap, such as aphids, leafhoppers, and whiteflies, host mutualistic bacteria in their guts that produce needed nutrients. These "endosymbionts" often live within specialized host cells and have been transmitted from mother to offspring for millions of years. Many insects even contain several types of bacteria in different places. So when Carol von Dohlen of Utah State University in Logan set out to study the two known bugs in the citrus mealybug, she expected to find them side by side or in separate cells.
But when Von Dohlen and her student Shawn Kohler examined the mealybugs' so-called "symbiotic spheres," long assumed to be host structures that housed bacteria, they became stumped. Using a technique called fluorescent in situ hybridization, they found evidence of both beta-proteobacteria and gamma-proteobacteria inside the spheres. But even using electron microscopes, they couldn't localize the beta-bacteria. Finally, microscope supervisor William McManus asked, "What if the symbiotic spheres ARE bacteria?" Von Dohlen and Kohler looked at each other. "When he said that, everything suddenly made sense," von Dohlen says.
Further work confirmed that the gamma-bacterial endosymbionts indeed live inside the beta-bacterial ones--and that both are transferred together from mother to offspring. The nature of the relationship between the two bacteria is unknown, Von Dohlen says, nor is it clear whether the innermost bug performs any services for the insect.
The paper is "a major new contribution," says bacteriologist Paul Baumann of the University of California, Davis. The demonstration that one bacterium can live inside another is "one of the most exciting findings in many years," he says. To Von Dohlen, "it goes to show that you can have this preconceived notion that will blind you to data that's in front of your face." And as for McManus, who opened her eyes, she says, "I instantly made him a co-author!"