Although it is known that Wolbachia — a bacterial genus present in 20 to 80 percent of all insects — is transmitted from female hosts to their offspring through their germ line, it has only been suspected that infectious transmission must occur between carriers and noncarriers without passing through any germ line. Now, using confocal microscopy, researchers at Princeton University in New Jersey have shown that Wolbachia introduced to a host eventually crosses several tissues to ultimately infect the germ line of flies. The investigators, led by Horacio M. Frydman of the department of molecular biology, injected the abdominal cavities of several Drosophila species of flies with Wolbachia and imaged the insects’ ovaries using a Zeiss confocal microscope. They report in the May 25 issue of Nature that, over the course of several days, the bacteria crossed the peritoneal sheath membrane surrounding the ovary, the muscle epithelium that encloses each ovariole and the somatic tissue that envelops the germ-line cells. The pattern of infection suggests that Wolbachia reaches the germ-line cells through the forward section of the ovariole, or germarium. Interestingly, the germarium is the location of several somatic stem cells, leading the researchers to believe that these cells provide a niche that presents a preferential infection site for Wolbachia. In experiments with long-term, maternally transmitted infections, they found that female Drosophila had higher bacterial levels in their germ-line cells than in, for example, follicle cells. The only exception was that the Wolbachia also strongly accumulated along the border of the germarium, similar to the behavior of transient infections. Frydman’s group notes that the mechanism by which Wolbachia enters and accumulates within the stem cell niche remains unclear, but that future research should reveal the answer to that question as well as to the cellular basis for horizontal transfer.