Fish guts reveal microbial alliance.

When most people clean a fish, they throw away the guts, but a team of biologists would rather take a close look at them.

To understand the roles of the many microbes that live naturally within mammalian intestines, Jeffrey I. Gordon of Washington University School of Medicine in St. Louis has long studied mice born and raised germfree (SN: 5/31/03,p. 344). Now, he and his colleagues are pursuing similar studies with zebra fish.

While these small vertebrates don't possess a stomach, much of the rest of their intestinal tract resembles the mammalian system. Equally important, zebra fish larvae are transparent, enabling the scientists to easily observe gut development.

In an upcoming Proceedings of the National Academy of Sciences, Gordon's team describes how to raise germfree zebra fish. The researchers report that the health of the sterile fish begins to deteriorate about a week after birth and that the fish die within a month rather than live several years.

However, if the germfree animals are inoculated with microbes from a typical zebra fish aquarium, they appear to have normal life spans.

Gordon and his colleagues compared gene activity in the guts of germfree fish with that offish with microbes and found differences in 212 fish genes. They also uncovered evidence that germfree zebra fish have difficulty processing nutrients and show abnormalities in their immune systems. Similar observations have been made in germfree mice.

The microbe-host interactions shared by the guts of zebra fish and mice probably evolved in a common ancestor long ago, according to the researchers. They now plan to use the germfree fish to identify the chemical signals with which microbes influence the gut.