SPACE CENTER, Houston (AP) -- Space shuttle Columbia's astronauts kept busy Sunday as they continued their 16-day mission of conducting scientific experiments.

Astronaut Laurel Clark worked on a study of how bacteria and yeast develop in space and how reduced gravity affects their response to antibiotics.

The experiment is one of several from the European Space Agency.

``The clumps haven't changed a lot. But instead of being in one area of the chamber they've spread out to more of a serpentine pattern. Within (the) large clumps, there are visible islands of very white tissue,'' Clark said, describing how the yeast cells she tested changed.

``It sounds like you're seeing some good changes up there,'' Mission Control told Clark.

The results will be compared with results of an experiment conducted on Earth.

``We're just all thrilled that everything is going as well as it is,'' Clark said.

Astronaut Ilan Ramon, the first Israeli to fly in space, was also happy the experiment he was shepherding _ an attempt to photograph desert dust drifting over the Mediterranean in order to assess its impact on the weather _ was finally seeing some results.