Astronauts weren't the only living things aboard the space shuttle Endeavour that landed safely this week – a precious payload of germs, grown and frozen in zero-gravity, also returned to Earth.

Researchers sent up sealed containers of Streptococcus pneumoniae bacteria, the germs responsible for many diseases in patients with weakened immune systems. David Niesel, a microbiologist at the University of Texas Medical Branch at Galveston, said the experiment will help scientists explore the risks of getting sick in space.

"There's a decline in people's immune function the longer they're in the space environment, and it's been shown that other bacteria also alter their properties in microgravity," Niesel said. "They grow faster, they tend to be more virulent and resistant to microbial treatment."

The S. pneumoniae bacteria are normally harmless, but Niesel said they never turn down opportunities to exploit weak immune systems and turn into full-blown disease. For astronauts on long spaceflights, he said, the germs could prove to be dangerous.

"Strep pneumoniae is a very potent pathogen in people who are immunosuppressed," he said. "It's the No. 1 cause of community-acquired pneumonia and a leading mediator of bacteremia [bacterial blood infections] and meningitis."

Having no well-equipped hospital in a small cabin millions of miles from Earth, Niesel and his colleagues wanted to know how S. pneumoniae behaved in space, as other shuttle missions have explored with different germs.

To do so, the researchers rocketed six refrigerated vials of bacteria into orbit, then had the space shuttle crew warm them up so that they could grow. After 15 hours and 30 minutes, the bacteria were chilled to -139 degrees Fahrenheit (-95 Celsius).

"That locked the bacteria at whatever stage they were at ? so we get a picture of what they were like in space at that time, which is the cool part," Niesel said. While the bacteria grew in space, Niesel and his team performed the same experiment on the ground for comparison.

"We should be able to see the differences that result when the bacteria see this unique space environment," Niesel said of the two perfectly synced experiments. ?"We think it will provide important information for understanding the adaptation of bacteria to unique environments and begin to answer the question of whether this species is a cause for concern for long-duration space travelers."