Microbes May Threaten Lengthy Spaceflights

Deborah Kimbrell of University of California at Davis is lead investigator for a NASA-supported experiment involving fruit flies, whose immune systems operate in ways that are surprisingly similar to those of humans. Her team sent 10 vials of fruit flies on a recent shuttle mission, along with fungus and bacteria to later infect them, to test their immunity.

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The research is not complete, but Kimbrell said that as with humans, the flies lost some of their ability to fight off infection after returning to Earth. She, too, believes that weightlessness itself plays a major role in disrupting the immune response.

The human immune system, which Pierson said is as complex as the central nervous system, includes an arsenal of responses to invading bacteria, viruses and other pathogens -- ranging from first responders such as neutrophils and microphages to more sophisticated T-cells and B-cells that act on a molecular level. Although research into the workings of these infection-fighters during space missions remains limited -- because of technical problems with collecting samples in space, very limited crew time and, more recently, deep funding cuts -- Pierson said many key infection-fighting molecules appear to be less available to people in space.

A NASA study published in 2005, for instance, found that neutrophil function in blood samples of 25 astronauts decreased significantly, in some cases by as much as 60 percent below preflight levels. Other research found that virus-specific antibodies increased significantly during flight, indicating that viruses were active and spreading.

Pierson and others say this appears to be caused by a number of factors -- including stress (known to decrease immunity), the fact that weightlessness changes the basic dynamics of cells, and the still-undetermined effects of increased exposure to radiation. Researchers said there have been anecdotal reports of increased fever blisters (a form of herpes), other minor outbreaks and colds.

But astronauts appear to have had sufficient immune reserves to avoid serious illness so far, and no missions have been compromised or cut short as a result, Pierson said.

Adding to the challenge is a recent finding that salmonella bacteria, a common food-borne pathogen, become more virulent in space. Researchers are now studying whether this phenomenon -- which also involves changes in how genes are expressed -- is specific to salmonella or occurs with other bacteria.

"Microbes do better and the host does worse," said Kimbrell. "A perfect, but unfortunate fit."