Life is a bit different in space, even for microbes. Research shows that the pattern of gene activity in some microbes differs in weightlessness, leading to differences in behavior. These differences could be behind a curious observation: the common food-borne pathogen salmonella becomes more virulent when grown in a form of simulated microgravity.
This news is little comfort to astronauts whose immune systems already function below par in weightlessness, making infection more likely. To help keep astronauts healthy and to better understand microbial infection in general, scientists want to know exactly which genes are affected by microgravity and why weightlessness--whether real or simulated--should cause these changes.
Above: A false-color micrograph of the disease-causing microbe salmonella. Image courtesy Avinash Abhyankar.
"Whenever you see the virulence of a microbe change in response to an environmental stimulus, that's a chance to learn something about how that pathogen causes disease," says Cheryl Nickerson, an expert in microbiology and immunology at Tulane University Health Sciences Center.
Nickerson and her colleagues hope that studying these changes could point out new ways to combat "bad" microbes with drugs and vaccines, both for the sake of astronauts exposed to extra-virulent microbes and for people here on the ground. Using modern advances in biotechnology and the weightlessness provided by the International Space Station (ISS), they will explore the changes in gene expression experienced by microbes in the true weightlessness of spaceflight.
Their first experiment, called "Yeast GAP", will send genetically engineered brewer's yeast (Saccharomyces cerevisiae) up to the space station aboard a Russian Progress rocket in early 2004.
Brewer's yeast is not itself pathogenic. Nevertheless, "yeast cells make a great 'model organism' for this research because they're easily handled, thoroughly studied, and their genome has been completely mapped," says Nickerson, the principal investigator of Yeast GAP. Furthermore, brewer's yeast shares much of its DNA with infectious species of microscopic fungi and protozoans. "Also, the yeast's genome is relatively simple, which makes the results easier to analyze," she says.
Left: The single-celled fungus Saccharomyces cerevisiae, also known as brewer's yeast. Image courtesy David Byres, Florida Community College at Jacksonville.
Still, the challenge is formidable. The brewer's yeast genome contains 6,312 genes, each of which produces one of the proteins that constitute the molecular machinery of the cell. To get a grip on this immense complexity, the researchers will send up 6,312 variants of the single-celled yeast. Each variant has a different gene "knocked out" and replaced with a unique "barcode" pattern of custom-made DNA. This barcode DNA does not encode a protein; it merely serves as a tag distinguishing that particular variant from all the others.
"We mix all these different strains of yeast in a special growth apparatus (called the Group Activation Pack, hence the acronym GAP) and see which ones grow well in weightlessness," explains Timothy Hammond, co-investigator for Yeast GAP and a kidney specialist (nephrologist) at Tulane University Health Sciences Center and the Veterans Affairs Medical Center in New Orleans.
Suppose a yeast variant is missing some particular gene--let's call it "gene X." And suppose that variant fails to grow as well in space as it does on the ground. Such a result would imply that the missing gene "X" is an essential part of the yeast's response to microgravity.
That little nugget of knowledge would then help guide future research: scientists could target their experiments to see how the protein produced by gene X relates to the changes in various microbes' behaviors in space--including microbes that cause disease.
Above: Growing cells remain suspended in microgravity--a difference from ground-based cultures that could be cueing differences in gene expression. Image courtesy NASA.
Why should any kind of cell behave differently in microgravity? No one's sure, but scientists have some ideas. For example: perhaps cells sense deformations in their sack-like membranes and respond to that signal. Cells cultured in 1-g normally settle to the bottom of their container and become flattened, while cells floating in weightlessness remain more round. That difference could be cueing changes in gene expression.
Nickerson and others are exploring this idea on the ground using a "microgravity simulator" developed by NASA's Johnson Space Center. Called the "rotating wall vessel bioreactor", it mimics the conditions of weightlessness for microbes by growing them inside of a slowly rotating liquid-filled chamber. The rotation of the liquid counteracts the slow sedimentation of the cells, thereby creating a constant "free-fall" of the cells through the culture medium. Cells feel a slight sheer as they move through the liquid--a difference from true weightlessness that could affect their behavior--but like cells in orbit, they avoid becoming flattened on the bottom of the container. (It was using this bioreactor that Nickerson first noticed the increased virulence of salmonella.)
Right: A commercially available version of the rotating wall bioreactor developed by NASA's Johnson Space Center. Image courtesy Synthecon, Inc.
Apparently, the bioreactor's approximation of weightlessness works rather well. An earlier experiment by Hammond showed that a strain of brewer's yeast grown on the ground in the bioreactor showed many of the same changes in behavior as yeast grown onboard the space shuttle. Exploring the similarities and differences in how cells react to this bioreactor environment versus true microgravity will be another important outcome of Yeast GAP, Hammond says. If the rotating bioreactor proves sufficiently similar to the orbital environment, it could provide a cheaper and more convenient way to study microbes in microgravity-like conditions.
Whether performed in true or simulated weightlessness, this line of research could help unravel the genetic basis of infection--a bit of knowledge that would help astronauts and land-lovers alike to live a little healthier.
Biological and Physical Research on the ISS -- NASA's Office
of Biological and Physical Research supports research on the International Space Station
Yeast GAP fact sheet -- from Marshall Space Flight Center
Antibiotics in Space -- (Science@NASA) Test tubes of bacteria produce more antibiotics in space than they do on Earth. Researchers aren't sure why ... but they aim to find out.
Patches for a broken heart -- (Science@NASA) Using a space-age device called a bioreactor, researchers have grown patches of tissue that beat and respond much like a human heart does.
BioServe Space Technologies -- a non-profit, NASA-sponsored Research Partnership Center (RPC) located at the University of Colorado in Boulder that developed the Group Activation Pack experiment apparatus used in the Yeast GAP experiment
Bioreactor brief for educators -- with instructions on how to build a classroom bioreactor
The genetics of brewer's yeast -- a thorough introduction by Fred Sherman of the University of Rochester Medical School
Background on brewer's yeast -- from the University of British Columbia
Synthecon -- a commercial company marketing the rotating bioreactor technology developed by NASA
Nickerson, C.A., Ott, C.M., Mister, S.J., Morrow, B.J., Burns-Keliher, L., and Piersons, D.L., "Microgravity as a Novel Environmental Signal Affecting Salmonella enterica Serovar Typhimurium Virulence," Infection and Immunity, 68(6), 3147-3152 (2000).
Wilson, J.W., Ramamurthy, R., Porwollik, S., McClelland, M., Hammond, T., Allen, P., Ott, C.M., Pierson, D.L., and Nickerson, C.A., "Microarray analysis identifies Salmonella genes belonging to the low-shear modeled microgravity regulon," Proc. Nat. Acad. Sci., 99(21), 13807-13812 (2002)
Johanson, K., Allen, P.L., Lewis, F., Cubano, L.A., Hyman, L.E., Hammond, T.G., "Saccharomyces cerevisiae gene expression changes during rotating wall vessel suspension culture," J Appl Physiol, 93, 2171-2180 (2002)
Nickerson, C.A., Ott, M.C., Wilsona, J.W., Ramamurthry, R., LeBlanca, C.L., Bentrupa, K.H., Hammond, T., Piersons, D.L., "Low-shear modeled microgravity: a global environmental regulatory signal affecting bacterial gene expression, physiology, and pathogenesis," J Microbiological Methods, 54, 1-11 (2003)