Space: A bad influence on microbes?At least one common disease-causing microbe
becomes more
virulent in simulated microgravity. Scientists studying this phenomenon
hope to gain a better understanding of infectious disease.
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Right: A false-color micrograph of the disease-causing microbe salmonella. Image courtesy Avinash Abhyankar. 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. "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 and
for people here on the ground. Using modern advances in biotechnology
and the weightlessness provided by the International Space Station
(ISS), they plan to explore the changes in gene expression experienced by
microbes in the true weightlessness of spaceflight. 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. 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 shear 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.) 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. |
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