It's unavoidable: Humans can't go
to space without taking trillions of microscopic microbes with them.
An experiment onboard the ISS aims to find out how bugs behave in
by Dr Tony Phillips
There are trillions of microbes orbiting
Earth onboard the International Space Station (ISS). And that's
just in the gut of one astronaut.
Astronauts, like everyone
else, carry microbes with them wherever they go. There are 1014
in the colon, trillions more on your hands, and in your mouth. The
math is simple: Microbes outnumber people, in space and on Earth,
by a staggering factor.
In fact, says Cheryl
Nickerson of Tulane University Health Sciences Center, "there are
more bacterial cells in your body than human cells."
Many are beneficial.
Some of the bacteria in our stomachs, for instance, produce vitamin
K needed for the proper clotting of blood. Others help digest food.
Even pathogens, in moderation, lend a hand by stimulating the immune
system. In short: people need bugs.
With NASA planning
to send people back to the Moon and on to Mars, researchers are
increasingly focused on the question, "what does space travel do
to the human body?" An inseparable question is, "what does space
travel do to microbes?"
There's already some
evidence that microorganisms behave oddly in a weightless environment.
In her laboratory at Tulane, Nickerson has floated some Salmonella
bacteria in a rotating wall bioreactor--a device designed by NASA
that simulates low gravity.
The bioreactor is a
fluid-filled habitat for bacteria. Shaped like a cylinder, it slowly
rotates, gently tumbling microbes inside. Bacteria in the bioreactor
never hit bottom, they hang suspended in their liquid growth medium,
much as they would in Earth orbit. "It isn't real microgravity,"
notes Nickerson, "but it does approximate some aspects of weightlessness."
tube full of bacterial colonies grown onboard the space
shuttle ("0-g") alongside a matched ground control ("1-g").
Production of the antibiotic Monorden was 200% greater
in the 0-g test tube.
In the bioreactor,
Salmonella changes. The 2nd-leading cause of gastric distress in
the USA gets even worse than usual. Its ability to cause disease
is increased, says Nickerson. Pseudo-weightlessness makes the Salmonella
more resistant to stomach acids and to heat. The bacteria also do
a better job eluding macrophages, which are disease-fighting cells
in your immune system.
Scary. But the news
isn't all bad. Some bacteria can produce helpful antibiotics, and
they seem to produce more in space than on Earth--as much as 200%
more according to mid-1990 space shuttle experiments sponsored by
pharmaceutical company Bristol-Meyers Squib and partner BioServ
"These changes are
a result of altered genetic expression," says Nickerson. Somehow
weightlessness signals the genes of these microbes, commanding them
to do things differently. In Salmonella, for instance, 163 genes
(out of about 4600 total) changed their levels of expression--becoming
more or less active than usual--inside the bioreactor. "The affected
genes covered the full range of cell function: metabolism, structure,
movement, virulence factors. You name it."
Researchers are only
beginning to understand these sweeping changes. Are some microbes
more sensitive to space travel than others? Which genes are most
altered? And how does this affect people?
An experiment on the
ISS, called "Yeast GAP," aims to find out. Nickerson is the principle
investigator (PI) for the experiment. She works closely on the project
with her Co-PI, Tim Hammond of Tulane University Health Sciences
Center and the Veterans Affairs Medical Center in New Orleans.
"Last month, we sent
16 vials of brewer's yeast to the space station onboard a Russian
Progress supply rocket," says Hammond. In orbit, space station science
officer Mike Foale gave the yeast some nutrient soup, and they began
to reproduce. The population of cells grew ten-fold in only 30 minutes--"that's
about five generations of yeast," notes Hammond. Then Foale flooded
the growth chamber with a fixative agent, to stop the population
explosion and preserve the cells for analysis back on Earth.
Nickerson and Hammond
"can't wait to get the yeast back" to their labs in Louisiana, because,
they believe, the cells are going to teach them a lot about genetic
activity in space. "These are no ordinary yeast cells," notes Nickerson.
"They've been genetically engineered" to reveal their secrets.
Hammond explains: "Yeast
cells have 6312 distinct genes, so scientists have created 6312
different varieties of yeast. Each variety has one gene 'knocked
out' and replaced with a barcode pattern of nucleotides." These
barcodes are like dog tags; by reading them, researchers can tell
which gene has been knocked out of a particular yeast cell.
The Yeast GAP apparatus. "GAP" is short for Group Activation
All 6312 types were
sent to the ISS, and all 6312 have had their opportunity to grow
Which varieties grew
best? Which ones fared poorly? Nickerson and Hammond will find out
when the samples are returned to Earth (on some future shuttle flight).
Using DNA microarray analysis, they will sort the yeast cells by
barcode and count them, and compare the results to identical samples
grown on Earth. This simple procedure will reveal genetic activity
and pinpoint the genes yeast needed most to thrive in orbit.
Simpleā€¦ but ingenious.
Yeast is a good organism
for this research for many reasons. Its genome has been completely
mapped. It's tough enough to withstand a trip to space. And although
the Yeast GAP strain is benign, some of its genes are similar to
those found in infectious microbes, so it serves as a model for
virulent bugs that might become more virulent in orbit. "Yeast even
has some genes in common with people," adds Nickerson. "So it can
teach us lessons about human responses to spaceflight, too."
Here on Earth, yeast
is perhaps the oldest domesticated organism, used for millennia
to bake breads and brew beer. In space, astronauts are going to
want to do those things, too, eventually. Knowing how yeast reacts
to weightlessness has practical value beyond genetics research.
Cooking. Sleeping. We do none of these things alone. Trillions of
microbes do them with us. And if we settle space, they're coming,
too, so we'd better find out how they like it.