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May
6, 2009: Salmonella, what's gotten into
you?
Researchers
have been asking themselves this question ever since Salmonella
bacteria grown on board the space shuttle returned to Earth
3 to 7 times more virulent than Salmonella grown
on the ground under otherwise identical conditions. Figuring
out why could help safeguard astronauts from disease and lead
to new treatments for food poisoning and other common ailments
on Earth.
 New
research by Cheryl Nickerson (Arizona State University) and
colleagues explains not only why Salmonella gets
"revved up" in space, but also how to calm it down
again.
"We
think space travel tricks Salmonella into behaving
as if it is in the human gut," Nickerson says. "It's
a mechanical phenomenon having to do with 'fluid shear.'"
Right:
A photomicrograph of Salmonella bacteria. Courtesy of Pacific
Northwest National Laboratory. [Larger
image]
Salmonella
microbes have the ability to sense the force of fluid moving
past their outer surfaces. This "fluid shear" acts
as a signal to the microbe, helping it to know where in the
human body it's located. Salmonella usually enters
the body by hitching a ride on food that a person eats. In
the middle of the tube-shaped intestines, the liquid-like
mixture of half-digested food and digestive juices churns
around quite a bit, so the amount of fluid shear is high.
But
as a Salmonella cell approaches the wall of the intestines,
it slips into the tiny spaces between microscopic, hair-like
protrusions called microvilli that cover the intestinal lining.
There, the cell becomes sheltered from the churning motion,
and fluid shear drops to very low levels. And it's there that
the bacterial cell can cross from the intestines into the
bloodstream to start an infection. So
it would make sense for a bacterium experiencing low fluid
shear to alter the activity of genes that help the bacterium
survive and cause infection.
Computer-based
simulations have shown that the amount of fluid shear experienced
by bacteria in the weightless environment of orbit is similar
to the amount in these tiny spaces at the intestinal wall,
Nickerson says. "Space flight is a low fluid shear environment."
Nickerson's
team looked at Salmonella from two shuttle flights
to the International Space Station: STS-115 in Sept. 2006
and STS-123 in March 2008. They found that 167 genes were
either more or less active in these keyed-up bacteria than
in the bacteria that hadn't flown. The team also identified
a "master switch" that regulates about one-third
of these genes, a protein called Hfq. Activity of this protein
was also affected by the low fluid shear conditions of spaceflight.
Above:
NASA astronaut Heidemarie Stefanyshyn-Piper activates a Salmonella
experiment during space shuttle mission STS-115. [Larger
image]
Now
that scientists know which genes and proteins help create
this virulence-boosting effect, they are working to use this
information to develop new strategies to combat Salmonella
food-borne illness, such as vaccines and therapeutics.
The
team has already found one promising way to combat Salmonella's
extra virulence: add a dash of ions. When Nickerson and her
colleagues grew the same strain of bacteria in a culture medium
that contained higher concentrations of five ions -- potassium,
chloride, sulfate, magnesium, and phosphate -- the virulence
of the bacteria due to spaceflight no longer went up!
"Cells
are funny things," Nickerson says. "If you give
them too much or too little of something they're used to having
around, they'll surprise you with how they respond."
As
it turns out, many of the genes activated by the low fluid
shear environment of spaceflight are involved in transporting
these ions in and out of the cells, so there could be a connection.
Research on this ion effect is still ongoing, Nickerson says,
but she speculates that it could eventually lead to new ways
to use these ions to ward off Salmonella infections.
"One
question people ask me is, 'Why in the world did you think
of looking at [Salmonella in space]?' I turn that
around and ask, 'Why would you not think of it!'" Nickerson
says. "Whenever scientists have studied microbes under
extreme conditions, we have found amazing new insights into
how they function. Space flight is another extreme environment
that's relatively untapped."
"To
me it was a no-brainer."
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
| more
information |
|
The
experiments described in this story were managed by
NASA's Ames Research Center at Moffett Field, Calif.,
in support of NASA's Exploration Systems Mission Directorate.
The Nickerson team's initial findings were published
in the Proceedings of the National Academy of Sciences,
and their most recent findings published in the journal
PLoS ONE.
NASA's
Future: US
Space Exploration Policy |
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