A germ that
causes food poisoning and other illnesses can be three times more dangerous in
space than on the ground, an experiment has shown.
The finding
spells out tougher challenges for astronauts taking trips to the moon or Mars,
as recent work also hints that the body's immune
system weakens during extended stays in space.
"Space
flight alters cellular and physiological responses in astronauts including the
immune response," said Cheryl Nickerson, a microbiologist at Arizona State University and leader of the experiment. "However, relatively little was known
about microbial changes to infectious disease risk in response to space
flight."
NASA's STS-115
space shuttle mission, launched in September 2006, carried Nickerson and her
colleague's Salmonella typhimurium bacterial experiment into space while
her group conducted an identical experiment on Earth. Their findings are
detailed in an upcoming issue of the Proceedings of the National Academy of
Sciences.
Flip the
switch
Bacteria
express different sets of genes in different environments to ensure their
survival. Inhospitable conditions, for example, can turn on a "master
switch" in some bacteria and allow the microbes to form
tough spores that can survive the extreme conditions of space.
Prior to
Nickerson and her team's study, the genetic behavior of Salmonella
typhimurium--the main culprit in cases of food poisoning and typhoid fever--was
unknown. The microbe poses a significant threat to astronauts during
spaceflight, especially because it is resistant to many antibiotic treatments.
The researchers'
experiment revealed that a genetic switch called "Hfq," which may
control more than 160 genes in S. typhimurium, turns on in space
and causes S. typhimurium to become three times more virulent
than on the Earth's surface.
Based on
what the space-faring bacteria did to animal models on the ground, Nickerson
and her colleagues think hard-to-control biofilms are responsible for the increased
danger.
"Biofilms
are associated with increased pathogenicity because the immune system can't
clear the bacteria effectively and antibiotics don't treat them effectively,"
Nickerson said. "The change that we observed [in space] is consistent with what
looks like formation of a biofilm. The ground-grown samples did not show
biofilm formation."
Tiny
passengers
To see how
zero-gravity affected the bacteria, the researchers sent a special
growth chamber aboard the space shuttle Atlantis with the astronaut crew of
STS-115.
When
astronaut Heidemarie Stefanyshyn-Piper activated the experiment in space,
Nickerson and her group started an identical version inside the orbital
simulator at NASA's Kennedy Space Center. The simulator ensured that both sets
of bacteria grew in the same conditions, aside from the difference of
zero-gravity.
"This
simulator is linked in real-time to the shuttle and duplicates the exact
temperature, humidity and growth conditions of the shuttle, with the exception
that [it is] not flying in space," Nickerson said. Both experiments
"froze" the bacteria in place at the same time, allowing the researchers
to see that Hfq was greatly activated by zero-gravity.
In spite of
the microbe's increased danger to astronauts, Nickerson and her group thinks the
Hfq genetic regulator could be used to control food-borne disease caused by S.
typhimurium--especially since no vaccine exists for it. The researchers plan
to conduct more investigations of disease-causing microbes in space to better
understand their risks and mechanisms during spaceflight.
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