Color-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells. The bacteria have been shown to be three times as virulent in space because of a genetic trigger called "Hfq."
Credit: Rocky Mountain Laboratories/NIAID/NIH
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.?
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.