Spaceflight boosts bacterial deadliness
Microgravity increases virulence of Salmonella in space.
It sounds like a plot device from a cheap science-fiction novel: bacteria that travel into space come back to Earth deadlier than before.
But that's what an international team of researchers found when they looked at the bacterium Salmonella typhimurium grown aboard last year's Atlantis mission: it came back more virulent than controls kept on Earth. Its trip into space made the pathogen three times more potent — the dose required to kill 50% of mice in a study population was a third that of controls.
Researchers aren't clear about exactly how or why this happens, but they suspect that the reduced liquid flow around bacterial cultures in low gravity may be to blame.
"The space-flight environment imparts a signal that can induce molecular changes in bacterial cells," write Cheryl Nickerson and her colleagues in a paper published in Proceedings of the National Academy of Sciences USA this week1. The result could explain some curious observations from space missions so far. How it will affect the design of future missions is as yet unclear. And whether space flight would make any and all bacteria more virulent — or less so — is unknown.
Down to Earth
Earlier ground-based studies, in which bacteria were kept neutrally buoyant in liquid to simulate low-gravity conditions, showed that virulence can be boosted. But there was doubt as to whether the 'gravity' effects in these experiments were what caused the result.
"It was very important to show that what was simulated on the ground occurs in space too," says study author Jörg Vogel, a researcher at the Max Planck Institute for Infection Biology in Berlin.
To test this, the research team grew tubes of Salmonella aboard Atlantis for 24 hours. Control samples on the ground were grown for the same duration in identical conditions as those on the shuttle — except for gravity.
Analysis back on Earth showed that the space travellers were different from the controls in the expression levels of 167 genes, and amounts of 73 proteins. A particular protein called Hfq — which is known to regulate RNA and have a role in bacterial virulence down on Earth - may be responsible for the increased potency, the researchers suggest.
Christophe Lasseur, who helps the European Space Agency to plan for life-support on manned missions, says some previous space missions have reported greater problems with bacteria — such as corrosion in the craft — than had been expected. This led to speculation that space flight changes bacteria, possibly through their increased exposure to radiation. The new work is the first clear evidence that bacteria can be affected by space conditions.
"It's a very good paper," says Lasseur. "We had some idea that the virulence could be increased but there was no evidence."
Nickerson, a microbiologist at Arizona State University in Tempe, thinks the change could be brought about by a reduction in 'fluid shear' — the movement of liquid around the bacteria. Previous microgravity studies on Earth with low shear have also increased Salmonella virulence, she says2.
Nickerson says the effects of fluid sheer have generally been neglected in Earth-based experiments, in part due to the difficulty of changing it.
"A lot of our knowledge has come from testing cells at extremes," she says. "It's exciting that this research can help crew stay healthy as they push the boundaries of manned space flight. What's equally exciting is the potential this research holds to improve health on Earth."
As for health in space, Jacob Cohen, who looks at future manned explorations of space for NASA in Washington DC, says it is unclear how big an impact this study will have on astronauts. "We haven't seen any increased virulence effect on the crew," he says. "Further study needs to be done to see how these changes might affect an actual mission."
"Space flight at the moment is rare for human beings," notes Vogel. "Looking 20 years ahead, I think about the companies that are going to offer flights for tourists; we'll have tonnes of people flying to space. Those people will inevitably carry lots of bacteria. We don't know how these bacteria will behave in those conditions."
- Wilson, J. W. et al. Proc. Natl Acad. Sci. USA doi/10.1073/pnas.0707155104 (2007).
- Nickerson, C., et al. Microbiol. Mol. Biol. Rev. 68, 345-361 (2004).
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