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Nickerson and lead author James Wilson, professors in ASU’s School of Life Sciences, have performed the first study of its kind to investigate the effect of space flight on the genetic responses and disease-causing potential, or virulence, of Salmonella typhimurium, the main bacterial culprit of food poisoning. Their results, published in the journal Proceedings of the National Academy of Sciences (www.pnas.org.cgi/doi/10.1073/pnas.0707), reveal a key role for a master regulator, called Hfq, in triggering the genetic changes that show an increase in the virulence of salmonella as a result of space flight.
The results of these studies hold potential to greatly advance infectious disease research in space and on Earth, and could lead to the development of new therapeutics to treat and prevent infectious disease.
To study the effects of space flight, Nickerson and colleagues sent specially contained tubes of salmonella in an experimental payload aboard the space shuttle Atlantis. The tubes of bacteria were placed in triple containment for safety, and posed no threat to the health and safety of the crew.
During the flight, astronaut Heidemarie M. Stefanyshyn-Piper activated growth of the bacteria in sealed hardware and ‘fixed’ the cultures after a day of growth to determine changes in gene and protein expression levels.
“The bacterial cultures were taken up into space and activated to grow in a separate compartment of the tubes, called the ‘growth chamber,’ ” Nickerson says. “The bacteria didn’t have access to the growth chamber until Heide pushed down on a plunger, which introduced the bacteria into the growth media. Then they were grown for 24 hours, and at the end of 24 hours, Heide pushed down on the plunger again, which either “fixed” the bacteria with chemicals that preserved the gene expression message, or else introduced fresh media to keep the bacteria growing to perform the virulence studies.”
As a synchronous control experiment back on Earth, Nickerson’s team grew an identical set of bacteria in the same type of tubes used for flight, and incubated them in a special room at the NASA Kennedy Space Center called the “orbital environmental simulator.”
“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 they are not flying in space,” Nickerson says. “In addition, we were also linked via real-time telecommunications with the shuttle crew when they were activating and terminating our experiments in flight, and we did the exact same things at the same time to the ground samples that the astronauts did to the flight samples – thus, we had perfectly matched, synchronous ground controls.”
After the bacteria returned to Earth, the group performed the first global analysis of salmonella to measure the effect of space flight on gene and protein expression and virulence. By measuring the gene and protein patterns, the researchers could hone in on the key molecular players necessary for virulence from among thousands of potential candidates.
“We chose to measure gene expression at the mRNA level, since the technique to do this, called microarray analysis, is a highly advanced and convenient way to quantitatively measure the expression of every gene in a single experiment,” says Wilson, who coordinated the team’s molecular profiling efforts for the Nickerson lab, and played a central role in the performance of these experiments, including data analysis. “It is a very powerful technique that was very applicable to the space flight experiment. The isolation of mRNA poses particular challenges, since it is very sensitive to degradation, but we designed the experiment using a fixative that preserved the mRNA very well.”
After logging millions of miles in space, the invaluable and well-traveled bacterial samples were analyzed back on Earth. For the protein profiling studies, the samples were taken to the University of Arizona’s core proteomics facility at its Center for Toxicology to measure the level of every protein that had been subjected to space flight.
“Working with the UA group was great, and we obtained very nice data that complemented the microarray analysis very well,” Wilson says. “Keep in mind also that our body of mRNA and protein expression data from this experiment is precious, since comprehensive analysis of an organism’s molecular genetic response to space flight is very rare.”
Compared to bacteria that remained on Earth, the space-traveling salmonella had changed expression of 167 genes. After the flight, animal virulence studies showed that bacteria that were flown in space were almost three times as likely to cause disease when compared with control bacteria grown on the ground.
The study discovered that an important regulatory protein, Hfq, may be a key molecule responsible for the increased virulence caused by space flight.
“Hfq is a protein that binds to and regulates a number of regulatory RNAs – which, in turn, control gene expression,” Nickerson says. “Our studies suggest that there may be a role for these regulatory RNAs in the cellular response to the physical and mechanical forces found in space flight, which are relevant to conditions that cells encounter here on Earth during the normal course of their life cycles.”
These results have important implications for human health, since salmonella (and other gut-related bacterial pathogens) are a leading cause of food-borne illness and infectious disease, especially in the developing world. Nickerson’s group further highlights Hfq as a potential therapeutic target, since no vaccine exists for salmonella food-borne infections in humans.
In addition, the space flight studies could shed new light on why salmonella has become increasingly resistant to antibiotic treatment.
“We also studied the morphology of the bacteria in response to space flight, and the change that we observed is consistent with what looks like formation of a biofilm,” Nickerson says. “The ground grown samples did not show biofilm formation. Biofilms are associated with increased pathogenicity, because the immune system can’t clear the bacteria effectively – and antibiotics don’t treat them effectively.”
The group will embark on another space shuttle mission likely next year to further understand the risks and mechanisms of infectious disease agents during space flight, and how microbes cause infections on Earth.
Joe">mailto:Joseph.Caspermeyer@asu.edu">Joe Caspermeyer, Joseph.Caspermeyer@asu.edu