Science fiction loves to play off the potential threat of threat of alien viruses. But a new study suggests that space travellers are much more likely to be threatened by germs from our own planet that become more virulent in space.
Warding off infections is a real priority for astronauts, especially if longer space missions to the Moon and Mars are to go ahead. People have a tendency to get sick in space and over half of the astronauts on the Apollo missions became ill during their trips or soon after their return to Earth.
Earlier research has shown that prolonged weightlessness weakens our immune systems by preventing key sets of genes from switching on. But that’s only part of the problem. A team of researchers from NASA, Arizona State University and 12 other institutions has shown that bacteria also react to zero-gravity conditions, by becoming more virulent, or able to cause disease.
This time last year, they sent cultures of Salmonella typhimurium – one of the most common causes of food poisoning – into space aboard the Space Shuttle Atlantis. To protect the vulnerable crew, the bacteria were safely housed in three layers of containment. All the astronauts had to do to start the experiment was to push down a plunger.
That dropped the isolated bacteria into a nutritious broth that fuelled their growth. After 24 hours of growth, the astronauts pushed more plungers to either pumped in more growth solution, or immobilised the bacteria with a chemical fixative, preserving their genetic activity for the journey home.
Back on Earth, other scientists were doing the same thing with their own Salmonella samples grown in the same conditions (albeit with extra gravity). They spoke with the Shuttle crew through radios to synchronise their experiments.
When the Shuttle returned, the team recovered the space-bound bacteria and analysed the pattern of genetic activity across their entire genomes using microarrays. This modern and powerful technique allows scientists to measure the activity of thousands of genes at the same time. The researchers also measured the levels of every protein in the samples.
The team looked at the entire Salmonella genome and found that the expression of 167 genes and the levels of 73 proteins had changed in the space-travellers. Clearly, the environmental changes of space-flight had triggered changes in the bacteria at the molecular level.
When fed to mice, the altered bacteria were three times more virulent than their Earth-bound counterparts. The infected mice succumbed to much lower concentrations of space-faring bacteria and in much shorter times.
These results support earlier studies which showed that Salmonella grown in conditions that simulated space flight turned into super-bugs. They were more virulent and more resistant to high temperatures, acidic conditions and white blood cells.
When the team looked at the bacterial cells under the microscope, they found that they hadn’t changed in size or shape. The major difference was that the space-faring Salmonella were much more likely to form biofilms, massive communities of bacteria that clump together and live within a network of substances that they themselves excrete.
Our immune systems and antibiotics find it much more difficult to clear infectious bacteria once they form these biofilms and this could explain why Salmonella seems to be much more dangerous after a jaunt in space.
One protein in particular – Hfq – plays a crucial role in these changes. It lords over the activity of a large suite of genes, including some involved in the creation of biofilms. It sticks to small RNA molecules that control the expression of other genes, and ensures that the larger mRNA molecules are successfully translated into proteins in the face of environmental stress.
The researchers suspected that Hfq is a key player that orchestrates a slew of genetic chances responsible for Salmonella’s hardier nature and increased virulence. They tested this idea by growing mutant Salmonella that lacked Hfq in a special chamber that mimicked incredibly low gravity. Sure enough, these mutants never developed the resistance to acid and white blood cells that normal Salmonella do.
Hfq is now an inviting target for drugs designed to guard the health of future astronauts, but these discoveries could also have implications for medicine on Earth.
So far, there is no vaccine for Salmonella and the bacterium is becoming increasingly resistant to antibiotics. Targeting Hfq and disrupting the bacteria’s ability to make biofilms could change that. Hfq is also very similar in a wide range of other bacteria so drugs that focus on it could find uses in treating a whole range of diseases.
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Reference: Wilson et al. Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq. PNAS doi/10/1073/pnas.0707155104.