Spacebugs

Salmonella, isolated from infected macrophrages. Photo credit: Pacific Northwest National Laboratory.

Friday, September 28th, 2007--

When you send Salmonella typhimurium up in space, it changes. And not for the better. This bacterium, which is the same type that's often responsible for food poisoning, comes back to Earth more virulent, according to a new report in the Proceedings of the National Academy of Sciences. When compared to Earth-bound Salmonella cultured the exact same way, the space bugs killed more mice and killed them more quickly.

But don't be alarmed, says Cheryl Nickerson, a scientist at Arizona State University in Tempe, AZ and author on the paper. "When we report an increase in virulence, there's a tendency for people to think, 'My God, these bacteria... are they uncontrollable, who's in danger?'...Absolutely no one is in danger," says Nickerson. For one thing, Nickerson says the bacteria did not show new resistance to antibiotics.

What changed in the Salmonella aboard NASA’s space shuttle mission STS-115 was the expression of 167 of the bacterium's genes. The researchers identified a protein, dubbed Hfq protein, that seems to regulate about a third of these genetic responses. "It was the first study to obtain the entire genetic response of a bacterium to space flight," Nickerson says.

The study authors say it isn't the lack of gravity that is causing the bacterial genes to behave differently. It's how the lack of gravity affects the way fluid moves over the bacterial cell's surface. "Cells can sense when they're in an environment where fluid is passing with great force over their surface," says James Wilson, also from Arizona State University and the lead author on the paper. This force is called fluid shear. A rock in a fast-moving river will have high fluid shear, Wilson says, while a rock sitting at the bottom of a pond would have low fluid shear.

Space creates a low fluid shear environment because the cells are suspended without gravity pulling the liquid over them. Salmonella may respond to this signal because of similar changes in fluid shear within the hosts they infect. When Salmonella are floating around in your blood stream, fluid shear is high, Wilson says. When bacteria float near cells they want to infect they sometimes get caught between a cell's microvilli (arm-like projections that stick out of the cell). Here fluid shear is low. "That's also an area where the bacteria interact with host cells," Wilson says. It could be that bacteria have evolved to recognize low fluid shear as a signal that they should behave a little differently, which might explain why their gene expression changes in space, he adds.

The researchers aren't exactly sure how the bacteria become more virulent, but it may be related to something called a biofilm. On Earth, when bacteria aggregate and coat themselves in a biofilm they are much harder to treat and are often more virulent, Nickerson says. The space Salmonella formed a coating that looked similar, but the researchers couldn't confirm it was a biofilm.

Understanding how virulence in bacteria changes in response to this extreme environment may help researchers develop ways to fight these bacteria here on Earth, Wilson says. Nickerson explains: "We are finding that the bacteria is showing its hand a little bit in terms of new ways that it causes disease."

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--Flora Lichtman