by Although lettuce and other leafy salad vegetables are vital parts of a balanced diet, a literal “rocket salad” may not be the healthiest choice for astronauts aboard the International Space Station (ISS).
New research has shown that microgravity, the condition in which ISS astronauts live, may make lettuce more vulnerable to pathogens — organisms that can sometimes lead to foodborne disease outbreaks. The research could have a significant impact on future manned space missions to the moon and, eventually, to Mars.
Currently, ISS Greens crew members can eat salad grown on the space station in temperature, water and light-controlled rooms. They can also eat items sent into orbit from Earth. However, the ISS is known to host many pathogens, such as bacteria and fungi, that can cause disease. And if some of these pathogens, such as E. coli and salmonella, colonize the tissues of the lettuce eaten, the consumer can become ill.
Although bacteria can grow anywhere, so contamination is always a risk, NASA and other space agencies as well as private space companies such as SpaceX are concerned that an outbreak of foodborne illness could cause a mission seriously off track and jeopardizing billions of dollars of investment.
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Salad does not fight in space
Scientists at the University of Delaware aimed to find out exactly how microgravity affects lettuce, and whether the environment can make salad vegetables more or less vulnerable to disease-causing organisms. They achieved their results by exposing the plants to rotation created by a device called a clinostat. This simulates ISS microgravity conditions for the samples.
Plants can sense gravity using their roots, and often show a type of differential growth called “gravitropism” as a result of how gravity pulls on them. Ultimately the team found that plants grown in simulated microgravity environments were more vulnerable to colonization by the Salmonella pathogen.
Small pores in stems and leaves are called “stomata” and plants use these stomata for gas exchange. As a defense mechanism, the pores tend to “close,” essentially, when a plant is exposed to a stressor such as bacteria.
However, for some reason, planets that grew in microgravity due to rotation opened their pores when exposed to bacteria, which meant that they were much more prone to salmonella invasion in space than which would be on Earth.
“The fact that they were still open when we presented them with things that looked like stress was really unexpected,” Noah Totsline, a team member and scientist at the University of Delaware, said in a statement.
Although the plants were spinning at speeds no greater than that of a rotisserie chicken – or this was not true microgravity – it was enough to confuse their sense of direction and affect their responses to stressors such as bacteria.
“Basically, the plant wouldn’t know which way up or down,” Totsline said. “We were kind of confused about their response to gravity.”
The team also used their experiment to test the use of a “helper bacteria” called B. subtilis UD1022, which is used to help plant growth and protect against bacterial colonization.
Rather than helping plants prevent salmonella in microgravity, the team found that UD1022 failed to protect plants, which could have resulted in the bacteria being able to trigger a biological response that would the plants to close their stomata.
“The failure of UD1022 to close stomata under simulated microgravity is surprising and interesting and opens up the tuna of other worms,” said Harsh Bais, a team member and plant biologist at the University of Delaware, in the statement. “I suspect that the ability of UD1022 to negate stomatal closure under the plant’s simulated microgravity may override the plant and UD1022’s ability to communicate with each other, helping Salmonella to invade the plant.”
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However, the team has an idea that could help offset the risk of contamination as a result of plants opening their stomata wider in microgravity environments.
“Starting with sterilized seeds is a way to reduce risks of having microbes on plants,” Kniel said. “But then there could be microbes in the space environment and they can get into plants that way.”
Another possible option is to modify the genetics of the plants to prevent them from opening their stomata wider when in space in the first place. Scientists at the Bais lab are already evaluating different lettuce species with different genetics and testing their response to microgravity.
“If we find, for example, one that closes its stomata compared to another that we have already tested that opens its stomata, then we can try to compare the genetics of these two different cultivars,” said Bais . “That’s going to pose a lot of questions for us in terms of what’s changing.”
The team’s research was published this month on the Springer Scientific Reports website.
