Space food: Lettuce grown in microgravity found more susceptible to pathogens

26 Jan 2024 --- Space researchers have successfully grown lettuce under conditions that imitated the weightless environment aboard the International Space Station (ISS). The University of Delaware, US, scientists exposed the plants to simulated microgravity by rotation, discovering that these plants, under the manufactured microgravity, were actually more prone to infections from a human pathogen, Salmonella.

With the ISS hosting many pathogenic bacteria and fungi, the scientists point to the danger that many of these disease-causing microbes at the ISS are very aggressive and can easily colonize the tissue of lettuce and other plants. Astronauts consuming lettuce that has been overrun by E. coli or Salmonella can get easily sick.

With billions of dollars poured into space exploration each year by NASA and private companies like SpaceX, the researchers are concerned that a foodborne illness outbreak aboard the International Space Station could derail a mission.

Space salad
It has been more than three years since the National Aeronautics and Space Administration made space-grown lettuce an item on the menu for astronauts aboard the ISS.

It has been more than three years since the National Aeronautics and Space Administration made space-grown lettuce an item on the menu for astronauts aboard the ISS.

Alongside their space diet staples of flour tortillas and powdered coffee, astronauts can munch on a salad, grown from control chambers aboard the ISS that account for the ideal temperature, amount of water and light that plants need to mature.

It has been more than three years since NASA made space-grown lettuce an item on the menu for astronauts aboard the ISS.Stomata, the tiny pores in leaves and stems that plants use to breathe, normally close to defend a plant when it senses a stressor like bacteria nearby, explains Noah Totsline, an alumnus of the University of Delaware’s Department of Plant and Soil Sciences who finished his graduate program last December. 

When the researchers added bacteria to lettuce under their microgravity simulation, they found the leafy greens opened their stomata wide instead of closing them.

“The fact that they were remaining open when we were presenting them with what would appear to be a stressor was really unexpected,” says Totsline.

Totsline, who is the lead author of both papers, worked with plant biology professor Harsh Bais as well as microbial food safety professor Kali Kniel and Chandran Sabanayagam of the Delaware Biotechnology Institute. 

The research team used a device called a clinostat to rotate plants at the speed of a rotisserie chicken on a spinner.

“In effect, the plant would not know which way was up or down,” Totsline says. “We were kind of confusing their response to gravity.”

It wasn’t true microgravity, he notes, but it did the job to help plants lose their sense of directionality. Ultimately, the researchers discovered that it appears Salmonella can invade leaf tissue more easily under simulated microgravity conditions than it can under typical conditions on Earth. 

Bacterial helpers
Additionally, Bais and other UD researchers have shown the usage of a helper bacteria called B. subtilis UD1022 in promoting plant growth and fitness against pathogens or other stressors such as drought.

They add the UD1022 to the microgravity simulation that on Earth can protect plants against Salmonella, thinking it might help the plants fend off Salmonella in microgravity. 

Instead, they found the bacterium actually failed to protect plants in space-like conditions, which could stem from the bacteria’s inability to trigger a biochemical response that would force a plant to close its stomata. 

“The failure of UD1022 to close stomata under simulated microgravity is both surprising and interesting and opens another can of worms,” Bais says. 

“I suspect the ability of UD1022 to negate the stomata closure under microgravity simulation may overwhelm the plant and make the plant and UD1022 unable to communicate with each other, helping Salmonella invade a plant.”

Findings from the paper, funded by NASA-EPSCoR, are published in Scientific Reports and in npj Microgravity.

Microbes everywhere
Microbes are prevalent. These germs are present on humans, on animals, on the food we eat and in the environment. Naturally, UD microbial food safety professor Kali Kniel highlights that wherever humans are, there is a potential for bacterial pathogens to coexist. 

According to NASA, around seven people at a time live and work on the International Space Station. It’s not the tightest environment — about as big as a six-bedroom house — but it’s still the kind of place where germs can wreak havoc.

“We need to be prepared for and reduce risks in space for those living now on the International Space Station and for those who might live there in the future,” Kniel comments. “It is important to better understand how bacterial pathogens react to microgravity in order to develop appropriate mitigation strategies.”

Findings from the study help prevent both future Earthbound and interplanetary problems with fresh produce.Kniel and Bais have a long history of bringing their subject areas of microbial food safety and plant biology together to study human pathogens on plants.

“To best develop ways to reduce risks associated with the contamination of leafy greens and other produce commodities we need to better understand the interactions between human pathogens on plants grown in space,” Kniel says. “And the best way to do this is with a multidisciplinary approach.”

Greater need for safe food on space
Extraterrestrial habitation may seem a far out concept, but feeding the next generation spacefaring explorers is a scientific goal that simultaneously supports future astronaut farmers as well as present-day producers grappling with climate issues.

Last September, Food Ingredients First met with scientists and artists exhibiting at the “Spacefarming: The future of food” event hosted by Next Nature Network in the Netherlands – with Mars-grown potatoes among the key futuristic solutions highlighted.

While it may be a while before humans can live on the moon or Mars, the University of Delaware research has some big potential impacts for cohabiting outer space.

According to a UN report, the Earth could be home to 9.7 billion people in 2050 and 10.4 billion people in 2100.

On top of that, Bais, the UD plant biology professor, says food safety and food security measures are already at their peak across the world. With the loss of agricultural land over time to grow food, “people are going to soon think seriously about alternate habitation spaces,” he underscores.

“These are not fiction anymore.”

In addition, the Centers for Disease Control and Prevention or the US Food and Drug Administration will regularly issue a recall on certain lettuce on Earth, telling people not to eat it because of a risk of E. coli or Salmonella.

With leafy greens being the food of choice for many astronauts and easy to grow in indoor environments — such as a hydroponic environment in the ISS — Bais emphasizes that it is important to make sure those greens are always safe to eat.  

“You don’t want the whole mission to fail just because of a food safety outbreak,” he remarks. 

Sterilized seeds and improved genetics 
If plants are opening their stomata wider in a microgravity environment and allowing bacteria to easily get in, the researchers posit the question — “what can be done?”

As it turns out, the answer isn’t that simple. “Starting with sterilized seeds is a way to reduce risks of having microbes on plants,” Kniel comments. 

“But then microbes may be in the space environment and can get onto plants that way.” 

Bais says scientists may need to tweak plants’ genetics to prevent them from opening their stomata wider in space. His lab is already taking different lettuce varieties that have different genetics and evaluating them under simulated microgravity. 

“If, for example, we find one that closes their stomata compared to another we have already tested that opens their stomata, then we can try to compare the genetics of these two different cultivars,” Bais said. “That will give us a lot of questions in terms of what is changing.” 

Any answers they find could help prevent future Earthbound and interplanetary problems with rocket salad.

By Benjamin Ferrer