Eating lettuce and salads in space is a bad idea, according to a new study

Salads and leafy greens, like lettuce, are essential components of a healthy diet on Earth, and they’re also crucial for astronauts embarking on space missions.

Since the National Aeronautics and Space Administration (NASA) introduced space-grown lettuce to the International Space Station (ISS) menu over three years ago, it has become a vital part of astronauts’ diets, complementing staples like flour tortillas and powdered coffee.

However, growing lettuce in space presents unique challenges, especially concerning food safety.

Problem: Lettuce pathogens in space

The ISS harbors numerous pathogenic bacteria and fungi, posing a significant risk to astronauts’ health.

These pathogens, including E. coli and Salmonella, can aggressively colonize plant tissues, such as lettuce, leading to potential foodborne illnesses.

With billions invested annually in space exploration by entities like NASA and SpaceX, an outbreak aboard the ISS could severely impact mission success.

A team of researchers from the University of Delaware, including lead author Noah Totsline, recently published studies in Scientific Reports and npj Microgravity, exploring this issue in depth.

They replicated the weightless environment of the ISS to study how lettuce plants respond to these conditions, particularly in terms of pathogen susceptibility.

Gravity, plant defense, and pathogen invasion

Plants typically use their roots to sense gravity, but the University of Delaware team disrupted this process by rotating the plants, simulating microgravity.

Surprisingly, they found that plants grown under these conditions were more prone to Salmonella infection.

The researchers attributed this to an unusual response in the plants’ stomata – the tiny pores responsible for gas exchange. Noah Totsline, an alumnus of UD’s Department of Plant and Soil Sciences, explained why in the study.

“The fact that they were remaining open when we were presenting them with what would appear to be a stress was really unexpected. In effect, the plant would not know which way was up or down,” Totsline said. “We were kind of confusing their response to gravity.”

This reaction inadvertently makes the plants more vulnerable to Salmonella invasion.

To achieve the simulation of microgravity, the team utilized a clinostat, a device that rotates plants similarly to a rotisserie chicken. “The plant would not know which way was up or down,” Totsline describes.

This disorientation, while not true microgravity, effectively helped the plants lose their sense of direction, making the conditions right for studying pathogen interaction.

Implications for space agriculture

The study’s findings highlight a critical area of concern for space agriculture: ensuring food safety under unique environmental conditions.

The propensity for increased Salmonella invasion in space-grown lettuce underlines the need for continued research and innovative solutions to protect astronauts’ health.

UD researchers, led by Professor Harsh Bais, have been investigating the role of a helper bacteria, Bacillus subtilis UD1022, in enhancing plant growth and resilience against various stressors, including pathogens and drought.

This bacterium, previously effective in protecting plants against Salmonella on Earth, was added to a microgravity simulation to assess its protective capabilities in space-like conditions.

Contrary to expectations, Bais and his team discovered that UD1022 did not safeguard plants in these simulated conditions.

Risks of lettuce microbes in space

This failure is attributed to the bacterium’s inability to induce a biochemical response in plants, crucial for closing their stomata and preventing pathogen invasion.

“The failure of UD1022 to close stomata under simulated microgravity is both surprising and interesting and opens another can of worms,” Bais said, expressing surprise at these findings.

“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,” Bais concluded.

Kali Kniel, a microbial food safety professor at UD, emphasizes the ubiquity of microbes and the consequent risk of bacterial pathogens in any human-inhabited space, including the International Space Station (ISS).

“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 said, stressing the importance of mitigating these risks.

With around seven astronauts working on the ISS at any given time, the environment, comparable to a six-bedroom house, is prone to microbial havoc.

“It is important to better understand how bacterial pathogens react to microgravity in order to develop appropriate mitigation strategies,” said Kniel.

Finding a solution for safe space agriculture

The collaboration between Kniel and Bais, blending expertise in microbial food safety and plant biology, is pivotal in studying human pathogens on plants in space.

“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 said. “And the best way to do this is with a multidisciplinary approach.”

As Earth’s population is projected to reach 9.7 billion by 2050 and 10.4 billion by 2100, according to a United Nations report, the demand for safe food in space becomes increasingly critical.

Bais points out the urgency of exploring alternate living spaces due to diminishing agricultural land on Earth. He notes, “people are going to soon think seriously about alternate habitation spaces. These are not fiction anymore.”

The significance of this research is underscored by the frequent food safety issues on Earth, like E. coli or Salmonella outbreaks in leafy greens.

With such greens being a primary food source for astronauts and easily cultivable in controlled environments like the ISS, ensuring their safety is paramount. Bais emphasizes, “You don’t want the whole mission to fail just because of a food safety outbreak.”

Sterilized seeds and genetic modifications

Addressing the challenge of plant vulnerability in microgravity environments, Kniel suggests starting with sterilized seeds to minimize microbial presence.

However, she acknowledges the possibility of microbes in the space environment contaminating the plants.

“Starting with sterilized seeds is a way to reduce risks of having microbes on plants,” Kniel said. “But then microbes may be in the space environment and can get onto plants that way.” 

Bais proposes genetic modifications to plants to prevent them from excessively opening their stomata in space.

His lab is evaluating different lettuce varieties under simulated microgravity to identify genetic factors that influence stomatal behavior.

“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.” 

The outcomes of this research could be crucial in preventing food safety issues with plants grown in space, ensuring the well-being of astronauts and paving the way for sustainable living beyond Earth.

Looking ahead, the work of Totsline and his team opens the door to further studies on how microgravity affects plant physiology and pathogen interaction.

These insights will not only benefit space missions but also enhance our understanding of plant biology and disease management in agriculture.

As we venture further into space, ensuring the safety and nutritional value of astronaut diets remains a top priority, guiding the future of space farming and habitation.

The full study was published in the journal Scientific Reports.

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