Lettuce and other leafy green vegetables are part of a healthy, balanced diet, even for astronauts on missions. In addition to staples of flour tortillas and ground coffee, astronauts can munch on salads, which are grown in a control room aboard the International Space Station based on the ideal temperature, amount of water and light the plants need to mature.

But there is a problem. There are many pathogenic bacteria and fungi on the International Space Station. Many of the pathogenic microorganisms present on the ISS are highly aggressive and easily colonize the tissues of lettuce and other plants. People can become sick if they eat lettuce infected with E. coli or salmonella.

Private companies such as NASA and SpaceX invest billions of dollars each year in space exploration, and some researchers worry that a foodborne illness outbreak on the International Space Station could derail missions.

In a new study published in the journal Scientific Reports and npj Microgravity, researchers at the University of Delaware grew lettuce in conditions that mimicked the weightless environment of the International Space Station. Plants are masters at sensing gravity, using their roots to find it. Plants grown at the University of Delaware are exposed to simulated microgravity through rotation. The researchers found that these plants in artificial microgravity were actually more susceptible to infection by the human pathogen Salmonella.

Noah Totsline, an alumnus of UD's Department of Plant and Soil Sciences, said stomata are tiny pores in plant leaves and stems used for breathing. When plants sense stressors such as bacteria nearby, they often close the stomata to protect the plant. When researchers added bacteria to lettuce in a microgravity simulation, they found that the leafy green's stomata opened instead of closing.

"When we showed them what seemed like a pressure, they stayed open, which was really a surprise to us," Totsline said.

Toteslein, first author on both papers, collaborated with Harsh Bais, professor of plant biology, Kali Kniel, professor of microbial food safety, and Chandran Sabanayagan of the Delaware Institute of Biotechnology. The team used a device called a gyrator to spin the plants at the speed of a rotisserie chicken on a spinner.

"Effectively, the plant doesn't know which direction is up or down," Totsline said. "We're kind of confusing how they respond to gravity."

It's not true microgravity, Totterslein said, but it can help plants become disoriented. Ultimately, the researchers found that Salmonella appeared to invade leaf tissue more easily under simulated microgravity conditions than under typical conditions on Earth.

In addition, Bais and other UD researchers discovered the role of a helper bacterium called B. subtilis UD1022 in promoting plant growth and improving plants' ability to resist pathogens or other stresses, such as drought.

They added UD1022 to a simulation of microgravity that protects plants from Salmonella on Earth, thinking it could help plants resist Salmonella in a microgravity environment.

Instead, they found that the bacteria were actually unable to protect the plants in space-like conditions, possibly due to the bacteria's inability to trigger the biochemical reactions that force plants to close their stomata.

"UD1022's inability to close stomata in simulated microgravity was surprising, interesting, and opened another window," Bais said. "I hypothesized that UD1022's ability to negate stomata closure in simulated microgravity might overwhelm the plant, preventing the plant and UD1022 from communicating with each other, thereby helping Salmonella invade the plant."

Foodborne pathogens on the International Space Station

Microorganisms are everywhere. These bacteria are found on us, on animals, on the food we eat, and in the environment.

So it's only natural that wherever there are humans, there's the potential for bacterial pathogens to coexist, says Kali Kniel, a professor of microbial food safety at UD.

According to NASA, approximately seven people live and work on the International Space Station at any one time. The environment isn't the tightest -- it's about the size of a six-bedroom house -- but it's still a place where the kind of bacteria can thrive.

"We need to prepare and reduce space risks for people living on the International Space Station now and those who may live there in the future. It is necessary to better understand the response of bacterial pathogens to microgravity so that appropriate mitigation strategies can be developed," Knier said.

The two researchers have long combined the two subject areas of microbial food safety and plant biology to study human pathogens on plants.

"To optimally reduce the risks associated with contamination of leafy greens and other produce, we need to better understand the interactions between human pathogens and plants grown in space. The best way to do this is through a multidisciplinary approach," Knier said.

The earth’s population continues to grow, and the demand for safe food in space is greater

It may be some time before humans can live on the moon or Mars, but UD's research has a huge potential impact on outer space habitation. According to a United Nations report, the population of the earth will reach 9.7 billion by 2050 and 10.4 billion by 2100. In addition, Bais, a professor of plant biology at the University of California, said food safety and food security measures around the world have reached their peak. "As farmland for growing food dwindles, people will soon be seriously considering alternative living spaces," he said. "These are not fictions anymore."

The Centers for Disease Control and Prevention or the Food and Drug Administration seem to be issuing recalls more often for certain types of lettuce on Earth, telling people not to eat it because of the potential for E. coli or salmonella contamination.

Bais said that leafy greens are the food of choice for many astronauts and are easy to grow in indoor environments such as the hydroponic environment on the International Space Station, so it is important to ensure that these leafy greens are always safe to eat. No one wants the entire mission to fail just because of a food safety incident.

Solution: Neutered seeds and improved genes

So, what should we do if the stomata of plants open wider in a microgravity environment, allowing bacteria to easily enter? It turns out the answer isn't that simple.

"Starting with sterilized seeds is one way to reduce the risk of microorganisms on the plant," Knier said. "But then the microorganisms may get into the space environment and get into the plant that way."

Bais said scientists may need to tweak the genes of plants to prevent them from opening their pores wider in space. His lab has begun taking different lettuce varieties with different genes and evaluating them in simulated microgravity conditions. "For example, if we find that one plant has stomata that close, and another plant that we've tested has stomata that open, then we can try to compare the genes of those two different cultivars. That will give us a lot of questions about what's changing."

Any answers they find could help prevent future problems with space salads.

Compiled from /scitechdaily