New York, Jan 23 (IANS): Lettuce and other leafy green vegetables that are part of a healthy, balanced diet could be more susceptible to bacterial infections in space than on Earth, according to a study.

It's been more than three years since NASA made space-grown lettuce an item on the menu for astronauts aboard the International Space Station.

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.

To see whether the salads are safe to eat in space, researchers at the University of Delaware in the US grew lettuce under conditions that imitated the weightless environment aboard the ISS.

The plants were exposed to simulated microgravity by rotation. The results, published in the journals Scientific Reports and in npj Microgravity, showed that the plants under the manufactured microgravity were actually more prone to infections from a human pathogen, Salmonella.

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, said Noah Totsline, from the varsity’s Department of Plant and Soil Sciences.

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 stress was really unexpected," Totsline said.

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

It wasn't true microgravity, Totsline said, 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.

In addition, the researchers showed that 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 added 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.

But sterilising seeds and improved genetics could be a solution, the researchers said.

"Starting with sterilised seeds is a way to reduce risks of having microbes on plants," said microbial food safety professor Kali Kniel. "But then microbes may be in the space environment and can get onto plants that way."