Salad is good for you, generally speaking, so growing fresh greens in orbit seems like a winning way for space farers to stay healthy. New research suggests that as nutritious as space salad might be, it could pose something of a risk to astronauts.

The problem is growing leafy plants like lettuce and spinach in space can come with a side dish of bacteria, according to a new study from a team at the University of Delaware. In tests on plants grown in simulated microgravity, they were shown to actually be more susceptible than normal to the Salmonella enterica pathogen.

We know that the International Space Station (ISS) is home to a lot of aggressive bacteria and fungi, and if these space microbes were to cause widespread sickness in an astronaut crew, it would mean lives were at risk.

Bacteria map
The researchers looked at the interaction between bacteria and plant stomata. (Totsline et al., Scientific Reports, 2024)

"You don't want the whole mission to fail just because of a food safety outbreak," says plant biologist Harsh Bais from the University of Delaware.

Bais and his colleagues used a device called a clinostat to perform some clever rotation tricks in the lab, putting lettuce plants into a similar state as they would be in microgravity. They then added S. enterica bacteria to the leaves.

What was interesting – and surprising – was that the tiny stomata pores in the lettuce opened up to allow the bacteria to invade. Ordinarily, the job of the stomata is to keep dangerous attackers out, while at the same time helping the plant to breathe.

The researchers then added a more helpful species of bacteria, one that typically protects plants from external stressors. Again, the defenses didn't work in microgravity – suggesting there's something about this state that disables the chemical reactions that the lettuce would normally use to keep itself safe.

"The fact that [the stomata] were remaining open when we were presenting them with what would appear to be a stress was really unexpected," says plant scientist Noah Totsline from the University of Delaware.

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

While previous studies have shown that space lettuce is as safe and nutritious as the equivalent plants grown on Earth, the new research indicates that it might struggle to ward off infection in the usual way.

Add in what we know about space bacteria being particularly nasty, and this is a potential problem. The team behind the study wants to see a lot more research into making sure our foodstuffs are safe outside of orbit, with genetic modifications one possibility.

"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," says University of Delaware microbiologist Kali Kniel.

"It is important to better understand how bacterial pathogens react to microgravity in order to develop appropriate mitigation strategies."

The research has been published in Scientific Reports and NPJ Microgravity.