Food poisoning bacteria become super-virulent in space, according to a study of salmonella that spent 12 days orbiting the Earth on the space shuttle Atlantis.

The research raises fears that diseases boosted by low gravity could pose unexpected medical problems on future long-haul space journeys or for astronauts on a proposed future moon base.

It is the first study to examine the effect of space flight on the virulence of a pathogen. "Given the proposed increase in both duration and distance from Earth for future manned space flight missions - including lunar colonisation and a mission to Mars - the risk for in-flight infectious diseases will be increased," said Cheryl Nickerson at Arizona State University.

Her team sent vials of salmonella bacteria into orbit on Atlantis's 12-day mission in September last year. They kept bacteria from the same strain in conditions as close to the space shuttle as possible on Earth. When they fed the samples to different groups of mice they found that the bacteria that had been in space were nearly three times as likely to kill the animals.

"Since spaceflight involves a number of environmental changes we do not know the exact part of spaceflight that caused the change in virulence or other phenotypes we observed in our experiment," said Professor Nickerson, "However, our collective data strongly suggests it is the low fluid shear growth environment, where turbulence and fluid action is minimal, that plays a major role in the response of salmonella to spaceflight."

The team also compared the pattern of gene expression in the space bacteria and those that had stayed on the ground. They found that the expression level in 167 different genes had been altered, they report in the Proceedings of the National Academy of Sciences.

"This experiment is a 'first of its kind' in spaceflight biological study. It is the first study to examine the effect of spaceflight on the virulence of a pathogen, and the first to obtain the entire gene expression response of a bacterium to spaceflight," said Prof Nickerson.

Although the team cannot be sure that the same increase in virulence occurs in other pathogens, the results will concern those planning future missions in which astronauts spend extended periods in space.

President Bush has committed the US to returning astronauts to the moon by 2020 and setting up a permanent moon base. This would require much more time in zero gravity and low gravity conditions which would mean more opportunities for astronauts to fall ill.

Two weeks ago a government advisory committee said the UK should reconsider its ban on human space flight and begin a crewed space programme. The committee, convened by the British National Space Centre, said there would be huge scientific, cultural and economic benefits to sending humans into space. Building up an astronaut corps from scratch would cost £50m to £75m over five years.