Microbes that cause salmonella came back from spaceflight even more virulent and dangerous in an experiment aboard the US space shuttle Atlantis, according to a study published on Monday.

The experiment by microbiologists at Arizona State University sent tubes with salmonella bacteria on a shuttle flight in September 2006 to measure how space flight might affect disease-causing microbes.

The salmonella sample that travelled millions of kilometers (miles) in orbit changed their pattern of certain genes compared to identical bacteria back on Earth, said the study in the journal Proceedings of the National Academy of Sciences.

Following the shuttle flight, studies using mice showed the salmonella bacteria aboard the shuttle were "almost three times as likely to cause disease when compared with control bacteria grown on the ground," said a university statement outlining the study.

After about three weeks, 40 percent of mice fed the salmonella from Earth were still alive while only 10 percent of those given the bacteria from space survived, according to the study.

The professors who carried out the research, Cheryl Nickerson and lead author James Wilson, said the results of the research could help advance understanding of salmonella and other infectious diseases -- in space and on Earth -- possibly paving the way for development of new treatments.

Salmonella and other related bacterial pathogens are a leading cause of food-borne illness and infectious disease, particularly in developing countries. No vaccine exists for salmonella and it has become increasingly resistant to antibiotic treatment, the study said.

The research revealed the key role of a regulatory protein, Hfq, which could be responsible for the enhanced virulence of the microbes after space flight, the authors said. The finding could shed light on how cells act on Earth.

The study also examined the morphology of the bacteria during space flight and found changes that indicate the formation of a biofilm, which was not observed in the samples grown on Earth. Biofilms are associated with increased virulence of bacteria.

The scientists plan to carry out more experiments on the same theme possibly on space shuttle flights in 2008.