French scientists say the prolific virulence and growth of bacteria in space, coupled with reduced production of antibodies, might limit future space travel.

The researchers from Nancy-University in Lorraine, France, said long-term space flights might compromised by microbial hitchhikers, such as bacteria. That's because space travel appears to weaken the human immune system, while increasing the virulence and growth of microbes, they said.

"When people think of space travel, often the vast distances are what come to mind first," said Jean-Pol Frippiat, one of the report's co-authors. "But even after we figure out a way to cover these distances in a reasonable amount of time, we still need to figure out how astronauts are going to overcome disease and sickness."

Frippiat and colleagues said they based their conclusions on studies showing that immune systems of both people and animals in space flight conditions are significantly weaker than their grounded counterparts. They also reviewed studies that examined the effects of space flight conditions and altered gravity on virulence and growth of common pathogens such as Salmonella, E.coli and Staphylococcus. Those studies, they said, show such bacteria reproduce more rapidly in space flight conditions, leading to increased risk of contamination, colonization and serious infection.

The research appears in The Journal of Leukocyte Biology.