A study has found that Salmonella grown during the Atlantis space-shuttle mission STS-115 in 2006 has suffered major changes in the expression of 167 genes and it became significantly more deadly than its earth-bred counterpart in experiments on mice. The study is to appear in the Proceedings of the National Academy of Sciences.

The study, which is first of its kind, was led by Arizona State University Professors Cheryl Nickerson and James Wilson who studied the popular cause of food poisoning, Salmonella typhimurium. They have found that spaceflight triggers a gene which regulates most of the molecular changes that the bacteria underwent. Apparently, the exposure to stress makes Salmonella much more virulent and dangerous.

Compared with earth-grown bacteria, those grown on the shuttle displayed major changes in the activity of 167 genes and in the production of 73 proteins. Furthermore, lower concentrations of the space-grown bacteria caused lethal infections in mice, and the space-grown bacteria killed more mice sooner than those grown on Earth.

Professor Cheryl Nickerson said that microgravity might cause bacterial cells mechanical stresses, because they are in a "state of buoyancy, floating suspended," which changes the flow of fluids over the surfaces of the cells, and hence the cells' behavior. "Being cultured in microgravity means the force of the liquid passing over the cells is low," she told AP. The cells "are responding not to microgravity, but indirectly to microgravity in the low fluid shear effects."

An technique called scanning electron microscopy also showed some Salmonella were starting to form biofilms, a protective slime layer, National Geographic reports. Biofilms are known to worsen some diseases and reduce the effectiveness of many antibiotics.

Salmonella, as well as other gut-related bacterial pathogens, are a leading cause of food-borne illness and infectious disease, especially in the developing world. The regulatory Hfq gene that the team found is a potential therapeutic target, since no vaccine currently exists for Salmonella food-borne infections in humans. In addition, the space flight studies may shed new light on why Salmonella and other bacteria have become increasingly resistant to antibiotic treatment.