Here's what the news reports said:
Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq. J. W. Wilson et al. (many authors), PNAS Sept. 27 2007. Link
"The researchers found 167 genes had changed in the salmonella that went to space..."
Dr. Cheryl Nickerson, the PI on the project, is quoted (misquoted?) as saying "These bugs can sense where they are by changes in their environment. The minute they sense a different environment, they change their genetic machinery so they can survive."
This left me thinking that the genotypes of the bacteria had consistently mutated (cosmic rays) in a way that made them more virulent. From an evolutionary perspective this seemed very improbable - both in having similar mutations arise in many bacteria and in having the bacteria direct their mutations to fit their changed environment.
Now I read the paper I discover that the genes hadn't changed at all. As the title indicates, what had changed was the expression of some genes (some turned up and some down) - this is a transient response to the altered culture conditions in space, not a genetic change. The cells in the culture that had been in space were more clumped together, in what may be the zero-gravity broth-phase equivalent of a biofilm. These bacteria caused more serious disease when injected into mice, probably because the clumping made it harder for the mouse immune system to kill them.