According to Astrogenetix, the Salmonella mutates in low gravity into a
more virulent form, and then they extract mutant DNA from it to make into
a vaccine that can defeat the less virulent earthly forms.
If anyone thinks that bringing organisms back to Earth that have mutated
into more virulent forms in space is a bad idea, I fully agree.
This sounds like Fred Hoyle's Comet-Borne Flu Germs being done on purpose.

The Andromeda Strain rides again - and so when it comes back and gets loose
the Salmonnella death rate goes up. Sounds like mad science at it's best.
Make a great movie - Salmonella deaths are up - Dustin Hoffman and Rene
Russo trace the outbreaks to returnimg astronauts from the station, Head of
the drug Company and Government types try to get them killed before they
spill the beans.

This is crazier than the Mutated Tomatoes experiments a few years back.