I love those analogies...
..."trip a switch" - So the great maker (I'm not religious btw) decided that bacterium have a magic 'no gravity=3x virulent' switch - sounds like something a BBC journo would say.
Just what is it about evolution for millenia on a 1x gravity planet that makes it more effective in a 0x gravity environment!? I thought if you take an organism out of its natural habitat it will not have developed any necessary skills/processes to be any better but will be much worse.
Now try the experiment with other dangerous bacterium, I'd say it happened by chance in this salmonella. In fact I'd think they did actually carry the tests out on lots of bacterium, and this single one showed the effects causing the sensational storyline! Journalists eh? Who needs em!? ;-)
Another possibility is that because the shuttle was travelling at thousands of miles an hour for days that the effects of time slowed where they were in relation to the earthbound parallel experiments, hence giving the bacterium 3x as long to form!? ;-) *joke*