Stepwise evolutionary model for E. coli O157:H7 from ancestral O55:H7 . In red letters are the possible events happening and where they occurred during the stepwise evolution. The circle in gray represents an intermediary A3 CC, which has not yet been isolated. SOR - sorbitol fermentation [if (+) fermenting, if (-) non-fermenting or slow fermenting]. GUD - β-D-glucuronidase activity.