E. coli if given age only. Of course, gram + versus gram - would be a clue, and we were told that occasionally a gram stain will show so few gram + bugs that you will assume it is a contaminant, but in that case, it is almost always saprophyticus.
Obviously, 'honeymoon' UTI's and UTI's in younger women (back-to-front wiping?) are more likely to be e coli. If you are given no gram stain or picture, go with e coli, regardless of sexual history.