I think (and please correct me if I am interpreting this incorrectly) the teaching of the Pharisees and Saducees is like yeast in the fact that their teachings (considered to be self serving and or man made laws) could easily rise and spread throughout the land in the minds and hearts of many.

JESUS was trying to get the people of the day to realize he was the bread of life and through him only could men find their way to the FATHER.