Fist of all the Pharisees are meant to represent Gods law and authority on the earth but due to corruption, evil, sin and power seeking desires Jesus calls them hypocrites and has a very negative view of them in the NT-

"The teachers of the law and the pharisees sit in Moses's seat. So you must obey them and do everything they tell you. But do not do what they do, for they do not practice what they preach." Matt 23 " The Seven Woes". (read all of Matt 23 for Jesus' full view on them).

The Pharisees also represent men/woman in general in the world, men who are not truly conformed to Christs ways and teachings.

They also represent a false understanding and practice of authority, power and influence. They represent God and the church as well here. 

They also represent a serious warning to all of mankind.

Yeast represents both negative and potentially positive things here- sin, corruption and evil and also those who spread God word or represent God. Yeast also represents growth. 

Yeast is a very small substance so if you become even slightly corrupted and conform yourself to the ways of the Pharisees or of the sinful world beware of this for yourself and for others especially if you are preaching the gospel which is what Jesus' disciples would be doing because their teachings and practices could make man grow into something wrong and corrupted. It also means do not lead others astray through false teachings or practices. One small substance which is yeast or one man can lead many- those who eat the bread, astray. For yeast (the preacher) is responsible for making other things grow, other things which are larger or more numerous than its original self (people in quantity). So Jesus is warning His disciples here of their responsibility towards Him and His true word and true practice and representation of it to themselves and for others. 

Jesus is also referring to the breaking of bread here a symbol later to be used to represent that of His body which was broken/sacrificed for all of mankind. (a key word here is "broke"). Also Jesus is not talking about physical food here but spiritual food though He did feed the people with physical food but for a deeper and more spiritual meaning or representation.

Jesus also mentions the importance of having a strong faith and not having a hardened heart and the importance of truly understanding His word. This message is not just for His disciples but for every man or woman.

(Jesus' chosen people are the Jews also known as the Israelites. He is a descendant of David of Israel and is also known as the King of the Jews and the King/Savior of the world.)

In the first feeding of the 5000:

The number 5 represents divine grace or atonement, the cross and Jesus Christ.

12 represents His chosen people all the 12 Tribes of Israel. It also represents His 12 disciples who would take His gospel to the whole world. 12 also represents mankind in full. So 12 represents the connection and relationship between all of man and God.

In the second feeding of the 4000:

4 represents the earth in full.  7 represents perfection and completeness.

Jesus shows us here then that His word or His bread and salvation is for all of mankind. That He is the savior of mankind and through Him we can have eternal life. If we accept Him as our one and only savior, that He is the true way and path in our lives, that we should live for Him and not for ourselves and conform ourselves to being more like Jesus Christ throughout out lives. We must aim to have the same values that Jesus taught written on our hearts and practiced throughout our lives and not be like the Pharisees but be true to God internally and externally. We must look and see and hear and understand the truth which is Jesus' word and practice His ways and teachings in truth.