What is the advatage of a culture plate over broth?

Answer 1

With a culture plate, you can isolate one colony so you have a pure culture. In a broth tube, if it gets contaminated, you don't have a way to know that.

Answer 2

The colony from a single cell is correct. With a broth, you have no way to isolate colonies, so you will end up with several different colonies. Contamination is not so much a problem as is multiple colonies.

Satellite colonies are an excellent example of this. Satellite colonies in genetic engineering experiments occur close to a primary colony on bacterial agar plates. Short version, the primary colony, which will have your selective marker, typically an AmpR gene, will locally deplete ampicillin from the plate. Amp is the only way you can select for transformed cells. In broth, if your amp (or any other drug) is depleted, you will end up with a hodgepodge of cells- some with and some without your plasmid, and ultimatley gene, of interest.

Answer 3

Purity is a good advantage, if you need to do other testing, you need to have pure colonies. Addtionally, you can determine characteristics about the bacteria by the colony morphology. Color, size of colony, and overall shape are important in making identification.