Your lab manual says that the nutrient agar control plate in this experiment, is "a comparison for growth quality." Explain why the results on the nutrient agar plate are necessary to justify your conclusions about the mannitol salt agar plate\.\*

Nutrient agar lets us know about the presence/absence of any kind of bacteria.

Mannitol salt agar let us know about the presence/absence of osmotolerant bacteria.

The difference between them lets us know which bacteria are osmotolerant and which ones are not.

Explanation:

The mannitol salt agar is useful to identify organisms with specific traits. It is a selective medium that only allows the growth of osmotolerant cells. It is used to identify bacteria such as Staphylococcus aureus.

The comparison between nutrient agar and mannitol salt agar is significant because it gives more information about the bacteria type. If no cell grows in these two mediums means that there are no bacteria at all. But if cells can grow in the nutrient agar and not in the mannitol salt agar, it means the growing bacteria are not tolerant to salty mediums. They are not osmotolerant.    

A comparison between the presence/absence of bacteria growing in each medium lets us know which species are osmotolerant and which ones are not.