One test of great importance in the identification of Staphylococci is growth characteristics on mannitol salt agar. This medium is selective due to the presence of 7.5% NaCl (twice the content of sea water) and differential due to the presence of phenol red. If the microbe can grow on the medium, it is tolerant of 7.5% salt. If the microbe uses the sugar mannitol, it produces acids that modify the pH indicator phenol red from its normal red appearance to yellow. Staphylococci generally grow in the presence of 7.5% NaCl, while Streps and other Gram positive cocci generall don't. Some Staphylococci will ferment lactose, but others will not. This test can be very useful in reducing the number of possible identities for the unknown, and so it is a tried and true test for Gram positive cocci.
To learn more about the mannitol salt agar use in identifying Gram positive cocci, go to MDM Exercise 18.
Follow your instructor's guidance in what to do and what to turn in. When you are done, you may be asked to take a quiz over this material.