Laboratory 10:  Biochemical Testing of Gram Positive Organisms

Catalase Testing and Phenylethylalcohol Agar (PEA) are not shown on this page.  A positive catalase is indicated by bubbling upon the addition of hydrogen peroxide.  Gram positive organisms grow readily on PEA while Gram negative organisms are inhibited.

Mannitol Salt Agar

A selective and differential media, only bacterial that are salt-tolerant grow on mannitol salt agar.  In our laboratory those organisms are primarily of the genus Staphylococcus.  In addition, the medial is differential because it contains both mannitol and the pH indicator phenol red.  If mannitol is fermented, acids are produced and the phenol red turns yellow.   This media represents one way that pathogenic Staphylococcus aureus (SA) can be distinguished from Staphylococcus epidermidis (SE).  Escherichia coli (EC) is Gram negative and not salt tolerant.  It will not grow on this media

Blood Agar

Left column:  Organisms that are alpha hemolytic produce a red cloudy zone around colonies
Center: Organisms termed beta hemolytic produce zones that are clear around colonies
Right column:  These are examples of organisms that are not hemolytic.