The main criterion for differentiation between Staphylococcus and Streptococcus genera is the catalase test. Staphylococci are catalase positive whereas Streptococci are Catalase negative. Catalase is an enzyme used by bacteria to induce the reaction of reduction of hydrogen peroxide into water and oxygen.

Catalase is considered an endoenzyme because it is produced and used within a cells metabolic machinery. Since it breaks down superoxide molecules it must work inside the cell.

Catalase +

The morphologically similar Enterococcus (catalase negative) and Staphylococcus (catalase positive) can be differentiated using the catalase test.

Catalase.

Staphylococci are catalase positive.

Streptococci are catalase negative.