Assume that your lab partner swabs the side of his face and uses the swab to inoculate a nutrient agar plate. The next day, he performs a Gram stain on the colonies. They are gram-positive cocci. You advise him that he should next look for

the ability to produce catalase enzyme

Explanation:

Gram positive cocci bacteria are oval shaped organisms when viewed under the microscope. The oval shaped cells either occur in clusters (staphylococci) or occur in chains (streptococci).

Gram positive staphylococci and Gram positive streptococci can be differentiated by the ability to produce catalase enzyme. While the latter have the ability to produce catalase, the former do not.

Positive catalase reaction involves the ability to degrade hydrogen peroxide by the catalase enzyme and it is usually indicated by evolution of oxygen bubbles.

Hence, after finding out that the organisms are Gram-positive, the next thing is to determine if they are staphylococcal or streptococcal by checking the ability to degrade hydrogen peroxide through the production of catalase.