Which organism is appropriate as a positive control in the PYR test?

• A. Staphylococcus aureus
• B. Streptococcus pyogenes
• C. Enterococcus faecalis
• D. Streptococcus pneumoniae

Answer: (A)

The key idea is that a positive control in an enzymatic test proves the assay is working properly. The PYR test detects the activity of the PYRase enzyme, which hydrolyzes a substrate to release a compound that the reagent turns red if the reaction occurs. Using a known positive organism ensures you can see this expected red color and confirm that the reagents and procedure are functioning correctly.

Staphylococcus aureus is used as the positive control because it reliably expresses PYRase under standard testing conditions, producing a clear red color with the PYR reagent. Seeing that positive color verifies that a true positive can be detected by the test, so any negative result in your actual samples can be interpreted with confidence about the procedure.

In contrast, relying on organisms that are not consistently PYR-positive as controls could fail to produce the expected color even when the test is functioning, leading to misinterpretation. Therefore, the chosen positive control is a organism with a consistent, robust PYRase activity that gives a definitive positive result.