Which of the following would be appropriate for positive and negative controls in the PYR test?

• A. Staphylococcus aureus and Staphylococcus lugdunensis
• B. Streptococcus pyogenes and Staphylococcus lugdunensis
• C. Streptococcus pyogenes and Enterococcus faecalis
• D. Streptococcus pneumoniae and alpha-hemolytic streptococci viridans group

Answer: (A)

In the PYR test, you validate the assay with one organism known to produce the enzyme (a positive control) and one known not to produce it (a negative control). A positive PYR result appears as a color change (red) after adding the reagent, while a negative result stays the same or changes only weakly.

Staphylococcus lugdunensis is a PYR producer, whereas Staphylococcus aureus is not. Using these two as the controls gives you one positive and one negative reference, which is exactly what you need to confirm the test is working properly. The other options pair two positives or two negatives, which wouldn’t reliably demonstrate that the assay can both detect and rule out PYR activity.