Which of the following would not be appropriate as a positive control in the PYR test?

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

Answer: (B)

A positive control must be an organism that reliably shows the expected PYR reaction under standard test conditions, proving the reagents and procedure are functioning. In PYR testing, Streptococcus pyogenes and Enterococcus faecalis are classic, dependable PYR-positive organisms; they consistently hydrolyze the substrate to yield the characteristic color change. Staphylococcus lugdunensis, however, does not provide a consistently reliable positive signal across strains and experimental runs, so using it as a positive control could lead to ambiguous or misread results. Because a positive control should demonstrate a robust, reproducible positive result, lugdunensis is not appropriate for that role, whereas the more consistently PYR-positive organisms serve as better controls.