Which organism yields a negative PYR test?

Which organism yields a negative PYR test?

• A. Streptococcus pyogenes
• B. Enterococcus faecalis
• C. Viridans streptococci
• D. Enterococcus faecium

Answer: (C)

The test hinges on detecting the PYR enzyme, pyrrolidonyl arylamidase. If an organism makes this enzyme, adding the PYR reagent leads to a color change indicating a positive result. Viridans streptococci typically lack this enzyme, so the reaction stays negative. This pattern helps separate the viridans group from organisms that do produce PYR, such as Streptococcus pyogenes (Group A strep) and Enterococcus species. In other words, a negative PYR result points toward viridans streptococci, while positive results would suggest the other organisms.

The test hinges on detecting the PYR enzyme, pyrrolidonyl arylamidase. If an organism makes this enzyme, adding the PYR reagent leads to a color change indicating a positive result. Viridans streptococci typically lack this enzyme, so the reaction stays negative. This pattern helps separate the viridans group from organisms that do produce PYR, such as Streptococcus pyogenes (Group A strep) and Enterococcus species. In other words, a negative PYR result points toward viridans streptococci, while positive results would suggest the other organisms.