Which organism yields a negative PYR test?

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Prepare for the Clinical Laboratory Science Bacteriology Test. Engage with multiple choice questions and flashcards with helpful hints and explanations. Ace your exam confidently!

Multiple Choice

Which organism yields a negative PYR test?

Explanation:
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.

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