PYR Test: Principle, Media, Procedure, Results, Uses


The PYR test is a biochemical test that detects the presence of pyrrolidonyl aminopeptidase (PYRase) enzyme in bacteria. PYRase is an enzyme that hydrolyzes the synthetic substrate L-pyrrolidonyl-β-naphthylamide (PYR) and releases β-naphthylamide, which reacts with a color reagent to produce a bright pink or cherry red color. The PYR test is used to differentiate and presumptively identify certain groups of bacteria based on their ability to produce PYRase.

The PYR test is especially useful for the identification of Streptococcus pyogenes (Group A Streptococcus), which is the causative agent of streptococcal pharyngitis (strep throat), scarlet fever, rheumatic fever, and other infections. S. pyogenes is PYR positive, while other β-hemolytic streptococci such as Streptococcus agalactiae (Group B Streptococcus) and Streptococcus bovis (Group D Streptococcus) are PYR negative. The PYR test can also help to distinguish Enterococcus spp. (PYR positive) from other non-enterococcal streptococci (PYR negative).

In addition, the PYR test can be used to identify some Gram-negative bacteria such as Citrobacter spp., Klebsiella spp., and Yersinia spp., which are PYR positive, from other Gram-negative bacteria such as Escherichia coli, Salmonella spp., and Shigella spp., which are PYR negative. The PYR test can also differentiate E. coli (PYR negative) from other indole-positive, lactose-positive, Gram-negative bacilli such as Enterobacter spp. and Serratia spp. (PYR positive).

The PYR test can be performed by two methods: the tube method and the rapid method. The tube method uses a liquid or solid culture medium that contains the PYR substrate and requires incubation for 4 to 24 hours before adding the color reagent. The rapid method uses a filter paper disk that is impregnated with the PYR substrate and requires only 2 to 10 minutes of incubation before adding the color reagent. Both methods are simple, inexpensive, and reliable for the detection of PYRase activity in bacteria.

The purpose of this article is to explain the principle, media, procedure, results, uses, quality control, precautions, applications, and limitations of the PYR test in detail. This article will help microbiologists, students, researchers, and health professionals to understand and perform the PYR test effectively and accurately for the identification of bacteria.