PYR

**Note on ambiguity:** "PYR" is most often tested on boards as the PYR test (microbiology). In other contexts, "PYR" can be shorthand for pyridoxine (vitamin B6), pyrimethamine, or pyruvate. The explanation below covers the microbiology meaning, followed by brief notes on the others.

**PYR test (microbiology)** - Principle: Organisms with pyrrolidonyl arylamidase hydrolyze the substrate L-pyrrolidonyl-β-naphthylamide. This releases β-naphthylamine, which reacts with the reagent N,N-methylaminocinnamaldehyde (cinnamaldehyde) to produce a bright cherry-red color. - Positive organisms: - Streptococcus pyogenes (Group A strep, GAS) - Enterococcus species (E. faecalis, E. faecium) - Negative organisms: Streptococcus agalactiae (GBS), S. pneumoniae, viridans streptococci, and the S. bovis/gallolyticus group (non-enterococcal group D).

**Where it fits in the workup of gram-positive cocci** - Catalase negative → Streptococcus or Enterococcus (catalase positive suggests Staphylococcus). - Beta-hemolytic and bacitracin-sensitive, PYR positive → S. pyogenes (GAS). - Beta-hemolytic and bacitracin-resistant, CAMP positive, hippurate hydrolysis positive → S. agalactiae (GBS). - Enterococcus: grows in 6.5% NaCl and bile esculin, and is PYR positive. Non-enterococcal group D strep (S. gallolyticus) grows in bile esculin but not in 6.5% NaCl, and is PYR negative. - Staphylococcus lugdunensis is also PYR positive. This is a useful distinguishing point from other coagulase-negative staphylococci, though it is a less common boards point.

**Clinical significance** - S. pyogenes: pharyngitis, impetigo, erysipelas, cellulitis, necrotizing fasciitis, scarlet fever, toxic shock-like syndrome. Post-infectious complications are rheumatic fever (after pharyngitis) and glomerulonephritis (after pharyngitis or skin infection). - Enterococcus: UTI, biliary infection, endocarditis, and intra-abdominal infection. It is intrinsically resistant to cephalosporins and has variable vancomycin resistance (VRE, via the vanA gene, which changes D-Ala-D-Ala to D-Ala-D-Lac).

**Other possible meanings of "PYR"** - Pyridoxine: vitamin B6. Its active form, pyridoxal phosphate (PLP), is a cofactor for transamination, decarboxylation (including GABA, serotonin, dopamine, and heme synthesis via ALA synthase), and glycogen phosphorylase. Isoniazid causes B6 deficiency, which leads to peripheral neuropathy and sideroblastic anemia, so give pyridoxine with it. - Pyrimethamine: a dihydrofolate reductase inhibitor in protozoa. It is used with sulfadiazine for Toxoplasma gondii and can cause megaloblastic anemia, so give leucovorin (folinic acid). - Pyruvate: the end product of glycolysis. It is converted by pyruvate dehydrogenase (cofactors: thiamine/B1, lipoic acid, CoA, FAD, NAD) to acetyl-CoA. PDH deficiency causes lactic acidosis and neurologic defects. The treatment is a ketogenic diet (high fat, with lysine and leucine).