Periodic Acid Schiff (PAS) stain is used to detect fungi; which statement is true?

Periodic Acid Schiff (PAS) stain is used to detect fungi; which statement is true?

• A. It does not stain fungal elements
• B. Stains fungal elements well and nocardia spp. do not stain well
• C. It stains bacteria best
• D. It stains viruses

Answer: (B)

Periodic Acid Schiff highlights polysaccharides and glycoproteins. Fungal cell walls are rich in polysaccharides like chitin and glucans, so after oxidation they form aldehydes that react with Schiff reagent to give a strong magenta stain, making fungal elements readily visible in tissue. Nocardia, while a tough organism to stain for some methods, has a different cell-wall composition and does not stain as well with PAS compared with fungi, so PAS is not reliable for identifying them. Viruses lack a cell wall and are not detected by PAS, and bacteria are not the primary target of PAS in routine practice. So the statement that PAS stains fungal elements well and nocardia spp. do not stain well is the best reflects how this stain is used.

Periodic Acid Schiff highlights polysaccharides and glycoproteins. Fungal cell walls are rich in polysaccharides like chitin and glucans, so after oxidation they form aldehydes that react with Schiff reagent to give a strong magenta stain, making fungal elements readily visible in tissue. Nocardia, while a tough organism to stain for some methods, has a different cell-wall composition and does not stain as well with PAS compared with fungi, so PAS is not reliable for identifying them. Viruses lack a cell wall and are not detected by PAS, and bacteria are not the primary target of PAS in routine practice. So the statement that PAS stains fungal elements well and nocardia spp. do not stain well is the best reflects how this stain is used.