The Periodic Acid Schiff (PAS) stain is a histological staining technique that is used to identify and visualize fungal organisms in tissues. It is based on the ability of the fungal cell wall to bind with the periodic acid and then subsequently with Schiff's reagent.
Under the microscope, fungal cells will appear as deep magenta structures against a blue or purple background. The PAS stain is particularly useful in identifying fungal infections, such as Aspergillosis or Candidiasis, and can be used to confirm a suspected fungal infection in biopsy specimens.