This stain is an excellent general stain that is useful in a clinical setting as most fungi take up the stain. It is especially good for detecting yeast cells and hyphae in tissues. Fungi are stained a bright pink-magenta or purple against an:
- orange background of picric acid is used (Fig 2.7)
- or a green background if light green counterstain is used
The PAS stain comprises of two steps, hydrolysis and staining. Periodic acid hydrolyzes the cell wall aldehydes, allowing the modified Schiff reagent to stain the cell wall carbohydrates.
Figure 2.7 PAS stained tissue section showing numerous yeast cells of Histoplasma capsulatum var. duboisii. © Kaminski’s digital image library of medical mycology.
Table 1.7 Advantages and disadvantages of PAS stain