Abstract

THE CLASSIC PERIODIC ACID-SCHIFF (PAS) STAIN FOR THE IDENTIFICATION OF PATHOGENIC FUNGI, OCCURS IN A SHORT pH RANGE. 25 YEARS EXPERIENCE WITH THE TECHNIQUE

Cudberto Contreras Pérez, Elizabeth González Durán, Patricia Gutiérrez García, Claudia Ríos Rosas, Edgar S. Marín Suro and José D. Méndez*

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Abstract

A study of the Periodic Acid-Schiff (PAS) stain is presented, evaluating three different reagents. The results obtained define an optimal pH range for the reaction, which is 1.26 to 3.0. The values, below and above the range, cause a detriment in the reaction, affecting the quality of the staining and the visualization of the fungi, in the different clinical samples. Cryptococcus neoformans and Candida guilliermondii were used as controls, to stain smears and to see the behavior of the Schiff's reagent at different pHs. We selected some patients from the common mycoses in our environment (dermatophytosis, candidosis, pityriasis versicolor and cryptococcosis), as well as some clinical isolates (microculture), to show the results obtained with the PAS stain. The staining is useful, in patients with negative previous results for pathogenic fungi, as well as to prepare teaching material, for training and teaching purposes of medical mycology. The reaction of fuchsin, HCl, sodium or potassium metabisulfite occurs under aqueous conditions and the different amounts of reagents, used in the three procedures, determine the final pH of the solution. The reagents are sensitive to light, being necessary to package them properly, ensure quality, color and refrigeration, which will have an impact on improving the useful life of the reagent, between 6 months and a year. The three procedures to prepare the reagent are excellent for use, but considering the chemical safety measures, the hot procedures (Coleman-Feulgen and Leukofucsin), have the disadvantage of generating vapors, which when inhaled have a potential toxic risk, due to its probable carcinogenic effect, Category 1B, H350, the use of cold-prepared aldehyde fuchsin is recommended, which avoids this problem. The results lead us to have a robust technique, with highly reliable characteristics for the staining of medically important fungi.

Keywords: Periodic Acid-Schiff (PAS) stain, pH, Pathogenic Fungi, Infection Diagnosis.