Yeast Phosphate Agar, 500 g

The systemic mycoses that are responsible for coccidiodomycosis, histoplasmosis and blastomycosis infections (1), although unrelated generically, morphologically and culturally, have one characteristic in common, that of dimorphism. The dimorphic organisms involved exist in nature as the saprophytic form, sometimes called the mycelial phase. For the isolation of Histoplasma from clinical material a series of six early morning specimens should be collected in sterile bottles. Immediate inoculation is recommended. The specimen is directly inoculated on medium like Sabouraud Dextrose Agar with and without antibiotics. Never hold the specimen at room temperature, as Histoplama does not survive at room temperature. Another procedure that may be useful for recovery of Histoplasma as well as Blastomyces from clinical specimens involves placing one drop of concentrated NH₄OH (ammonia) on one side of an inoculated plate. Yeast Phosphate Agar was developed by Smith and Goodman (4) for primary recovery of B.dermatitidis, H.capsulatum and other dimorphic pathogenic fungi from clinical specimens. The medium is to be used with ammonium hydroxide. Ammonium hydroxide is a selective agent that aids in recovery of dimorphic pathogens by inhibiting bacteria, yeasts and saprophytic fungi (2, 3). Yeast extract provides nitrogenous nutrients and vitamin B complex to support fungal growth. Phosphates buffer the medium. A drop of ammonia added to the surface of the inoculated plate inhibits bacteria, yeasts and saprophytic fungi present in clinical specimens without affecting dimorphic fungi like Blastomyces and Histoplasma . Phenol red changes colour of the medium from orange yellow to pink on addition of ammonia. Phenol red also shows loss of alkalinity as the ammonia volatilizes and the pH falls below 7.0. Clinical specimens suspected of being from cases of Histoplasmosis and Coccidiodomycosis must be manipulated in an exhaust protective cabinet in order to minimize the risk of inhalation of infective particles (2). Storage and Shelf-life: Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label. References: 1. Baker F. J. and Breach M. R., 1980, Medical Mycology, Medical Microbiological Techniques, London, Tonbridge. 2. Haley L. D. and Callaway C. S., 1978, Laboratory Methods in Medical Mycology, HEW Publication No. (CDC) 78-8361, Centre for Diseases Control, Atlanta, Ger. 3. Murray P. R., Baron J. H., Pfaller M. A., Jorgensen J. H. and Yolken R. H., (Eds.), Manual of Clinical Microbiology, 8th Ed., 2003, American Society for Microbiology, Washington, D.C. 4. Smith and Goodman, 1974, Am J. Clin. Pathol., 62:276.