Advantage of Fontana-Masson stain in capsule-deficient cryptococcal infection

We describe light and electron microscopic findings in three cases of pulmonary infection caused by capsule-deficient Cryptococcus neoformans (CDCN) and emphasize the value of the Fontana-Masson silver (FMS) stain in diagnosing cryptococcosis. Capsule-deficient C. neoformans infections, including coccidioidomycosis, histoplasmosis, typical cryptococcosis, blastomycosis, candidiasis, and pneumocystosis were used as controls. Several stains including mucicarmine, alcian blue, and FMS were done on tissue sections from all of the above mentioned infections. It was found that while the FMS stain was positive only in the organisms of CDCN and typical C. neoformans, the mucicarmine was positive only in typical C. neoformans. The negativity of the mucicarmine in the organisms of CDCN reflected the absence of capsular material as demonstrated by ultrastructural study. Therefore, when fungal cultures are not available or are negative, and tissue mucicarmine is negative, the FMS stain is a valuable alternative for use on tissue sections. It may be used, along with routine fungal stains, when a yeast-forming fungal infection is suspected.