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Pneumocystis: A Polysaccharide Anomaly

Pneumocystis (Pc) is an opportunistic fungal pathogen that is well known for its ability to cause severe pneumonia in immunosuppressed individuals. The diagnosis of Pneumocystis pneumonia is commonly made by confirming the presence of the organism in specimens by staining with the fluorescent dye Calcofluor White, a non‐specific fluorochrome that is best known for staining fungal and plant cell walls by binding to β‐linked polysaccharides such as chitin and cellulose respectively. Previously, it was assumed that Calcofluor White, also known as Fluorescent Brightener 28 (FB28), was staining Pc by binding to chitin in Pc cyst form cell walls. However, recent genomic analysis of three species of Pc reveals that none of them encode the enzymes chitin synthase or chitinases needed for chitin synthesis and degradation, respectively. The absence of these genes means that Pc does not contain chitin in its cell walls. Therefore, we investigated what Calcofluor White is staining in Pneumocystis.