Fluorescent staining of Histoplasma capsulatum.

In an earlier paper from this laboratory (Gordon, 1958) it was demonstrated that fluorescein-labeled specific antiserum employed as a cellular stain will differentiate Candida albicans and Candida tropicalis from other yeasts and yeastlike organisms, including the remaining members of the genus Candida. The present report describes the preparation and use of a similar reagent for detection and ident fication of the yeast phase of Histoplasma capsulatum, which must be differentiated from morphologically similar fungi and protozoa that invade the human body. Although typical forms of the pathogenic yeasts are usually separable, it is often difficult to identify them specifically or to differentiate them from artifacts. Many reports, for example, of ambiguous large forms of H. capsulatum and small forms of Blastomyces dermatitidis have appeared, as have instances of proved or possible dual infection (Binford, 1955; Weed, 1955; Starr et al., 1955; Moore, 1955). Histoplasma duboisii characteristically produces forins similar to those of B. dermatitidis both in tissue and in culture, as do some strains of H. capsulatum to a lesser extent (Drouhet and Schwarz, 1956). The latter may also be simulated by Cryptococcus neoformans (Weed, 1955; Baker, 1957; Seabury, 1958), Sporotrichum schenckii (Kligman and l3aldridge, 1951), Torulopsis glabrata (Lopez Fernandez, 1952), and certain protozoa, particularly Leishmania donovani (Starr et al., 1955). The periodic acid-Schiff stain (Pillsbury and Kligman, 1951) is an excellent tool for detecting fungi in tissues, but often is less satisfactory than the routine hematoxylin and eosin for species identification. H. capsulatum and S. schenckii, for instance, may look very much alike when stained with the Schiff reagent. The fluorescent antibody reagent differentiates among species of fungi.