Cultivate and differentiate fungi with Oxoid Sabouraud Dextrose Agar, a modification of Sabouraud agar (Carlier¹). The medium is often used with antibiotics for the isolation of pathogenic fungi from material containing large numbers of other fungi or bacteria.

Georg et al.² aseptically added 0.5g cycloheximide, 20,000 units penicillin and 40,000 units streptomycin to each litre of autoclaved, cooled medium. Cryptococcus neoformans, Aspergillus fumigatus and Allescheria boydii are sensitive to cycloheximide; Actinomyces bovis and Nocardia asteroides are sensitive to penicillin and streptomycin. Alternatively, one may add 0.4g chloramphenicol and 0.05g cycloheximide to each liter of reconstituted medium before autoclaving (Ajello³). The same microorganisms are sensitive to this new combination (see Dermasel Selective Supplement SR0075E).

Williams Smith & Jones⁴ employed Oxoid Sabouraud Dextrose Agar, containing 20,000 units penicillin and 0.04g neomycin per liter, for the count of yeasts in the alimentary tract of the pig. Hantschke⁵ used colistin, novobiocin and cycloheximide to isolate Candida albicans. Dolan⁶ used gentamicin, chloramphenicol and cycloheximide for the selective isolation of pathogenic fungi.

Oxoid Sabouraud Dextrose Agar may also be used as the basis of a Pagano-Levin medium⁷ for the isolation of Candida albicans. 0.1g of triphenyltetrazolium chloride (as a filter sterilised solution) is added to each liter of autoclaved molten medium cooled to 55°C. The medium is usually made inhibitory to most non-pathogenic fungi and bacteria by the addition of antibiotics, as above. After incubation for three days at 25°C, Candida albicans colonies are unpigmented or pale pink while other Candida species and other fungi form deeper pink or red colonies. The test is adequate for screening purposes, but other diagnostic criteria should also be utilized for the identification of Candida albicans⁸

The combination of cycloheximide and chloramphenicol inhibits many pathogenic fungi². However, the mycelial phase of Histoplasma capsulatum, Paracoccidioides brasiliensis, Sporothrix schoenckii and Blastomyces dermatitidis is not inhibited by these antibiotics when incubated at 25-30°C ⁹.

Please check relevant health and safety documentation before working with cyclohexamide. Some of the pathogenic fungi may produce infective spores which are easily dispersed into the laboratory. Such organisms should be examined only within a protective cabinet.

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