Question
Sabouraud dextrose agar
Answer
Sabouraud Dextrose Agar (SDA) is the standard, most widely used culture medium for the isolation and cultivation of fungi in the clinical mycology laboratory.
Composition: peptone, dextrose (glucose) at a relatively high concentration, and agar, at an acidic pH (~5.6) — both the high sugar content and the acidic pH favour fungal growth while relatively suppressing most bacterial contaminants, which generally prefer a more neutral pH.
Modifications: antibiotics (chloramphenicol and/or gentamicin) are often added to further suppress bacterial contamination, and cycloheximide is added to inhibit rapidly-growing saprophytic/environmental contaminant moulds when isolating slower-growing pathogenic dermatophytes or dimorphic fungi (though cycloheximide also inhibits some pathogenic fungi, e.g., Cryptococcus neoformans and some Candida species, so its use must be selective depending on the suspected organism).
Incubation: typically at 25–30°C for moulds/dermatophytes (mimicking environmental temperature) and at 37°C for demonstrating the yeast phase of dimorphic fungi or for routine yeast identification (e.g., Candida); incubation periods range from a few days (for rapidly growing yeasts) to several weeks (for slow-growing dermatophytes or dimorphic moulds).
Clinical use: the primary, first-line medium for culturing virtually all clinically significant fungi from clinical specimens — skin/hair/nail scrapings (dermatophytes), sputum/blood/tissue (systemic fungal pathogens), and vaginal/oral swabs (Candida) — with colony morphology, growth rate, and pigmentation providing important presumptive identification clues before confirmatory microscopic/biochemical/molecular testing.