How to Make a Differential Culture Medium for Mixed Yeast and Mold Growth
When both yeasts and molds are present in a clinical specimen (e.g., sputum, urine, BAL), a selective and differential approach is required to isolate and identify each:
35–37°C → Promotes yeast (especially for dimorphic fungi).
Use CHROMagar Candida:
Differentiates common Candida spp. based on colony color (e.g., C. albicans = green, C. tropicalis = metallic blue, C. glabrata = pink).
Subculture individual colonies based on morphology to separate pure isolates for identification and antifungal susceptibility testing.
Fig. Yeasts in Gram staining of sputum
Example: Mixed Growth Interpretation
Fig. Yeast and mold mixed growth on SDA
Finding in Culture
Interpretation
Action
Creamy, moist colonies on SDA
Likely yeast (e.g., Candida)
Subculture on CHROMagar
Powdery/fuzzy colonies
Likely mold (e.g., Aspergillus)
LPCB for morphology
Growth only on SDA+cycloheximide
Likely yeast or dimorphic pathogen
Proceed with ID
No growth on cycloheximide plate but growth on plain SDA
Likely saprophytic mold suppressed by cycloheximide
Consider non-pathogenic
Keynotes on Yeasts vs Molds
Yeasts are unicellular fungi that reproduce by budding, while molds are multicellular, forming filamentous hyphae and spores.
Yeast colonies appear moist, creamy, and bacteria-like; mold colonies are dry, fuzzy, or woolly on culture media.
Microscopically, yeasts show oval budding cells or pseudohyphae; molds show septate or aseptate hyphal structures.
Fig. Mold growth on the SDA plate
Sabouraud Dextrose Agar (SDA) supports the growth of both yeasts and molds; adding cycloheximide inhibits environmental molds but allows pathogenic yeasts.
CHROMagar Candida helps differentiate Candida species by distinct colony colors, useful for mixed candidal growth.
For mixed specimens, inoculate on two SDA plates: one with chloramphenicol only, and another with chloramphenicol + cycloheximide.
Incubate at two temperatures: 25–30°C favors mold growth, while 35–37°C favors yeast or dimorphic forms.
Perform direct microscopy (KOH/LPCB/Gram) before culture to assess the presence of yeast cells or hyphae in the specimen.
Fig. Aspergillus typical structure in LPCB preparation
Subculture individual colony types to pure media to isolate and identify each organism separately for further testing.
Interpretation of mixed growth depends on morphology, growth pattern, media response, and patient context (e.g., an immunosuppressed host).
