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Microbiology Testing FAQs — Methods & Media | HiMedia USA
Should I add antibiotics to PDA?
Yes, adding chloramphenicol (25-50 mg/L) or other broad-spectrum antibiotics to PDA is recommended when testing samples with high bacterial loads (cannabis, food, soil, environmental). Antibiotics inhibit bacterial growth that would overgrow and obscure fungal colonies. For cannabis testing, chloramphenicol supplementation is standard practice. Pre-made PDA with antibiotics: order PDA with Chloramphenicol from HiMedia. HiMedia offers M1941 Potato Dextrose Agar w/ Chloramphenicol. Do not add antibiotics when culturing bacteria-fungi interactions.
What is the incubation temperature and time for PDA?
Standard incubation for PDA: 25-28°C (room temperature) for 5-7 days. Specific applications: (1) Cannabis TYMC: 25-28°C for 5-7 days (some states allow 3-5 days), (2) Food mold testing: 25°C for 5 days per AOAC, (3) Pharmaceutical mold testing: 20-25°C for 5-7 days per USP <61>, (4) Clinical mycology: may extend to 14 days for slow-growing pathogenic fungi. Check plates daily after day 3. Prolonged incubation (>7 days) increases contamination risk but may recover fastidious fungi.
Why is yeast and mold testing required for cannabis?
Yeast and mold testing ensures product safety and quality. Concerns: (1) Aspergillus species produce aflatoxins (carcinogenic) and can cause aspergillosis in immunocompromised users, (2) High mold counts indicate poor cultivation/storage practices, (3) Mold growth produces musty odors and off-flavors, (4) Some states require specific Aspergillus testing (A. fumigatus, A. flavus, A. niger, A. terreus) due to health risks. Inhalation of moldy cannabis is particularly dangerous. Most states mandate TYMC testing for all cannabis products before sale.
How is PDA used for cannabis testing?
For cannabis Total Yeast & Mold Count (TYMC): (1) Homogenize 1g cannabis flower in 9mL sterile diluent, (2) Make serial dilutions (10^-1, 10^-2, 10^-3), (3) Plate 0.1mL or 1mL onto PDA with chloramphenicol (25 mg/L to inhibit bacteria), (4) Incubate at 20-25° C for 5-7 days, (5) Count yeast and mold colonies, calculate CFU/g. State limits vary: California <1,000 CFU/g for flower. For concentrates/edibles: stricter limits <100-1,000 CFU/g. PDA is the industry-standard medium for cannabis TYMC testing.
What is Potato Dextrose Agar used for?
Potato Dextrose Agar (PDA) is a general-purpose medium for cultivation of yeasts and molds. Primary uses: (1) Cannabis testing - Total Yeast & Mold Count (TYMC) per state regulations, (2) Food mycology - mold identification and enumeration, (3) Pharmaceutical microbial limits (USP <61>/<62>), (4) Environmental monitoring for fungal contamination, (5) Antifungal susceptibility testing, (6) Fungal culture maintenance. The acidic pH (5.6) and high dextrose content favor fungal growth while inhibiting most bacteria.