What to Do If a Mold Incubator Shows Growth in a Blank Control?
News 10 12 月, 2025
If a mold incubator shows microbial growth in a blank control (i.e., mold or contaminants appear in an uninoculated dish), the core cause is residual contamination inside the incubator, improper aseptic technique, contaminated consumables, or secondary contamination from the surrounding air.
Below is a prioritized troubleshooting guide including root causes, solutions, and preventive measures designed for practical laboratory use.
I. Start With 3 Fast Checks (Identify the root cause within 24 hours)
1. Is the blank consumable itself contaminated? (Most overlooked)
Check: culture plates, media, sterile water, sealing film.
Verification method:
Incubate an unopened sterile plate (or freshly sterilized media) in a biosafety cabinet without placing it into the incubator.
- If it still grows mold → consumables are contaminated
- If only incubated blanks grow → contamination comes from incubator or handling
Solutions:
- Replace consumables with certified sterile batches.
- Autoclave homemade media at 121°C for 20 min; use immediately after cooling.
- Avoid exposing sterilized media to air; ensure biosafety cabinet airflow ≥0.3 m/s and UV-sterilize for 30 min before use.
2. Is there visible contamination inside the incubator? (Most common)
Check:
- Corners, shelf gaps, door seals
- Moisture residues, media spills
- Humidifier water tank for cloudiness/odor
- Vent filters (incl. HEPA) for blockage or mold
Verification:
Swab surfaces (interior walls, door seals, shelves), inoculate onto blank media, and incubate.
If same contaminant appears → incubator residual contamination.
3. Was contamination introduced during handling? (Human error)
Common issues:
- Incubator door open too long (>30 sec)
- Hands/gloves touching plate edges
- Biosafety cabinet not disinfected or UV lamp ineffective
Verification:
Prepare two blank sets:
- Group A: sealed in biosafety cabinet, then incubated
- Group B: opened for 10 seconds in room air, then sealed
If B grows but A does not → handling/environmental contamination.
II. Solutions (Based on identified contamination source)
1. Incubator contamination — Full cleaning & disinfection
Step 1: Physical cleaning
Power off, cool down, then wipe interior, shelves, and door seals with 75% ethanol or 0.5% sodium hypochlorite.
Autoclave removable shelves or bake at 80°C for 2 hours.
Step 2: Deep disinfection
- UV irradiation for 60 min, or
- Hydrogen peroxide fumigation (30% H₂O₂, 10–15 mL/m³, sealed 24 h, then ventilated)
Step 3: Humidifier/water tank disinfection
Flush with 5% citric acid for 30 min, rinse 3×, refill with sterile water (change weekly).
HEPA filter check:
If airborne particle count is high or filter is >1 year old → replace (contact manufacturer to avoid seal damage).
2. Consumable contamination
- Choose certified sterile disposable products
- Store autoclaved media at 4°C ≤7 days
- Freshly prepare sterile water/buffers; discard after 3 days
3. Handling/environmental contamination
- Keep incubator door open ≤20 sec
- Only open plates inside a biosafety cabinet
- Replace gloves every hour or sanitize with 75% ethanol
- Maintain clean laboratory environment; use HEPA air purifier if necessary
III. Validation Test (Confirm contamination is resolved)
Prepare 3 groups of blanks (3 plates each):
| Group | Condition | Purpose |
|---|---|---|
| 1 | Sealed in biosafety cabinet, not incubated | Test consumables |
| 2 | Sealed then incubated | Test incubator |
| 3 | Opened for 10 sec then incubated | Test handling/air |
Pass criteria: No growth in all groups → contamination resolved.
If Groups 2 or 3 still grow → re-clean incubator or re-check HEPA system.
IV. Long-Term Prevention
- Weekly: replace humidifier water, wipe interior with 75% ethanol
- Monthly: deep disinfection; inspect HEPA filter pressure drop
- Quarterly: manufacturer calibration (temp/humidity) and seal inspection
- Keep blanks separate from sample plates (blanks on top shelves)
V. Common Mistakes to Avoid
- UV alone is not enough (cannot penetrate residues or mold spots)
- Never use tap water in humidifiers
- Shelf gaps often hide contaminants — sterilize regularly
- Don’t ignore door seal aging or incubator leaks
If contamination persists after all steps, the incubator may have structural issues (seal failure, sensor contamination) — contact manufacturer for full disassembly and maintenance.