How to Use the Contamination Guide
Use the checker above to quickly diagnose contamination based on appearance and location. The full reference guide below covers all major contamination types with causes, identification tips, and prevention strategies.
🍄
Fungal / Mould Contamination
Most common contamination in microbiology labs
Identification
- Fluffy, woolly, or powdery growth
- Colours: white, grey, green, black, orange
- Grows on plate surface or broth rim
- Spreads rapidly over 2–5 days
Prevention & Action
- Work in laminar flow cabinet (BSC)
- Add cycloheximide (50–100 µg/mL) to media
- Seal plates with parafilm if incubating long-term
- Discard contaminated cultures immediately
- Clean incubators with 70% ethanol regularly
🦠
Bacterial Contamination
Foreign bacteria in pure culture or sterile media
Identification
- Unexpected colony morphology on plates
- Turbid broth in uninoculated controls
- Slimy, spreading, or mucoid colonies
- Different colony size from expected strain
Prevention & Action
- Use selective media with appropriate antibiotics
- Flame inoculation loops until red-hot
- Always use sterile technique — open tubes briefly
- Autoclave all media and equipment properly
- Use positive and negative controls every experiment
🧬
Bacteriophage Contamination
Phage lysis — devastating and hard to eliminate
Identification
- Culture clears suddenly (lysis) after growing well
- Plaques (clear zones) on bacterial lawn
- OD600 drops unexpectedly during growth
- Recurring culture crashes in same lab area
Prevention & Action
- Decontaminate all surfaces with 1% SDS or 10% bleach
- Autoclave all liquid waste before disposal
- Never reuse phage-contaminated equipment
- Use phage-resistant strains if available
- Isolate phage work in dedicated area
🫙
Media / Autoclave Failure
Contamination from poorly sterilised media
Identification
- Growth in uninoculated (blank) media controls
- Multiple cultures contaminated simultaneously
- Contamination appears within 24 hr of pouring
- Autoclave indicator tape did not change colour
Prevention & Action
- Use autoclave indicator tape and biological indicators
- Verify 121°C, 15 psi, 20–30 min cycle
- Do not overfill autoclave — leave space for steam
- Always run uninoculated media controls
- Check autoclave maintenance records
🔀
Cross-Contamination Between Strains
Mixed cultures — wrong strain in your sample
Identification
- Two distinct colony types on the same plate
- PCR or sequencing gives unexpected results
- Culture grows on antibiotic it should be sensitive to
- Unexpected phenotype in assay results
Prevention & Action
- Use separate, labelled equipment per strain
- Never work with two strains open simultaneously
- Verify strain identity by colony PCR regularly
- Re-streak from glycerol stock to get pure colonies
- Use unique antibiotic markers for each strain
General Contamination Prevention Checklist
- Aseptic technique: Always work near a flame or inside a biosafety cabinet (BSC).
- Personal hygiene: Wash hands, wear gloves and lab coat. Change gloves between strains.
- Surface decontamination: Wipe bench with 70% ethanol before and after every experiment.
- Controls: Always include uninoculated media controls to detect media/autoclave failures.
- Labelling: Label every tube, plate, and flask with strain name, date, and antibiotic resistance.
- Incubator hygiene: Clean incubators monthly with 70% ethanol. Do not store spilled cultures.
- Waste disposal: Autoclave all liquid waste and contaminated solid waste before disposal.
- Glycerol stocks: Always maintain backup glycerol stocks at −80°C for all important strains.
Related Microbiology Tools