⚠️ Diagnosis
Introduction
Contamination is one of the most common and disruptive problems in a microbiology lab. This free guide helps you identify the type of contamination you are dealing with — bacterial, fungal, phage, cross-contamination, or sterilisation failure — using a rapid symptom-based checker, and provides evidence-based causes, identification criteria, and remediation steps used by research labs, clinical settings, and industrial microbiology facilities.
About This Tool
The Lab Contamination Guide combines a symptom-based checker with a full visual reference covering the five most common contamination types encountered in microbiology labs: fungal/mould, bacterial, bacteriophage, media/autoclave sterilisation failure, and cross-contamination between strains. Rather than requiring specialised equipment, the checker lets you describe what you're observing at the bench — where it appeared, what it looks like, and when it showed up — and returns a probable diagnosis with a recommended course of action, backed by the full reference cards below for deeper troubleshooting.
Understanding the Checker Inputs
The checker asks for three observations, each based on what's visible without additional lab testing:
- Location — where the contamination was noticed: an agar plate, liquid broth, both, or uninoculated blank media.
- Appearance — the visual character of the growth or change: fuzzy or powdery growth, slimy colonies, unexpected turbidity, a surface pellicle, a colour change, or unexpected clearing.
- Timing — when the contamination first appeared relative to inoculation: overnight, after 2–5 days, in a blank control, or within a few hours.
Taken together, these three parameters are usually enough to narrow a contamination event down to one of five categories.
Contamination Type Reference
Five reference cards covering identification signs and prevention/action steps for each major contamination category. Use these alongside the checker result above to confirm your diagnosis.
Diagnostic Logic Explanation
Unlike a molarity or Tm calculator, this tool does not use a mathematical formula. Instead the checker uses categorical pattern matching: each combination of location, appearance, and timing is cross-referenced against a lookup table of contamination signatures compiled from standard microbiology troubleshooting practice.
For example, broth that clears rapidly after growing well matches the signature for bacteriophage lysis, while growth in an uninoculated blank matches the signature for a sterilisation failure. If your specific combination isn't in the table, the checker returns general troubleshooting guidance instead of a specific diagnosis, since a novel combination cannot be assigned to a category with confidence.
Worked Example
Sample Input
- Location: Liquid broth / flask
- Appearance: Culture cleared / lysed unexpectedly
- Timing: Very rapidly — within a few hours
Step-by-Step
- Location "broth" + appearance "clear" + timing "rapid" are combined into the lookup key
broth|clear|rapid. - This key is checked against the signature table.
- It matches the signature for a culture that grows to visible turbidity and then clears suddenly.
Final Result
Likely Contamination Type: Bacteriophage Lysis
Interpretation
Rapid clearing after healthy growth is the classic signature of lytic phage infection, not bacterial contamination (which increases turbidity rather than clearing it). The recommended action is immediate decontamination of surfaces with 1% SDS or 10% bleach, autoclaving all liquid waste before disposal, and isolating phage work to a dedicated area.
Result Interpretation
The checker's output has two parts: a likely contamination type and a recommended action. Treat the diagnosis as your most probable starting hypothesis, not a certainty — visual and timing cues correlate strongly with contamination type but do not replace microscopy, plating on selective or differential media, or PCR-based strain verification when confirmation matters, such as before resuming a critical experiment or reporting a biosafety incident. The recommended action is the immediate troubleshooting step; the reference cards above give more targeted prevention guidance for the specific type you're facing.
Practical Applications
Use this guide any time you observe unexpected growth, culture behaviour, or experimental anomalies that suggest the presence of a contaminant. It is particularly valuable when troubleshooting recurring contamination events, training new lab members in contamination recognition, auditing aseptic technique after a failed experiment, or preparing standard operating procedures (SOPs) for contamination prevention and response.
Scientific Notes & Limitations
This tool is a rapid triage aid, not a diagnostic laboratory test. It draws on well-established general patterns in microbiology troubleshooting, but real contamination events can present atypically — mixed contamination, slow-growing organisms, or unusual media formulations can all produce results outside the five categories described here. Use confirmatory methods (Gram staining, selective media, 16S rRNA sequencing, or phage plaque assays) whenever the identification will affect a downstream decision, such as discarding a valuable strain or reporting a biosafety incident.
Practical Tips
General contamination prevention checklist for day-to-day lab work:
- 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.
Common Mistakes
1. Pouring plates on an open bench. Even briefly, an open plate in still lab air collects fungal spores. Always pour inside a laminar flow BSC or directly next to a flame.
2. Skipping blank media controls. Without an uninoculated control run alongside your culture, autoclave and media failures go undetected until experimental data is already compromised.
3. Not maintaining glycerol stocks. When contamination occurs, recovery depends on having a verified clean frozen stock to re-streak from. Labs that do not maintain −80°C stocks risk losing strains permanently after a contamination event.
Frequently Asked Questions
How do I tell the difference between fungal and bacterial contamination on an agar plate?
Fungal contamination appears as fluffy, woolly, or powdery growth that may be white, grey, green, black, or orange, and spreads rapidly over 2–5 days with visible aerial mycelium rising above the agar. Bacterial contamination produces discrete smooth, slimy, or mucoid colonies that are opaque, more uniform in shape, and typically appear within 12–24 hours at standard incubation temperatures. Under a stereo microscope, fungal colonies show thread-like hyphal structures while bacterial colonies appear dense and granular.
What causes a bacterial broth culture to suddenly clear after growing well?
A culture that reaches visible turbidity and then rapidly clears is the classic hallmark of bacteriophage lysis. Lytic phages infect bacterial cells, replicate inside them, and lyse the cells to release new phage particles, clearing the broth within hours. This is distinct from bacterial contamination, which causes increased turbidity rather than clearing. If OD600 values rise and then drop sharply, or if this pattern recurs across multiple cultures in the same lab area, a phage contamination event is highly likely. Decontaminate immediately with 1% SDS or 10% bleach and autoclave all liquid waste before disposal.
What should I do if my uninoculated (blank) media control shows growth?
Growth in uninoculated media controls indicates a sterilisation failure, not a technique problem. The most common cause is an incomplete autoclave cycle — incorrect temperature (should be 121°C), insufficient pressure (15 psi), too short a run time (20–30 min), or an overloaded chamber that prevented steam penetration. Discard the entire media batch and run a biological indicator (Geobacillus stearothermophilus spore strip) to validate the autoclave before preparing any new media.
How can I prevent fungal contamination when pouring agar plates?
Fungal spores are ubiquitous in lab air and settle readily on open plates during pouring. The most effective prevention is to pour plates inside a laminar flow biosafety cabinet. If a BSC is unavailable, work quickly near a Bunsen burner flame to create an upward convection current. Allow molten agar to cool to approximately 50°C before pouring to minimise condensation on lids, and seal long-term incubation plates with parafilm. Consider adding cycloheximide (50–100 µg/mL) to media when culturing bacteria that are not sensitive to it.
How do I confirm that my pure culture has become cross-contaminated with a second strain?
Cross-contamination is confirmed by observing two distinct colony morphologies on the same agar plate — differences in size, shape, colour, opacity, or edge texture. Streak single colonies of each morphotype separately onto fresh selective plates, then verify strain identity by colony PCR using strain-specific primers or 16S rRNA sequencing. Cross-contamination can also manifest as unexpected growth on antibiotic plates the target strain should be sensitive to, or anomalous phenotypic results in a functional assay. Always maintain glycerol stocks at −80°C so you can re-streak a verified pure culture if cross-contamination is confirmed.