Microtox® Test: Bacterial Bioluminescence Assay for Toxicity Screening

The Microtox® test is a standardized, rapid (5–30 min) in vitro bioassay using the marine bacterium Vibrio fischeri (strain NRRL B-11177) to assess acute toxicity.

Principle

• V. fischeri naturally bioluminesces (luciferase reaction).
• Toxicants inhibit metabolic activity → light output decreases.
• % inhibition vs. control = toxicity measure.
• EC50 (effective concentration causing 50% light loss) calculated.

Protocol

1. Reconstitute freeze-dried bacteria.
2. Expose to sample dilutions + diluent control.
3. Incubate at 15°C (standard).
4. Read luminometer at 5, 15, 30 min.
5. Software calculates EC50, confidence intervals.

Applications

• Water/wastewater effluent screening.
• Chemical/product toxicity ranking.
• Sediment pore water, soil leachates.
• Drinking water contamination events.
• Process control (industrial).

Advantages

• Fast, low cost, small sample volume.
• Broad sensitivity (organic, inorganic, heavy metals).
• Standardized (ISO 11348, ASTM D5660).
• Portable instrument (Microtox® M500, FX).

Limitations

• Bacterial system ≠ mammalian/human toxicity.
• Does not identify specific toxicant.
• Interferences: color, turbidity, volatility, pH, salinity (adjustments needed).
• Not for chronic/sublethal endpoints.

Regulatory Use

Screening tool; not a standalone regulatory endpoint. Complements specific chemical analyses and higher-tier bioassays (daphnia, fish).