Microtox bioassay

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Microtox is a quick, in vitro toxicity test that uses light-emitting bacteria called Allivibrio fischeri. When these bacteria encounter toxic substances, their respiration is affected and they emit less light. The resulting decrease in luminescence is measured and expressed as percent inhibition, which correlates with the sample’s toxicity. The test was created in 1979 by Azur Environmental (formerly Microbics) as a cost-effective alternative to fish and daphnid tests and has since become a standard method for checking toxicity in water, soils, and sediments. Over the years the Microtox technology has changed hands, with Modern Water acquiring Microtox and related technologies in 2011 from Strategic Diagnostics Incorporated.

How it works
Microtox uses a freeze-dried culture of Allivibrio fischeri that is reconstituted in a dedicated solution. The bioluminescence from the bacteria is measured with a photometer. In addition to liquid samples, the system can test solids like soils and sediments by using eluates or pore water to bring contaminants into contact with the bacteria. The platform offers several instruments, including the Model 500 laboratory photometer, the Continuous Toxicity Monitor (CTM) for site-wide, real-time testing, and the DeltaTox II portable unit for on‑site assessments.

Sample types and preparation
The technology can be applied to drinking water, stormwater, industrial effluents, soils, and sediments. Most samples only need salinity adjustment to about 2% using a non-toxic osmotic solution. If needed, samples can be dechlorinated and their pH adjusted carefully. Highly turbid or very colored samples may require clarification, settling, or centrifugation to improve clarity. Soil and sediment testing often use elutriates (water or solvent extracts of the solid) to obtain a representative contact with the bacteria, because contaminants may bind to particles. Direct solid-phase contact is possible but can introduce more interference from color, turbidity, or particle loss.

Tests and data interpretation
There are five major Microtox tests, with three used for soil and sediment analysis: Basic Test, 100% Test, and Solid-Phase Test. The Basic Test is a robust protocol for unknown or highly toxic samples, using multiple dilutions and controls. The 100% Test uses the full sample concentration and is more sensitive to technique, making it good for screening but sometimes less precise. The Solid-Phase Test exposes bacteria directly to solids and can show higher apparent toxicity due to direct contact. The Comparison Test and Inhibition Test are used for samples with lower toxicity; they employ multiple replicates at a single concentration and different timing corrections.

Results are expressed as EC50 or IC50 values (the concentration causing 50% inhibition), and other EC/IC values can be calculated. The Microtox Omni software, developed by Azur Environmental, guides instrument setup, data analysis, statistics, and reporting. It includes templates for common test formats and can be customized, with a test tutor and the ability to work with multiple databases for cross-site comparisons.

Applications and considerations
Microtox is widely used to evaluate the toxicity of both freshwater and marine samples, sediments, and industrial discharges. It supports environmental monitoring, regulatory use (such as establishing apparent effects thresholds, sediment quality standards, and permitting criteria), and can serve as an early warning system for toxic spills. While many studies show positive correlations between Microtox results and toxicity to fish, crustaceans, or algae, some concerns remain. Luminescence-based responses may not always predict organism survival, and tests using sediment extracts may not fully represent the whole sediment’s toxicity or bioavailability. Interference from particulates, color, or residual chlorine can affect results, so proper sample preparation and, when needed, dechlorination are important.

In short, Microtox offers fast, cost-effective screening of toxicity across water, soils, and sediments, providing useful data to guide further testing and environmental decision-making.