Mtt Assay
The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay is a colorimetric method that quantifies cell viability, proliferation, and cytotoxicity by measuring the reduction of yellow MTT to purple formazan crystals in metabolically active cells. This guide provides a source-bounded, practical framework for researchers, lab technicians, and students who need to design, execute, and interpret MTT experiments with rigor and reproducibility. The core principle relies on mitochondrial and cytosolic dehydrogenases that convert MTT to formazan, with the absorbance directly proportional to the number of living cells NCBI Bookshelf. Understanding the assay’s strengths and limitations is critical for obtaining reliable data, especially when comparing treatment effects across cell lines or conditions.
The MTT assay is widely applied in drug screening, toxicity testing, and basic cell biology. For example, it has been used to evaluate the neuritogenic effects of seaweed extracts on Neuro-2a cells and to assess cytotoxic combinations in cancer models Neuritogenic effects of Ulva lactuca aqueous extract on Neuro-2a cells. This guide covers core concepts, decision points, a stepwise workflow, quality checks, common mistakes, and interpretative limits, all grounded in published protocols and studies.
At a Glance
| Aspect | Key Information |
|---|---|
| Purpose | Measure cell viability, proliferation, or cytotoxicity |
| Principle | Reduction of MTT to formazan by living cells, detected at 540,570 nm |
| Typical time | 4,24 hours from cell seeding to final reading |
| Required equipment | Microplate reader, CO₂ incubator, sterile hood, centrifuge |
| Critical steps | Cell seeding density, MTT incubation time, solubilization completeness |
| Data output | Optical density (OD), often expressed as % viability vs. control |
| Main limitation | Does not distinguish between viability, proliferation, or metabolic activation, interference possible |
Core Concepts and Principle
The MTT assay exploits the ability of nicotinamide adenine dinucleotide phosphate (NADPH) dependent oxidoreductases and dehydrogenases in living cells to reduce the tetrazolium salt MTT into insoluble formazan crystals. The crystals are then solubilized, and the resulting colored solution is measured spectrophotometrically. The absorbance at 540,570 nm is linearly related to the number of viable cells under optimized conditions. Importantly, dead cells or cells with compromised metabolism do not reduce MTT, providing a selective readout Neuritogenic effects of Ulva lactuca aqueous extract on Neuro-2a cells.
The reduction occurs primarily in the mitochondria but also in endosomes and lysosomes. The amount of formazan produced depends on the metabolic rate, which can vary with cell density, passage number, and culture conditions. Researchers should note that the assay reflects metabolic activity rather than direct cell count, however, when calibrated properly, it serves as a reliable surrogate for viability. Studies such as those examining fedratinib induced cell cycle arrest in pancreatic cancer use MTT to quantify the effect of drug combinations Fedratinib induces non-classical G2/M arrest via mitotic kinase inhibition and synergizes with mitomycin C to suppress pancreatic ductal adenocarcinoma.
Decision Points and Assay Design
Cell Type and Seeding Density
The choice of cell line influences background reduction and optimal seeding density. Adherent cells require uniform attachment, suspension cells need gentle handling to avoid clumping. Pilot experiments should test a range of densities (e.g., 1,000 to 10,000 cells per well in a 96 well plate) to identify the linear range where MTT reduction is proportional to cell number.
MTT Concentration and Incubation Time
The standard MTT concentration is 5 mg/mL in phosphate buffered saline (PBS), added to culture medium at one tenth volume. Incubation times typically range from 2 to 4 hours, but longer incubations may lead to formazan crystal toxicity or saturation. For some primary cells or low metabolic activity, extending incubation up to 6 hours is acceptable if initial validation shows linearity.
Solubilization Protocol
After formazan formation, the crystals must be fully dissolved. Common solubilizing agents include dimethyl sulfoxide (DMSO), isopropanol with HCl, or a sodium dodecyl sulfate (SDS) acid solution. Incomplete dissolution causes high variability. The choice of solvent can affect absorbance, so the same method must be used throughout an experiment. The assay is often applied in combination studies such as capmatinib and paclitaxel for triple negative breast cancer, where consistent solubilization is critical for comparing treatment groups Capmatinib and paclitaxel combination: a novel strategy for overcoming challenges in triple negative breast cancer treatment.
Controls and Replicates
Include positive controls (e.g., cells treated with a known cytotoxic agent), negative controls (untreated cells in complete medium), and blank wells (medium without cells). Each condition should be run in triplicate at minimum. Technical replicates account for pipetting variation, biological replicates (separate experiments on different days) are essential for statistical power.
Practical Workflow
Seed cells. Trypsinize adherent cells, count, and adjust to desired concentration. Dispense 100 µL per well in a 96 well plate. Allow cells to attach overnight in a CO₂ incubator at 37°C.
Treat cells. Remove medium and add fresh medium containing test compounds or vehicle controls. Incubate for the desired treatment duration (e.g., 24, 48, or 72 hours).
Add MTT solution. Prepare MTT reagent (5 mg/mL in PBS), filter sterilize, and protect from light. Add 10 µL to each well containing 100 µL medium. Gently shake to mix.
Incubate for formazan formation. Place plate in the incubator for 3 to 4 hours. Monitor for purple precipitate, if not visible, extend incubation by 1 hour.
Remove medium (optional). For adherent cells, carefully aspirate the supernatant without disturbing the crystals. For suspension cells, centrifuge the plate at 300 × g for 5 minutes, then remove supernatant.
Dissolve formazan. Add 100 µL of solubilization solution (e.g., DMSO) to each well. Place on an orbital shaker for 15 minutes or until crystals are fully dissolved.
Read absorbance. Use a microplate reader set to 570 nm. A reference wavelength of 630 nm can subtract background. The EMBL EBI training resources emphasize consistent plate reading conditions for reproducible results EMBL EBI Training.
Calculate viability. Subtract blank absorbance from all wells. Express treatment groups as percentage of the untreated control mean. For example, % viability = (OD treatment / OD control) × 100.
Quality Checks and Controls
Linearity verification. Run a serial dilution of cells (e.g., 500 to 20,000 per well) to confirm that absorbance increases linearly with cell number. The correlation coefficient (R²) should be above 0.95.
Time course. Measure absorbance at multiple time points after MTT addition to ensure that the reaction has not plateaued or declined due to cytotoxicity from the MTT itself.
Vehicle control. Ensure that the solvent used for test compounds (e.g., DMSO at ≤0.1%) does not affect viability by including a vehicle only group.
Positive control. Use a known cytotoxic agent such as 1% Triton X 100 or staurosporine. The positive control should reduce viability to below 20% of the untreated control.
Interference check. Test compounds that are strong reducing agents (e.g., antioxidants, polyphenols) can directly reduce MTT, causing false signals. A cell free control with medium plus compound plus MTT is necessary. If color appears in the cell free well, alternative assays such as resazurin or ATP based methods should be considered. The Galaxy Training Network offers resources on data normalization that can help identify such artifacts Galaxy Training Network.
Common Mistakes and How to Avoid Them
Inconsistent cell seeding. Pipette carefully by gently mixing the cell suspension before each dispense. Use a multichannel pipette for 96 well plates.
Air bubbles. They scatter light and inflate absorbance. Tap the plate gently after pipetting or use a brief centrifugation (200 × g for 1 minute) before reading.
Excessive MTT incubation. Long incubations can kill cells and cause formazan crystals to detach, lowering absorbance. Stick to validated times.
Incomplete solubilization. Formazan crystals are not fully dissolved if the solution remains cloudy. Increase shaking time or use a stronger solvent such as acidified isopropanol.
Edge effects. Evaporation from outer wells can alter medium volume and cell health. Fill perimeter wells with sterile water or medium and do not use them for data collection. Bioconductor documentation includes quality control packages that can flag such artifacts in high throughput screens Bioconductor.
Not accounting for compound color. If the test compound itself is colored (e.g., certain natural extracts), measure absorbance of compound plus medium without cells and subtract appropriately.
Limits of Interpretation and Uncertainties
The MTT assay provides a snapshot of metabolic activity but does not directly measure cell number, cell death, or specific mechanisms. For example, compounds that stimulate mitochondrial activity without affecting proliferation will produce higher formazan signal, misleadingly suggesting greater viability. Similarly, some apoptotic cells initially retain partial metabolic activity, leading to underestimation of cytotoxicity. Therefore, the MTT assay should be confirmed with orthogonal methods such as trypan blue exclusion, flow cytometry with annexin V/PI staining, or lactate dehydrogenase (LDH) release assays.
The linear range of the assay is narrow, outside this range, absorbance plateaus and does not reflect changes in viable cell number. Researchers must validate the linear range for each cell type and treatment duration. Additionally, serum components in culture medium can reduce MTT, so the medium is often replaced with serum free or reduced serum conditions during the MTT step. The NCBI Sequence Read Archive, while primarily for sequencing data, emphasizes the importance of metadata and replication, principles that apply equally to cell based assays NCBI Sequence Read Archive.
When interpreting results from samples with high background reduction, such as macrophages or highly metabolic cells, consider using alternative tetrazolium salts (e.g., XTT, MTS) that produce water soluble formazan. Finally, the assay is end point and does not allow real time monitoring, careful time point selection is essential. Studies on breast cancer biomarkers using contrast enhanced ultrasound and shear wave elastography remind us that all assays have specific contexts and limitations Evaluating Breast Cancer Biomarkers Using Contrast enhanced Ultrasound and Shear Wave Elastography.
Frequently Asked Questions
1. Can I use the MTT assay with suspension cells?
Yes, but you must centrifuge the plate after MTT incubation to collect the formazan crystals before removing the medium. Alternatively, use a water soluble tetrazolium salt like MTS to avoid the centrifugation step.
2. Why is my absorbance too high even in untreated wells?
High absorbance may result from too many cells seeded per well, over incubation with MTT, or incomplete solubilization if reading occurs in the presence of residual medium. Reduce seeding density or shorten incubation time, and ensure thorough solubilization.
3. What should I do if my compound directly reduces MTT?
Perform a cell free control as described in Quality Checks. If the compound produces color, switch to an ATP based assay (e.g., CellTiter Glo) or a different tetrazolium salt that is less susceptible to direct reduction.
4. How many replicates are sufficient for statistically reliable data?
Technical triplicates within a plate are standard, but biological replicates (at least three independent experiments on different days) are necessary to account for day to day variability. Use an appropriate statistical test, such as ANOVA with post hoc comparisons, when analyzing multiple treatment groups.
References and Further Reading
- NCBI Bookshelf , Comprehensive cell biology and assay protocols.
- Neuritogenic effects of Ulva lactuca aqueous extract on Neuro 2a cells , Example of MTT in neutraceutical research.
- Capmatinib and paclitaxel combination: a novel strategy for overcoming challenges in triple negative breast cancer treatment , MTT use in drug combination studies.
- Fedratinib induces non classical G2/M arrest via mitotic kinase inhibition and synergizes with mitomycin C , MTT in cancer cell cycle research.
- Seaweed derived extracts and their combinations display immunomodulatory activity in a porcine cell culture model , MTT application in immunology.
- Structural design and cytotoxic profiling of a novel 1,3,4 thiadiazole derivative , MTT for cytotoxicity screening.
- EMBL EBI Training , Resources for experimental design and data analysis.
- Galaxy Training Network , Workflow tutorials for data normalization.
- Bioconductor , Software packages for high throughput assay quality control.
- Evaluating Breast Cancer Biomarkers Using Contrast enhanced Ultrasound and Shear Wave Elastography , Example of complementary imaging assays.