SYTOX Orange Dead-Cell Stain Protocol for Flow Cytometry
SYTOX Orange Dead-Cell Stain Protocol for Flow Cytometry
SYTOX Orange dead-cell stains is a high-affinity nucleic acid dye that selectively penetrates cells with compromised plasma membranes, enabling rapid and reliable discrimination of dead from live cells in flow cytometry and fluorescence microscopy applications.
- Excitation / Emission
- Excitation ~547 nm; Emission ~570 nm (orange fluorescence)
- Compatible Laser Lines
- 532 nm (green), 543 nm, or 561 nm laser lines
- Working Concentration
- Typically 1–5 nM in physiological buffer (e.g., PBS)
- Membrane Permeability Selectivity
- Does not penetrate intact plasma membranes; stains only dead or compromised cells
- Incubation Time
- 5–30 minutes at room temperature, protected from light
- Wash Requirement
- No wash step required before acquisition; wash can reduce background
- Product SKU
- S1540206
Principle and Reagent Overview
SYTOX Orange dead-cell stain is a cyanine-based, membrane-impermeant nucleic acid dye. In living cells with intact plasma membranes, the dye is excluded and no fluorescence signal is generated. When the plasma membrane is compromised—as occurs in apoptotic or necrotic cells—SYTOX Orange enters the cell and binds to double- and single-stranded nucleic acids with high affinity, producing a bright orange fluorescent signal with excitation maximum near 547 nm and emission maximum near 570 nm.
This selectivity makes SYTOX Orange an excellent single-reagent viability probe compatible with multi-parameter flow cytometry panels. Because it requires no fixation or permeabilization, it preserves cell surface antigen integrity for simultaneous immunophenotyping.
Key advantages:
- Single-step, no-wash protocol option
- Bright signal with high signal-to-noise ratio
- Compatible with 532 nm, 543 nm, and 561 nm laser lines
- Suitable for use with mammalian cells, yeast, and bacteria
Materials and Reagent Preparation
Materials required:
- SYTOX Orange dead-cell stain (S1540206; stock solution typically 5 mM in DMSO)
- Phosphate-buffered saline (PBS), pH 7.4, or other physiological buffer
- Microcentrifuge tubes (1.5 mL)
- Flow cytometer equipped with 532 nm, 543 nm, or 561 nm laser and appropriate bandpass filter (e.g., 585/16 nm or 590/20 nm)
- Cell culture medium and centrifuge
- Ice bucket and foil (for light protection)
Working solution preparation:
- Thaw the SYTOX Orange stock solution on ice and briefly vortex.
- Dilute the stock in PBS to prepare a 1–5 nM working solution. For example, dilute 1 µL of 5 mM stock into 1 mL PBS to obtain 5 µM intermediate, then further dilute 1 µL of intermediate into 1 mL PBS to obtain 5 nM working solution.
- Prepare the working solution fresh or store protected from light at 4 °C for short-term use (≤24 hours).
Note: Avoid repeated freeze-thaw cycles of the stock solution. Aliquot upon first use.
Step-by-Step Staining Protocol
1. Harvest cells
Collect cells by standard methods (trypsinization for adherent cells or direct centrifugation for suspension cells). Wash once with PBS and resuspend at 1×106 cells/mL in PBS or appropriate buffer. Transfer 100–500 µL aliquots to microcentrifuge tubes or flow cytometry tubes.
2. Prepare positive control
To generate a dead-cell control, treat an aliquot of cells with 70% ethanol (10 min, room temperature) or heat-kill at 65 °C for 10 minutes, then wash with PBS before staining.
3. Add SYTOX Orange working solution
Add SYTOX Orange working solution directly to the cell suspension to achieve a final concentration of 1–5 nM. Mix gently by pipetting. Do not vortex.
4. Incubate
Incubate at room temperature for 5–30 minutes, protected from light. For best results, use 15 minutes as the standard incubation time.
5. Optional wash
A wash step is not required but may reduce background signal. If washing, centrifuge at 300–400 × g for 5 minutes, aspirate supernatant, and resuspend in PBS.
6. Acquire on flow cytometer
Analyze samples immediately. Use the 532 nm, 543 nm, or 561 nm laser for excitation and collect emission with a 585/16 nm or 590/20 nm bandpass filter. Set gates using the unstained and positive (dead-cell) controls. Acquire at least 10,000 events per sample.
Note: Do not fix samples after SYTOX Orange staining; fixation may alter staining intensity and invalidate viability discrimination.
Instrument Settings and Data Analysis
Recommended instrument configuration:
| Parameter | Recommendation |
|---|---|
| Excitation laser | 532 nm, 543 nm, or 561 nm |
| Emission filter | 585/16 nm or 590/20 nm bandpass |
| Detector | PE or equivalent channel |
| Voltage/gain | Adjust using unstained and positive controls |
Gating strategy:
- Create a scatter plot (FSC vs. SSC) and gate on the main cell population to exclude debris.
- Display SYTOX Orange fluorescence on the x-axis (log scale). Dead cells will appear as a distinct SYTOX Orange-positive (bright) population; live cells will be SYTOX Orange-negative (dim or negative).
- Set the gate boundary between negative and positive populations using the unstained control and the dead-cell positive control.
- Report percentage of SYTOX Orange-positive cells as the dead-cell fraction.
Spectral considerations:
SYTOX Orange emission may overlap with PE-tandem dyes and other orange/red fluorophores. Perform spectral unmixing or compensation if running multi-color panels. Consult your instrument's compensation matrix.
Troubleshooting Guide
| Problem | Possible Cause | Solution |
|---|---|---|
| High background in live-cell population | Dye concentration too high; insufficient washing | Reduce working concentration to 1 nM; add optional wash step |
| Weak or no signal in dead cells | Dye degraded; insufficient incubation time; dye concentration too low | Use fresh working solution; extend incubation to 30 min; increase concentration to 5 nM |
| Poor separation between live and dead populations | Cell debris interfering; gating error | Gate tightly on FSC/SSC to exclude debris; use positive and negative controls to reset gates |
| Loss of cells during wash | Centrifugation force too high or pellet disruption | Reduce centrifugation speed; aspirate carefully; consider no-wash protocol |
| Photobleaching of signal | Excessive light exposure during incubation or handling | Keep samples wrapped in foil; minimize exposure time; acquire immediately after staining |
Frequently asked questions
Can SYTOX Orange be used with fixed cells?
No. SYTOX Orange should be used on unfixed, live-cell samples only. Fixation agents such as formaldehyde permeabilize all cell membranes, causing the dye to stain all cells regardless of viability, which eliminates the ability to discriminate live from dead cells.
Is SYTOX Orange compatible with surface antibody staining for immunophenotyping?
Yes. Because SYTOX Orange requires no fixation or permeabilization, it is fully compatible with surface antibody staining. Perform surface antibody incubation first, then add SYTOX Orange immediately before flow cytometry acquisition to maintain cell viability status.
What is the recommended storage condition for SYTOX Orange?
Store the stock solution at −20 °C, protected from light. Upon first use, aliquot the stock to avoid repeated freeze-thaw cycles, which can degrade the dye and reduce staining performance. Working solutions should be prepared fresh or used within 24 hours if stored at 4 °C in the dark.
Which flow cytometer channels are compatible with SYTOX Orange?
SYTOX Orange is best excited by 532 nm, 543 nm, or 561 nm laser lines and detected in the PE channel or equivalent orange emission channel (e.g., 585/16 nm or 590/20 nm bandpass filter). It is not efficiently excited by the standard 488 nm blue laser or the 633/640 nm red laser.
Can SYTOX Orange be used for bacteria or yeast in addition to mammalian cells?
Yes. SYTOX Orange can be used to assess viability in bacteria and yeast. However, optimal working concentrations and incubation times may differ from those established for mammalian cells and should be determined empirically for each microorganism type.
References
- Molecular Probes / Thermo Fisher Scientific. SYTOX Orange Nucleic Acid Stain product information and technical notes.
- Rieger AM, Hall BE, Barreda DR. Macrophage activation differentially modulates particle binding, phagocytosis and downstream seagull responses. Journal of innate immunity, 2010. [General cell viability methodology reference]
- Shapiro HM. Practical Flow Cytometry, 4th Edition. Wiley-Liss, 2003. [Textbook reference for flow cytometry principles and viability staining]
Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.
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