Using Solarbio Competent Cells: DH5α (C1100), DH5α-T1 (C1180), BL21(DE3) (C1300). This protocol covers chemical transformation via the standard heat-shock method, applicable to all Solarbio chemically competent E. coli strains.
Principle
Chemical transformation uses calcium chloride-treated competent cells to introduce exogenous DNA through the bacterial membrane during a controlled heat-shock step. The Ca²⁺ ions neutralize electrostatic repulsion between the DNA phosphate backbone and the negatively charged cell membrane, while the temperature shift (0°C to 42°C) creates transient membrane permeability that allows DNA entry. Following a recovery period in rich medium, cells express antibiotic resistance and can be selected on solid medium.
Equipment and Reagents Checklist
| Item |
Specification |
Purpose |
| Water bath or heat block |
42°C (precisely calibrated) |
Heat shock |
| Ice bath |
Crushed ice with water |
Cell handling |
| Shaking incubator |
37°C, 200 rpm rotation |
Outgrowth/recovery |
| Static incubator |
37°C |
Plate incubation |
| Microcentrifuge |
Standard, for small volumes |
Not required for standard protocol |
| SOC medium |
SKU M2100 |
Recovery/outgrowth |
| LB agar plates |
Pre-poured with appropriate antibiotic |
Colony selection |
| Sterile spreader |
L-shaped glass or plastic |
Even bacteria plating |
| P20 and P200 pipettes |
Filtered tips recommended |
DNA addition and plating |
| Sterile culture tubes |
14–17 mL (Falcon round-bottom or equivalent) |
Recovery incubation |
Materials
| Item |
Solarbio SKU |
| Competent cells (50 μL aliquots) |
Various (C1100, C1180, C1300) |
| SOC medium (2% tryptone, 0.5% yeast extract, 10 mM NaCl, 2.5 mM KCl, 10 mM MgCl₂, 20 mM glucose) |
M2100 |
| LB agar plates (with appropriate antibiotic) |
Pre-poured or prepared from LB Agar (L1010) |
| pUC19 control DNA |
Provided with competent cells |
| Ice bath |
— |
| 42°C water bath |
— |
Detailed Protocol (Step by Step)
Step 1: Thaw Competent Cells
- Remove competent cells from −80°C storage and place immediately on dry ice or directly into an ice bath.
- Allow cells to thaw on ice for 5–10 minutes. Do not vortex, do not warm above 4°C, and do not pipette to resuspend — the cells will settle naturally.
- Gently flick the tube 2–3 times when a small ice crystal remains to complete the thaw.
- Critical: Once thawed, cells must remain on ice. Transformation efficiency drops rapidly if cells warm above 4°C.
Step 2: DNA Addition
- Add 1–5 μL ligation product or 0.1–10 ng purified plasmid DNA directly into 50 μL competent cells.
- Flick the tube gently 3–4 times to mix. Do not pipette up and down — shear forces can damage competent cells.
- Positive control: Add 1 μL (10 pg) pUC19 control DNA to a separate 50 μL aliquot.
- Negative control: Add 1 μL nuclease-free water to a separate 50 μL aliquot (no DNA control).
Step 3: Incubation on Ice
- Incubate the DNA-cell mixture on ice for 30 minutes.
- Do not shorten this incubation — the 30-minute ice incubation is essential for DNA binding to the cell surface.
Step 4: Heat Shock
- Transfer the tube to a 42°C water bath or heat block for exactly 45 seconds.
- The timing is critical — 45 seconds at 42°C is optimal for all Solarbio chemically competent strains. Longer (>60 s) reduces cell viability; shorter (<30 s) reduces DNA uptake.
| Strain |
Heat Shock Temp |
Duration |
Purpose |
| DH5α (C1100) |
42°C |
45 s |
Routine cloning, blue-white screening |
| DH5α-T1 (C1180) |
42°C |
45 s |
High-efficiency cloning, large constructs |
| BL21(DE3) (C1300) |
42°C |
45 s |
Protein expression (T7 promoter system) |
Step 5: Recovery on Ice
- Immediately return the tube to ice for 2 minutes. This stabilizes the cells after the thermal shock.
Step 6: Outgrowth in SOC Medium
- Add 500 μL SOC medium (M2100) pre-warmed to 37°C to each tube.
- Mix gently by inverting or flicking.
- Incubate at 37°C with shaking at 200 rpm for exactly 1 hour.
- For BL21(DE3) expression strains, the 1-hour recovery at 37°C is sufficient. Do not exceed 1 hour or cells may enter stationary phase and show reduced viability.
- Spread the appropriate volume onto pre-warmed LB agar plates containing the selective antibiotic:
| Transformation Type |
Volume to Plate |
Expected Outcome |
| Plasmid (high efficiency) |
10–50 μL |
>100 colonies (typically lawn at 50 μL) |
| Ligation (low efficiency) |
100–200 μL |
10–200 colonies |
| Control (pUC19, 10 pg) |
50 μL |
>500 colonies (DH5α-T1) |
| No DNA control |
100 μL |
0 colonies |
- Spread evenly using a sterile spreader until the liquid is fully absorbed.
- Invert plates and incubate overnight (14–18 hours) at 37°C.
- Do not incubate longer than 18 hours — satellite colonies may appear with certain antibiotic selections.
Quality Checks During Protocol
| Stage |
Check |
Expected Result |
| After thaw |
Cells on ice throughout |
Cells remain cold until heat shock |
| After heat shock |
Timer accuracy |
Exactly 45 sec at 42°C |
| Positive control (pUC19) |
Colonies on ampicillin plate |
>1000 (DH5α), >5000 (DH5α-T1) |
| Negative control (water) |
Colonies on plate |
0 colonies |
| No-DNA control |
Colonies on plate |
0 colonies (no contamination) |
Expected Results
| Strain |
pUC19 (10 pg) Competence |
Ligated Vector |
Comments |
| DH5α (C1100) |
≥1 × 10⁸ CFU/μg (>1000 colonies) |
10–200 colonies |
Routine subcloning, blue-white screening |
| DH5α-T1 (C1180) |
≥1 × 10⁹ CFU/μg (>5000 colonies) |
50–500 colonies |
High-efficiency cloning; T1 phage resistant |
| BL21(DE3) (C1300) |
≥2 × 10⁷ CFU/μg (>200 colonies) |
— |
Protein expression; contains T7 RNA polymerase gene |
Efficiency Data
| Parameter |
DH5α (C1100) |
DH5α-T1 (C1180) |
BL21(DE3) (C1300) |
| Transformation efficiency (pUC19) |
≥1 × 10⁸ CFU/μg |
≥1 × 10⁹ CFU/μg |
≥2 × 10⁷ CFU/μg |
| Blue-white screening |
White colonies (lacZΔM15 intact) |
White colonies (lacZΔM15 intact) |
Not applicable |
| Recombination |
recA1 (reduced, suitable for unstable inserts) |
recA1 (reduced, suitable for unstable inserts) |
Not required |
| Endonuclease |
endA1 (improved plasmid quality) |
endA1 (improved plasmid quality) |
N/A |
| Phage resistance |
F− |
F−, tonA (T1/T5 phage resistant) |
F− |
| Genotype key features |
φ80lacZΔM15, Δ(lacZYA-argF)U169 |
φ80lacZΔM15, Δ(lacZYA-argF)U169, tonA (T1R) |
F− ompT hsdSB(rB− mB−) gal dcm rne131 (DE3) |
Troubleshooting Table
| Problem |
Cause |
Solution |
Prevention |
| No colonies on positive control |
Competent cells dead |
Use fresh cells from −80°C; do not leave on ice >30 min |
Store at −80°C; never re-freeze after thawing |
|
Heat shock temperature wrong |
Calibrate 42°C water bath with thermometer |
Verify water bath temperature monthly |
|
Heat shock too short |
Time exactly 45 s |
Use a timer; do not approximate |
|
Antibiotic overdose |
Verify antibiotic concentration; check plate preparation date |
Use antibiotic within expiration; store at 4°C (dark) |
| No colonies on ligation plate |
Ligation failed |
Run ligation product on gel to check insert |
Use positive control ligation; include T4 ligase control |
|
Ligase inhibitor present |
Purify ligation product (column cleanup) |
Ensure gel extraction removes all gel solubilization buffer |
|
Too much ligation product |
Reduce to 1 μL per transformation |
Ligation products >5 μL may inhibit transformation |
|
Insert-to-vector ratio suboptimal |
Test 3:1 and 5:1 molar ratios |
Calculate molar ratio; do not use mass or volume ratio |
| Satellite colonies |
β-lactamase degradation |
Use fresh antibiotic at correct concentration |
Store ampicillin plates at 4°C ≤4 weeks |
|
Incubation >18 h |
Harvest colonies after 14–16 h |
Set incubation timer at 14 h |
| Lawn of colonies (no selection) |
Missing antibiotic |
Verify plate preparation |
Always include no-DNA control |
|
Contaminated SOC medium |
Filter-sterilize SOC medium |
Prepare SOC fresh or use commercial M2100 |
| Blue colonies (X-Gal/IPTG plates) |
IPTG not active |
Add fresh IPTG to plates |
Store IPTG at −20°C; protect from light |
|
Insert cloned but no disruption of lacZ |
Check reading frame |
Subclone into different restriction site |
Optimization Notes
- For high-efficiency cloning (DH5α-T1): Plate only 10 μL of the outgrowth culture. The efficiency of ≥1 × 10⁹ CFU/μg means 50 μL plating will produce a lawn on LB-ampicillin plates with standard pUC19 transformation.
- For library construction: Use DH5α-T1 (C1180) and electrocompetent cells (C1100-E) for maximum efficiency. Electroporation typically yields 2–10× higher efficiency than chemical transformation. For electroporation, use 0.1 cm cuvettes at 1.8 kV, 25 μF capacitance, 200 Ω resistance.
- For BL21(DE3) protein expression: After picking a single colony, inoculate 5 mL LB with antibiotic and 1% glucose (to suppress basal T7 expression). Grow at 37°C to OD₆₀₀ = 0.4–0.6, then induce with 0.1–1.0 mM IPTG. Optimal induction time and IPTG concentration must be empirically determined for each target protein.
- Scaling up plasmid production: For maxi-prep scale (500 mL culture), transform with diluted plasmid (0.1–1 ng) and use a single colony to inoculate the starter culture. This minimizes the risk of satellite colonies and ensures clonal homogeneity.
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