Preparing Chemically Competent E. coli by the CaCl2 Method
Standard CaCl2-based preparation of chemically competent E. coli cells, suitable for heat-shock transformation with plasmid DNA. Replaces commercial competent-cell kits ($80–400 per tube) with in-house prep at ~$0.10 per transformation. The most consequential cost-accessibility win available in the open-reagent pipeline: eliminates the commercial gatekeeper on competent cells. Yields transformation efficiencies of 10⁶–10⁷ cfu/µg plasmid — sufficient for all routine cloning and reagent-production workflows. Written for a novice audience.
Version History
Version 0.1.1 Viewing Latest
Effective: 2026-04-20All `procedure N` references converted to Markdown hyperlinks pointing at https://librebiotech.org/?action=show&id=N — enables in-app click-through to referenced sibling procedures. Text content otherwise preserved.
Version 0.1.0
Effective: 2026-04-20Initial release. Atomic technique for the open-reagent stack. Based on the Mandel & Higa 1970 method (~55 years old, field-standard). Fresh original prose; foundational references cited. Completes the fully self-supporting production pipeline — users can now prepare their own competent cells rather than purchasing commercially.
Procedure Details
- PC1 / CL1 containment required if the host strain carries a GMO plasmid during competent-cell prep (typical workflow prepares cells fresh from an untransformed parent strain — no GMO present until the downstream transformation).
- BSL-1 host. Standard E. coli strains (BL21, DH5α, XL1-Blue) are non-pathogenic. Autoclave all waste.
- CaCl2 (calcium chloride). Low toxicity; mild skin irritant. Standard gloves. Solutions are safe to handle.
- Ice-cold temperatures throughout. Work on ice or in a cold room; room-temperature cells lose competence quickly.
- Glycerol. Safe; non-toxic. Sterile filter or autoclave solutions before use.
- Freezer safety. Handling vials removed from −80°C: insulated gloves; brief exposure only.
Buffer preparation (prepare in advance, store at 4°C for months):
- Transformation buffer — 60 mM CaCl2 + 15% glycerol + 10 mM PIPES pH 7.0 (alternative: 10 mM Tris-HCl pH 8.0). Sterile-filtered (0.22 µm). Pre-chilled to 4°C before use.
- Storage buffer — 85 mM CaCl2 + 15% glycerol (for aliquoting and freezing). Sterile-filtered. Pre-chilled.
- Ice — plenty. Every step requires cold temperatures.
Strain selection:
- DH5α is the generic cloning workhorse — high transformation efficiency, suitable for general plasmid propagation. First choice for competent cell prep if you'll make only one strain.
- BL21(DE3) is the protein-expression workhorse — required for Model D reagent production. Lower transformation efficiency than DH5α but essential for IPTG-inducible and T7-promoter systems.
- XL1-Blue — good efficiency, carries lacZα for blue-white screening.
Mental model: CaCl2 treatment makes E. coli membranes transiently permeable to DNA, enabling subsequent heat-shock transformation. The treatment takes cold-shocked cells, swells them in calcium-rich buffer, and freezes them in glycerol for long-term storage without losing competence.
Expected yield: 20–30 × 100 µL aliquots per 250 mL starter culture. Each aliquot supports one transformation. At 10⁶ cfu/µg plasmid, each aliquot yields thousands of colonies from 1 ng plasmid input.
- Day 0 (hands-off overnight): streak strain on LB agar; grow single-colony starter overnight.
- Day 1 (~3 h hands-on): pick colony into 250 mL LB, grow to OD600 0.4 (~2 h), harvest, two ice-cold buffer washes, resuspend in storage buffer, aliquot, flash-freeze.
- Total: ~3 hours active time, with a ~2 hour growth phase in the middle suitable for setting up the next day's experiments.
- Makes: 20–30 aliquots of 100 µL competent cells, stable for months at −80°C.
-
Microcentrifuge
Centrifugation
Specs: 4,000–13,000 × g, 4°C cooled ideally
Swing-bucket or fixed-angle; 50 mL adapter for harvest -
Micropipette
Liquid handling
Specs: P200, P1000; filter tips
Pre-chilled tips in an ice bucket -
Spectrophotometer
Measurement
Specs: OD600 at 1 mL cuvette volume
Harvest-timing gate -
Vortex mixer
Mixing
Specs: Standard benchtop
Do NOT vortex cells during prep — listed for completeness only -
Biosafety cabinet
Safety
Specs: Class II Type A2 or equivalent
For aliquotting; strictly sterile technique -
Freezer (−80 °C)
Storage
Specs: Ultra-low temperature
Long-term aliquot storage -
Shaking incubator
Thermal regulation
Specs: 37°C, 225 rpm, 250 mL flask positions
Growth phase
-
E. coli BL21(DE3)
(NEB)
C2527IBacterial strain
Qty: 1 fresh colony
Or DH5α / XL1-Blue / any standard lab strain; untransformed parent -
Buffer
Reagent
Qty: 20 mL
60 mM CaCl2 + 15% glycerol + 10 mM PIPES pH 7.0; sterile-filtered; pre-chilled -
Buffer
Reagent
Qty: 3 mL
85 mM CaCl2 + 15% glycerol; sterile-filtered; pre-chilled -
Superior Broth
(Athena Enzyme Systems)
0105Growth media
Qty: 55 mL (5 overnight + 50 prep)
LB broth equivalent
Protocol Parameters Captured per-assay on each run; exported as ISA-Tab Parameter Value columns
| Name | Type | Required | Default | Unit | Description |
|---|---|---|---|---|---|
starter_dilution_factor |
number | — |
100
|
— | Overnight-to-fresh dilution factor. 1:100 standard for OD600 0.4 in ~2 h. |
harvest_od600 |
number | — |
0.4
|
— | OD600 at time of harvest. 0.4 optimal; 0.5 acceptable; above 0.5 rapidly loses competence. |
cacl2_conc_mM |
number | — |
60
|
— | CaCl2 concentration in transformation buffer. 60 mM standard; 100 mM improves efficiency slightly. |
cacl2_incubation_min |
number | — |
30
|
minute (UO:0000031) | Ice incubation time in CaCl2 buffer. 30 min standard; longer (60 min) improves efficiency slightly. |
storage_glycerol_pct |
number | — |
15
|
— | Glycerol concentration in storage buffer. 15% standard cryoprotectant; lower risks cell damage; higher increases viscosity. |
aliquot_volume_ul |
number | — |
100
|
microliter (UO:0000101) | Volume per frozen aliquot. 100 µL standard — one transformation's worth per aliquot. |
Procedure Steps (Version 0.1.1)
Streak the parental E. coli strain (e.g. DH5α, BL21(DE3), XL1-Blue) onto an LB agar plate without antibiotic. Incubate overnight at 37°C.
Pick a single well-isolated colony into 5 mL LB broth (no antibiotic). Grow overnight at 37°C, 225 rpm — this is your starter culture.
Pre-chill: transformation buffer, storage buffer, 250 mL Erlenmeyer flask with 50 mL LB, microcentrifuge rotor, sterile 1.5 mL microcentrifuge tubes. All should be at 4°C before starting.
Dilute the overnight starter 1:100 into 50 mL LB (no antibiotic) in a pre-chilled 250 mL flask.
Incubate at 37°C, 225 rpm, until OD600 reaches 0.4 (typically 1.5–2 hours for BL21(DE3); faster for DH5α). Measure OD600 per LibreBiotech procedure 62. Do NOT exceed 0.5 — over-grown cells lose competence rapidly.
Transfer the flask to an ice bucket and chill on ice for 10 minutes. This cold shock is important — it primes the cells for buffer exchange.
Transfer the culture to two pre-chilled 50 mL conical tubes. Centrifuge at 4,000 × g for 10 minutes at 4°C. From this point on, every step is cold-chain — keep everything on ice except while centrifuging.
Discard supernatant. Resuspend each pellet gently in 10 mL ice-cold transformation buffer by swirling the tube on ice (do NOT vortex; do NOT warm). Pipette gently with a pre-chilled 10 mL pipette to ensure even resuspension.
Incubate on ice for 30 minutes. This is the critical CaCl2-exposure step that makes cells permeable.
Centrifuge at 4,000 × g for 10 minutes at 4°C. Discard supernatant.
Resuspend each pellet in 1.5 mL ice-cold storage buffer (containing 15% glycerol). Gentle swirl + gentle pipetting. The final cell density is high — solution will be cloudy.
Combine both resuspensions if working in parallel (total 3 mL), or keep separate if desired.
Aliquot 100 µL into each of 20–30 pre-chilled sterile 1.5 mL microcentrifuge tubes on ice. Work quickly but carefully — the cells are sensitive to warming.
Flash-freeze aliquots as quickly as possible. Option A: submerge tubes in liquid nitrogen for 30 seconds. Option B: dry-ice ethanol bath for 1 minute. Option C: place directly at −80°C (acceptable if A/B unavailable, but efficiency is slightly lower).
Transfer frozen aliquots to a labelled box at −80°C. Record the preparation in LibreBiotech: one Sample record per aliquot, with annotations for strain, prep date, storage location, and efficiency (populated after QC transformation).
After 24 hours at −80°C, perform the efficiency QC transformation on one aliquot per procedure 60. Record the efficiency value as an annotation on every aliquot from this batch. Reject the batch and remake if efficiency is <10⁵ cfu/µg.
Expected outcome. 20–30 frozen aliquots of 100 µL competent cells at −80°C. Transformation efficiency (measured by control transformation with pUC19 or similar): 10⁶–10⁷ cfu/µg for DH5α; 10⁵–10⁶ cfu/µg for BL21(DE3). Even "lower" efficiencies here are well above what procedure 60's workflow requires (1–10 ng plasmid).
Efficiency QC (mandatory for first batch; good practice for every batch). Transform one aliquot with 0.1 ng of a known-good control plasmid (pUC19 or similar) per procedure 60. Plate 10 µL, 50 µL, and 200 µL onto selection plates. Count colonies the next day. Calculate efficiency: (colonies × total vol ÷ plated vol) ÷ ng plasmid = cfu/µg. A value below 10⁵ cfu/µg suggests the prep failed — remake fresh.
Use. Follow procedure 60 for transformation. Do not refreeze thawed aliquots — use fresh from −80°C each time, thaw on ice, and discard unused portion.
Storage. −80°C freezer. Shelf life >1 year; slow loss of competence beyond that.
Troubleshooting.
| Symptom | Likely cause | Fix |
|---|---|---|
| Efficiency <10⁵ cfu/µg | Cells warmed during prep; stale CaCl2 buffer | Remake with fresh ice-cold buffer; strict cold chain throughout |
| Aliquots yellow/brown on thaw | Bacterial lysis during freeze | Gentler pipetting; flash-freeze immediately after aliquotting |
| No colonies at all in control transformation | Strain lost viability; antibiotic on plate is degraded | Streak fresh strain from master stock; fresh antibiotic plates |
| Transformation works only with high DNA amounts | Low-efficiency prep | Accept batch as "low efficiency" and use only for abundant-plasmid transformations; plan to remake |
| Satellite colonies around larger ones | Antibiotic degradation; not the prep | Fresh plates |
References
- Mandel M, Higa A (1970). Calcium-dependent bacteriophage DNA infection. J Mol Biol 53(1):159–62. (Foundational competent-cell paper). DOI paper
- Cohen SN, Chang ACY, Hsu L (1972). Nonchromosomal antibiotic resistance in bacteria: genetic transformation of E. coli by R-factor DNA. PNAS 69(8):2110–4. DOI paper
- Hanahan D (1983). Studies on transformation of Escherichia coli with plasmids. J Mol Biol 166(4):557–80. (Optimised buffer and procedure refinements). DOI paper
- LibreBiotech procedure 60 — Heat-shock Transformation (downstream use of competent cells). Link protocol