SOB (Super Optimal Broth) medium is a nutrient-rich microbial growth medium, formulated by Douglas Hanahan in 1983 (Hanahan, 1983). SOB or its glucose-supplemented version SOC is often used during the transformation of E. coli competent cells with plasmids (added in transformed cells after heat shock/electroporation). It results in better survival and higher transformation efficiency as compared to regular LB medium. The original recipe, described by Hanahan (Hanahan, 1983) contains 2% Bacto-tryptone (w/v), 0.5% Bacto-yeast extract (w/v), 10 mM Sodium chloride (NaCl), 2.5 mM Potassium chloride (KCl), 10 mM Magnesium sulfate (MgSO4), and 10 mM Magnesium chloride (MgCl2).
SOB medium is prepared without Mg++ by dissolving 20 g bacto-tryptone, 5 g bacto-yeast extract, 2 ml of 5M sodium chloride, and 2.5 ml of 1M KCl in water to a final volume of ≈ 1000 ml (980 ml). Magnesium salts (i.e., MgSO4 and MgCl2, 10 ml each of 1M MgSO4 and 1M MgCl2) are added to the medium after autoclaving to prevent any chemical reactions and precipitation of the medium components.
Reagents and solutions
5M Sodium chloride (NaCl)
1M Potassium chloride (KCl)
1M Magnesium sulfate (MgSO4) (Sterile)
1M Magnesium chloride (MgCl2) (Sterile)
5N NaOH (for pH adjustment)
Deionized / Distilled water
Equipment and disposable
Erlenmeyer flask / Beaker
Hot plate / Magnetic stirrer with a heating device
2% (w/v) Bacto-tryptone
0.5% (w/v) Bacto-yeast extract
10 mM Sodium chloride
2.5 mM Potassium chloride (KCl)
10 mM Magnesium sulfate (MgSO4)
10 mM Magnesium chloride (MgCl2)
Preparation of 1000 ml SOB medium
Step 1: To prepare 1000 ml of SOB medium, weigh out 20 g bacto-tryptone, and 5 g bacto-yeast extract in a 2-litre Erlenmeyer flask/beaker. Add 800 ml deionized/Milli-Q water and mix it. The solution will appear translucent yellowish with undissolved media ingredients.
◊ One can use manual shaking using a glass pipette to mix the ingredients. Magnetic stirrer makes the dissolving process easy and convenient.
◊ Do not dissolve in 1000 ml of deionized / Milli-Q water. In most cases, solution volume increases when a large amount of solute is dissolved in a solvent.
◊ Make sure no lumps are remaining after making the suspension of medium components.
Step 2: Dissolve all ingredients completely by heating to boiling while stirring.
◊ Medium with completely dissolved ingredients will appear as a transparent yellowish solution.
◊ Once all ingredients are dissolved and the medium appears transparent, stop the heating. Don’t unnecessarily heat the medium.
◊ Make sure to dissolve any residual powder sticking to the glass.
Step 3: Cool down the medium to room temperature. Add 2.5 ml of 1M KCl and mix. Adjust the pH 7.0 with 5N NaOH (~0.2 ml).
The SOB medium is not very highly buffered. The pH of the medium drops, as the growing culture reaches the near saturation phase.
Step 4: Adjust the volume to 980 ml with deionized / Milli-Q water. Mix it again.
Step 5: Transfer the medium to an autoclavable bottle or insert the cotton plug and cover the mouth Erlenmeyer flask with aluminum foil.
Step 6: Sterilize the solution by autoclaving for 20 minutes at 15 lb/in2 (1.05 kg/cm2) on liquid cycle.
Step 7: Let the solution cool down to room temperature. Add 10 ml of 1M MgSO4 and 10 ml of 1M MgCl2 aseptically and mix by shaking the bottle.
◊ Antibiotics and nutritional supplements should be added only after the solution has cooled to 40°C or below. Many supplements, like antibiotics, are degraded at high temperatures.
◊ After autoclaving, all sterile solutions should be handled inside the laminar flow hood to maintain sterility.
The solution can be stored at room temperature for a few days. Keep the medium at 2 – 8 °C for longer storage.
|Follow the table to prepare SOB medium of various volumes (100 ml, 500 ml, 1,000 ml and 10,000 ml).|
|Reagents / Volume||100 ml||500 ml||1,000 ml||10,000 ml|
|Bacto-tryptone||2 g||10 g||20 g||200 g|
|Bacto-yeast extract||0.5 g||2.5 g||5 g||50 g|
|Sodium chloride||0.5 ml||1 ml||2 ml||20 ml|
|1M KCl||0.25 ml||1.25 ml||2.5 ml||25 ml|
|1M MgSO4||1 ml||5 ml||10 ml||100 ml|
|1M MgCl2||1 ml||5 ml||10 ml||100 ml|
|Water||Adjust the final volume to 100 ml||Adjust the final volume to 500 ml||Adjust the final volume to 1000 ml||Adjust the final volume to 10000 ml|