Antibiotic Stock Solutions 

(many comments from Stan Maloy)





Stock Solution2 


(Rich Media)

(Minimal Media)


Na Ampicillin3 

Amp, Ap 

50 μg/ml 

15 μg/ml4 

20 mg/mL 50% EtOH  


Cam, Cm 

20 μg/ml 

5 μg/ml 

20 mg/mL EtOH  


Dox, Dx 

2 μg/ml 


1 mg/mL (500x)  

Gentamycin SO4 

Gent, Gen, Gn 

10 μg/ml 


2 mg/mL  

Kanamycin SO45 

Kan, Kn 

50 μg/ml 

12.5 μg/ml 

50 mg/mL (100x)  


Rif, Rf 

100 μg/ml 


20 mg/mL MeOH  

Tetracycline HCl6 

Tet, Tc 

20 μg/ml 

10 μg/ml 

4 mg/mL  



80 μg/ml 


24 mg/mL  



200 μg/ml 


15 mg/mL 1% acetic acid (75x)  


Spec, Spc, Sp 

100 μg/ml 

50 μg/ml 

20 mg/mL  

Streptomycin SO47 

Strep, Str, Sm, St 

200 μg/ml 

1000 μg/ml 

40 mg/mL  


Zeo, Zn, Zc 

50 μg/ml 


100 mg/mL  

1. The solid form of the antibiotics can be added directly to sterilized media that has been cooled to approximately 55°C. If kept at 4°C tetracycline, chloramphenicol, and streptomycin plates are usually good for several months, but kanamycin plates and ampicillin plates may only last for several weeks. A sensitive and resistant control should always be included each time antibiotic plates are used. 

2. For liquid media or for a few plates, a stock solution of the antibiotics can be prepared and stored at -20°C. All the antibiotic stock solutions can be prepared in sterile dH2O except as indicated. The stock solutions are generally 200x the concentration in rich medium, with exceptions indecated. 

3. High level expression of β-lactamase can destroy ampicillin in the medium surrounding the AmpR colonies, allowing AmpS satellite colonies to grow. Satellite colonies can be decreased by using 2x ampicillin when selecting for plasmids. Alternatively, Timentin prevents growth of satellite colonies. 

4. Anaerobically, grow cells on minimal medium containing 1 μg/mL (15 is too high). 

5. To select for a KanR gene on a multicopy plasmid use LB + 10 mM Tris HCl pH 7.4 + 250 μg/ml Kanamycin SO4 or Neomycin SO4. High levels of KanR are not expressed if the pH of the medium is less than 7.2. 

6. Certain strains are only resistant to half this concentration of tetracycline.  

7. For streptomycin plates add 1% Nutrient Broth to the minimal media (i.e., 1 ml NB/100 ml medium). 

Last Update: Monday November 14 2016
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Eric Kofoid eckofoid at