IGEM:Brown/2007/Lab Protocols/Preparing Antibiotic Stock Solutions
Beta-lactam-antibiotics are not very stable when dissolved. Slow but steady degradation happens even when frozen to -20°C. Therefore commercial beta-lactam-antibiotics are sold as dried powder and reconstituted just before use! If possible adjust stock solution pH to stability optimum and choose suitable storage temperature! Due to enhanced solubility Ampicillin-Na is used for cell culture instead of Ampicillin. Optimal efficacy and stability for Ampicillin is at pH=4-5! If you try to adjust Na-Ampicillin solution to pH=5 you will unfortunately and probably get precipitation. Literature recommends pH=6.8 for enhanced stability of Na-Ampicillin solutions! Especially if you use demineralizised water from a ion exchanger, which sometimes has pH=9-10 adjusting the pH can be important! Preferred storage temperature is -70°C!
See also here: Effects of Storage Temperature and pH on the Stability of Eleven P-Lactam Antibiotics in MIC Trays. JOURNAL OF CLINICAL MICROBIOLOGY, Mar. 1985, p. 366-370.
And even more: Interestingly also undissolved crystal Na-Ampicillin seems to degradate at 4°C under dark storage conditions!! In our lab old Na-Ampicillin from 4°C was dissolved in pH=9 water and for horrible onr month cloning, transfection and ligations failed completely. We bought new and problems where gone. Please save yourself from this fate!!
I also observed impaired activity of Kanamycin after several months at -20°C!
50 mg/ml is suitable concentration for stocks. Add 200 µl to 100 ml LB-Agar/LB-Medium (100 µg/ml). 200 µg/ml for Ampicillin works also fine.