Stability of minocycline, doxycycline, and tetracycline stored in agar plates and microdilution trays


When the agar dilution method is used for performing antimicrobic susceptibility tests, Mueller-Hinton agar plates may be prepared with varying concentrations of the drugs to be studied. With most antimicrobics, the plates can be prepared in advance and stored in the refrigerator for as long as four weeks. The present study documents the instability of minocycline when stored under refrigeration in agar plates. Doxycycline and tetracycline also lost activity but at a slower rate. Degradative changes occurred most rapidly at the lower concentrations; at 16 μg/ml or greater, the drugs were relatively stable. When diluted in Mueller-Hinton broth and stored at −60°C in microdilution trays, the three tetracyclines could be held up to six weeks with no loss of bioactivity. Concentrated stock solutions were kept at −60°C for six months with no loss of potency.

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Literature Cited

  1. 1.

    Barringer, W. C., Schultz, W., Sieger, G. M., Nash, R. A. 1974. Minocycline hydrochloride and its relationship to other tetracycline antibiotics. American Journal of Pharmacy146:179–191.

    CAS  Google Scholar 

  2. 2.

    Barry, A. L., Effinger, L. J., Badal, R. E. 1976. Short-term storage of six penicillins and cephalothin in microdilution trays for antimicrobial susceptibility tests. Antimicrobial Agents and Chemotherapy10:83–88.

    PubMed  CAS  Google Scholar 

  3. 3.

    Barry, A. L., Jones, R. N., Gavan, T. L. 1978. Evaluation of the Micro-Media System for quantitative antimicrobial drug susceptibility testing: A collaborative study. Antimicrobial Agents and Chemotherapy13:61–69.

    PubMed  CAS  Google Scholar 

  4. 4.

    Bennett, J. V., Brodie, J. L., Benner, E. J., Kirby, W. M. M. 1966. Simplified, accurate method for antibiotic assay of clinical specimens. Applied Microbiology14:170–177.

    PubMed  CAS  Google Scholar 

  5. 5.

    Reller, L. B., Schoenknecht, F. D., Kenny, M. A., Sherris J. C. 1974. Antibiotic susceptibility testing ofPseudomonas aeruginosa: Selection of a control strain and criteria for magnesium and calcium content of media. Journal of Infectious Diseases130:454–463.

    PubMed  CAS  Google Scholar 

  6. 6.

    Ryan, K. J., Needham, G. M., Dunsmoor, C. L., Sherris, J. C. 1970. Stability of antibiotics and chemotherapeutics in agar plates. Applied Microbiology20:447–451.

    PubMed  CAS  Google Scholar 

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Correspondence to A. L. Barry.

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Barry, A.L., Badal, R.E. Stability of minocycline, doxycycline, and tetracycline stored in agar plates and microdilution trays. Current Microbiology 1, 33–36 (1978).

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  • Tetracycline
  • Tray
  • Doxycycline
  • Minocycline
  • Agar Dilution Method