1. Variants arise readily in strains of Penicillium notatum. Certain of these produce large amounts of penicillin; others very little. Care in the selection and maintenance of strains is required for optimal production of the active substance.

  2. The amount of penicillin produced varies with the medium and the method of cultivation employed.

  3. Many methods of assaying penicillin exist. The one adopted can be a matter of personal choice provided a standard of known value (in Oxford or other units) be included in every test and the value given in reference to this standard.

  4. Penicillin is highly unstable under most conditions, but when dried and sealed in vacuo it retains its potency for comparatively long periods of time under varied conditions of temperature.

  5. As shown by Fleming (1) in his original article, penicillin is highly active against the gram-positive bacteria and far less so against the gram-negative organisms However, the more purified preparations have a considerable activity against gram-negative bacilli, particularly Salmonella enteritidis.

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