1. The action of penicillin on staphylococci has been studied in vitro by photoelectric turbidimetric methods.

  2. Penicillin is bactericidal for the staphylococcus and causes lysis of the organism.

  3. There is a close correlation between the concentration of penicillin and its activity.

  4. The number of organisms in the initial inoculum appears to affect the activity of penicillin but the constituents of the culture medium do not.

  5. More than one hundred-fold variations in the sensitivity of strains of staphylococci to penicillin have been demonstrated.

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