Abstract
The regularity with which pneumococci become resistant to the action of sulfapyridine under experimental conditions in vitro and in vivo 1-4 and the occurrence of such strains in infections in man, have led to the development of a method for their recognition through the use of blood agar plates, which is described below. The results of its application to stock laboratory cultures, to specimens obtained from patients who responded poorly to treatment with sulfapyridine, and to specimens from pneumonia patients submitted for routine bacteriological examination indicate that this test is suitable for use in connection with the control of chemotherapy.
Method. Specimens from pneumonia patients, or subcultures from such specimens, are inoculated on 3 blood agar plates (citrated horse blood, 5% in nutrient agar) to 2 of which sulfapyridine is added sufficient to make concentrations of 5 mg % and 10 mg %. The third blood agar plate, without sulfapyridine, serves as a control. Equal sized loopfuls of material are used in order to make the inocula as nearly as possible the same on all 3 plates. The growth on the plates is compared after 24 hours in the incubator, and pneu-mococcus colonies on the control plate without sulfapyridine are identified as to type by capsule swelling reaction and as to bile solubility by spraying dried bile over the colony. 5 The susceptible pneumococcus on the plates containing sulfapyridine produces some partial (alpha type) hemolysis where the inoculum is heaviest, but separate colonies are rarely visible and of minute size. Stained preparations from hemolyzed areas show aberrant bacterial forms, and subcultures sometimes show viable pneumococci. Pneumococcus growth on the plates containing sulfapyridine is seldom obscured by overgrowth of other organisms as these are also markedly inhibited.
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