The Use of Sodium Azide (NaN 3 ) and Crystal Violet in a Selective Medium for Streptococci and Erysipelothrix rhusiopathiae

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Abstract
A conc. of 1-2,000 Na azide in 5% blood agar having a pH of 6.8 was found to be most effective in inhibiting organisms other than the streptococci. Na azide in a medium at pH 6.8 is a more effective inhibiting agent than when used in a medium at pH 7.4. A conc. of 1-500,000 crystal violet appears to be most effective in inhibiting organisms not inhibited by Na azide. Pathogenic streptococci grew well in 1-500,000 crystal violet; Staphylococcus aureus did not grow in this conc. of the dye. A combination of 1-2,000 Na azide and 1-500,000 crystal violet in 5% blood agar having a pH of 6.8 inhibited almost all organisms except the streptococci. Diplococcus pneu-moniae, Erysipelothrix rhusiopathiae and the micrococci were not effectively inhibited on the combination medium. Hemolytic properties of the streptococci grown on Na azide blood agar are somewhat different from those observed on ordinary blood agar. E. rhusiopathiae is not inhibited by concs. of crystal violet and Na azide which inhibit all other organisms studied including the streptococci. A blood agar medium having a pH of 6.8 and containing 1-1,000 Na azide, 1-100,000 crystal violet and 5% blood is recommended for the selective isolation of E. rhusiopathiae from contaminated material.

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