Columbia CNA Agar

Selective Agar medium used for the isolation of Gram positive staphylococci and streptococci from clinical specimens. Eliner et al. added colistin nalidixic acid to Columbia Agar to suppress the growth of gram negative bacteria. The addition of 5-10% sheep red blood cells to the base demonstrates hemolytic patterns of the staphylococci and streptococci.

Formula per liter*:Casein Digest Peptone: 5.0g; Corn Starch: 1.0g; Peptic Digest of Animal Tissue: 8.0g; Sodium Chloride: 5.0g; Yeast Enriched Peptone: 10.0g; Colistine: 0.015g; Nalidixic Acid: 0.010g; Agar: 14.0g. *Grams per liter may be adjusted or formula supplemented to obtain desired performance.

Preparation: Mix 44 grams of the medium in a liter of purified water until evenly dispersed. Heat with repeated stirring and boil for one minute to dissolve completely. Distribute and autoclave at 121°C for 15 minutes. Prepare 5 to 10% blood agar by adding the appropriate volume of sterile defibrinated blood to melted sterile agar medium, cooled to 45-50°C. Dispense into petri dishes or tubes.

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