Columbia CNA Agar Base is a modification of the Columbia Agar Base with the selective antimicrobial agents colistin sulfate and nalidixic acid added (CNA). These agents inhibit the growth of Enterobacteriaceae and Pseudomonas while they allow the growth of yeast, staphylococci, streptococci and pneumococci. Colistin breaks the cell membrane of Gram-negative microorganisms, especially Pseudomonas species. The nalidixic acid blocks the DNA replication of susceptible bacteria and acts against many Gram-negative bacteria. The growth of most anaerobic bacteria is promoted by growth nutrients and stimulants such as nitrogen, vitamins, minerals and amino acids contained in the Peptone mixture and beef extract. Corn starch increases growth of Neisseria spp., and enhances the hemolytic reactions of some streptococci. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent. Blood is an additional source providing the microorganisms with growth factors and is the basis for determining haemolytic reactions. Hemolytic patterns may vary according to the blood or base medium types used. For example, defibrinated sheep blood allows the recovery of Thermophilus species and gives best results for group A streptococci. Some Gram-negative microorganisms, like Gardnerella vaginalis, and some species of bacteria can grow in Columbia CNA Agar with added blood.
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