Polymixin Nalidixic Acid Agar (Columbia CNA Agar)

Polymixin Nalidixic Acid Blood Agar or Columbia CNA Agar is selective and differential medium used for the isolation and differentiation of gram-positive microorganisms from clinical and nonclinical materials. Ellner et al formulated this medium with the same formula as Columbia Agar Base with the addition of colistin and nalidixic acid to inhibit the growth of gram-negative bacteria and to support the growth of Staphylococci, haemolytic Streptococci and Enterococci when supplemented with 5% blood1.  Sheep blood allows detection of haemolytic reactions and supplies X-factor necessary for the growth of many bacterial species. It should be noted that this medium has relatively high carbohydrate content and, therefore, beta-hemolytic streptococci may produce a greenish hemolytic reaction that may be mistaken for alpha haemolysis. The addition of the antimicrobial agents, colistin (or polymyxin B) and nalidixic acid, renders the medium selective for gram-positive microorganisms2. Colistin and nalidixic acid disrupt the cell membrane of gram-negative organisms, whereas nalidixic acid blocks DNA replication in susceptible gram-negative bacteria3.

Quality Control

The following organisms are routinely used for testing for this medium.
Test OrganismsResults
Escherichia coli ATCC® 25922 Inhibited
Neisseria meningitidis ATCC® 13090 Inhibited
Staphylococcus aureus ATCC® 25923 Growth, beta/gamma
Staphylococcus epidermidis ATCC® 12228 Growth, gamma
Streptococcus pneumoniae ATCC® 6303 Growth, alpha
Streptococcus pyogenes ATCC® 19615 Growth, beta

User Quality Control

Check for signs of contamination and deterioration. Users of commercially prepared media may be required to perform quality control testing with at least one known organism to demonstrate growth or a positive reaction; and at least one organism to demonstrate inhibition or a negative reaction (where applicable).

Ingredients g/L

(Final pH 7.3 +/- 0.2 at 25°C)
Biopeptone 20.0
Tryptic digest of beef heart 3.0
Corn starch 1.0
Sodium chloride 5.0
Colistin sulphate 0.010
Nalidixic acid 0.015
Agar 15.0
Sheep Blood 5%
3 total citations

  1. Ellner et al, 1966, Am. J. Clin. Path., 45:502
  2. Murray P. R., Baron J. H., Pfaller M. A., Jorgensen J. H. and Yolken R. H., (Ed.), 2003, Manual of Clinical Microbiology, 8th Ed., American Society for Microbiology, Washington, D.C.
  3. Estevez, 1984, Lab. Med., 15:258
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