Columbia colistin-nalidix agar is a modification of the Columbia Agar Base with the selective antimicrobial agents Colistin Sulfate and Nalidixic Acid added.
Organisms cultured
The agar inhibit the growth of Enterobacteriaceae and Pseudomonas while they allow the growth of yeast, staphylococci, streptococci and pneumococci. Some Gram-negative microorganisms, like Gardnerella vaginalis can grow in it also.
Mechanism
- Colistin breaks the cell membrane of Gram-negative microorganisms, especially Pseudomonas species.
- 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.
Composition
| Composition | Amount (g/l) |
|---|---|
| Peptone Mixture | 20.00 |
| Nalidixic Acid | 0.015 |
| Sodium Chloride | 5.00 |
| Colistin Sulfate | 0.01 |
| Beef Extract | 3.00 |
| Bacteriological Agar | 15.00 |
| Corn Starch | 1.00 |
| Final pH 7.3 ± 0.2 at 25ºC |
