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24: Columbia NaladixicAcid Agar (CNA)

Objectives

  • To learn about CNA agar

CNA 

CNA is a selective, differential agar medium used for isolation of gram positive bacteria in a variety of specimen types.  It is used frequently in clinical laboratories.

CNA agent

The selective/ inhibitory agent of CNA is the antibiotic naladixic acid, a quinolone drug similar to Cipro or Levaquin. This medium is basically blood agar, containing 5% sheep’s blood mixed with either TSA base or Columbia agar base. The differentiation of blood agar is due to the ability of many bacteria to hemolyze blood cells, using chemicals called hemolysins..

Hemolytic Patterns

There are 3 categories of hemolytic patterns—alpha, beta, and gamma.

  • BETA hemolysis is the complete breakdown of RBCs, producing a clear yellow zone (the color of the base media without blood added).
  • ALPHA hemolysis occurs when the hemoglobin within the RBCs is converted to methemoglobin, when released by the lysed RBCs. This produces a brown or green zone (light green to dark green) around the colonies.
  • GAMMA hemolysis indicates the lack of hemolytic ability.

The best way to read hemolysis is to hold the plate up against a light source (sun or lights).

 


NOTE: SOME bacteria gram - bacteria may grow on CNA--although not well--particularly if you --particularly if you let cultures sit for more than a couple of days. Usually those species will show as pinpoint colonies. 

 

    

alpha hemolysis        

 

          

                                                            beta hemolysis

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