[Microbiology] Columbia CNA With 5% Sheep Blood Agar

Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Columbia CNA With 5% Sheep Blood Agar

Purpose

Columbia CNA with 5% Sheep Blood Agar is used to isolate and differentiate staphylococci, streptococci, and enterococci, primarily from clinical specimens.

Principle

Columbia CNA with 5% Sheep Blood Agar is an undefined, differential, and selective medium that allows growth of Gram-positive organisms (especially staphylococci, streptococci, and enterococci)

and stops or inhibits growth of most Gram-negative organisms (Figure 2-9). Casein, digest of animal tissue, beef extract, yeast extract, corn starch, and sheep blood provide a range of carbon and energy sources to support a wide variety of organisms. In addition, sheep blood supplies the X factor (heme) and yeast extract provides B-vitamins. The antibiotics colistin and nalidixic acid (CNA) act as selective agents against Gram-negative organisms by affecting membrane integrity and interfering with DNA replication, respectively. They are particularly effective against Klebsiella, Proteus, and Pseudomonas species. Further, sheep blood makes possible differentiation of Gram-positive organisms based on hemolytic reaction (Figures 7-10 to 7-12).

2-9 COLUMBIA CNA WITH 5% SHEEP BLOOD AGAR This

plate was inoculated with four organisms—two Gram-positive

cocci, and two Gram-negative rods. Only the Gram-positive

organisms (left and top quadrants) grow well on the Columbia

CNA agar. The two Gram-negatives either didn’t grow (bottom)

or grew poorly (right). Further, the top Gram-positive is -

hemolytic, whereas the one on the left is nonhemolytic. 

Suggested Reading

- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011