Columbia CNA w/ 5% Sheep Blood Agar
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Why is this test performed?
Used as a undefined, differential, selective medium that allows isolation of Gram positive cocci. It inhibits growth of Gram negative.
SELECTIVE: Colistin & Nalidixic Acid will not kill Streptococcus or Staphococcus organisms.
DIFFERENTIAL: Hemolysis of RBC.
How is the agar inoculated?
Plate is divided into 4 sections. Spot inoculate each section with test organism.
What special ingredients are added to agar to perform the test?
Colistin & Nalidixid Acid & Sheep Blood.
How can we tell the diff btwn pos & neg results? What do they tell us about the bacteria used?
POSITIVE: If there's growth, then it is Gram + cocci. Not inhibited by colistin & nalidixic acid.
- Alpha hemolysis (α): Partially hemolyzes RBC, produces a green discoloration of the medium.
- Beta hemolysis (β): Completely hemolyzes RBC, producing a clear zone surrounding the colony.
- Gamma hemolysis (γ): Doen NOT hemolyze RBC, there is no change in the medium.
NEGATIVE: If no growth, then it's Gram -. (Inhibited by colistin & nalidixic acid.)
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