Which of the following would not be found in Columbia CNA with 5% sheep blood agar plates?
A Casein
B Agar
C Sodium chloride
D Phenylethyl alcohol
E Nalidixic acid
The correct answer and explanation is:
The correct answer is E) Nalidixic acid.
Columbia CNA with 5% sheep blood agar is a selective and differential medium commonly used in microbiological laboratories to grow Gram-positive organisms while inhibiting Gram-negative bacteria. It contains several key components to serve these functions.
- A) Casein: Casein is a protein found in milk and is often included in media to provide essential nitrogen and carbon sources for bacterial growth. It can support the growth of a wide variety of organisms, including those on Columbia CNA.
- B) Agar: Agar is a gelling agent that solidifies the medium, allowing bacteria to grow on a stable surface. It is a fundamental component of most solid microbiological media, including Columbia CNA.
- C) Sodium chloride: Sodium chloride is included to maintain the osmotic balance of the medium. This helps prevent cells from bursting or shrinking due to osmotic pressure, allowing for normal bacterial growth.
- D) Phenylethyl alcohol (PEA): Phenylethyl alcohol is included in Columbia CNA to selectively inhibit the growth of Gram-negative bacteria. It does this by interfering with the permeability of the outer membrane of Gram-negative organisms. This allows for selective growth of Gram-positive bacteria like staphylococci and streptococci.
- E) Nalidixic acid: Nalidixic acid is an antibiotic that targets bacterial DNA replication. While it is used in some selective media, it is not a component of Columbia CNA with 5% sheep blood agar. This medium relies on phenylethyl alcohol for selective inhibition, not nalidixic acid.
In summary, nalidixic acid is not a component of Columbia CNA with 5% sheep blood agar because the medium uses phenylethyl alcohol as its selective agent, and nalidixic acid would not typically be included in this type of medium.