Why are crystal violet agar 7.5 sodium chloride agar considered selective media?

Answer 1

7.5% NaCl inhibits growth of certain bacteria. This type of selective media allows for the growth of Staph. A high concentration of NaCl alters the osmotic balance between the media and the bacterium, NaCl plasmolyzes the bacterium due to the hypertonicity of the medium. The circular shape of Staph allows it to better resist plasmolysis. Crystal Violet is a dye that inhibits the growth of certain Gram-positive bacteria, it does this by binding to the thick peptidoglycan layer preventing cross-linking and thus preventing cell growth.

Answer 2

Crystal violet agar and 7.5% sodium chloride agar are selective media because they contain specific ingredients that inhibit the growth of certain bacteria while allowing the growth of others. Crystal violet inhibits the growth of Gram-positive bacteria, while 7.5% sodium chloride agar creates a high osmotic environment that inhibits the growth of most bacteria except halophiles.