Media types - basic principle

MacConkey medium P. vulgaris is a Gram- bacteria that is not inhibited by crystal violet and forms a colony. It is a lactose non-fermenter and therefore no bile is precipitated and red coloration appears around the colony. E. coli is a Gram- bacteria that is not inhibited by crystal violet and forms a colony. It is a lactose fermenter and produces acids to precipitate bile salts in the media, giving the red coloration to the medium around the colony. E. aerogenes is a Gram- bacteria that is not inhibited by crystal violet and forms a colony. It is a lactose fermenter and produces acids to precipitate bile salts in the media, giving the red coloration to the medium around the colony. S. aureus is a Gram+ bacteria and its growth is inhibited by crystal violet. Therefore no colonies are formed. MacConkey agar is a culture medium that contains crystal violet dye, bile salts, lactose and pH indicator. It is a selective medium because crystal violet inhibits the growth of Gram + bacteria while allowing Gram - bacteria to grow. It is also a differential medium because it differentiates between lactose fermenters and lactose non-fermentors. Lactose fermenters produce acids that precipitate bile salts and the absorption of the red dye gives the medium red color around the colonies. Lactose non-fermenters have no such red coloration around the colonies.

Blood agar medium P. aeruginosa does not cause hemolysis of RBC and are called gamma or non-hemolytic. E. coli shows a greenish-brown zone around the colony that is characteristic of alpha hemolysis or partial hemolysis of RBC. S. agalactiae shows a clear zone around the colony that is characteristic of beta hemolysis or complete hemolysis of RBC. Blood agar is an enriched medium containing 5% sheep blood to encourage the growth of fastidious bacteria that do not grow on simple nutrient medium. It is also a differential medium used to detect bacteria that produce hemolysin to cause hemolysis (alpha, beta) of RBC.

Mannitol salt medium E. aerogenes is not salt tolerant and does not grow on this medium. It is also a mannitol non-fermenter and therefore there is no change in the color of the medium E. coli is not salt tolerant and does not grow on this medium. It is also a mannitol non-fermenter and therefore there is no change in the color of the medium S. aureus is a salt tolerant bacterium that grows on this medium. It is also a mannitol fermenter that produces acid and changes the color of the medium to yellow Mannitol salt agar is a selective culture medium that contains high salt (7.5% NaCl) allowing only salt tolerant bacteria to grow. It is also a differential medium that contains mannitol as the carbohydrate source and phenol red as a pH indicator to detect acid produced by the bacteria. It distinguishes between bacteria that ferment mannitol to produce acid and convert the color of the medium to yellow. Mannitol non-fermenters do not change the color of the medium.