Selective & Differential Media

Selective & Differential Media

Selective Medium: culture medium that allows the growth of certain types of organisms, while inhibiting the growth of other organisms

-	dyes in the medium (e.g.: methylene blue in EMB & crystal violet in MacConkey's) or high salt concentration in the medium (e.g.: 7% salt in MSA) inhibit the growth of unwanted microorganisms

Differential Medium: culture medium that allows one to distinguish between or among different microorganisms based on a difference in colony appearance (color, shape, or growth pattern) on the medium.

-	dyes in the medium (e.g.: eosin/methylene blue in EMB) or pH indicators change the color of the medium as sugars in the medium (e.g.: lactose in EMB & MacConkey's and mannitol in MSA) are fermented to produce acid products

EMB (Eosin Methylene Blue) Agar

-	selective for: gram-negative bacteria

growth of gram-positive bacteria (e.g.: Staphylococcus aureus in the image below) is inhibited by the eosin & methylene blue dyes in the media

-	differential for: lactose fermentation

gram-negative Enterobacteria Escherichia coli and Enterobacter aerogenes ferment lactose

E. coli produces colonies with a characteristic green metallic sheen on EMB agar

E. aerogenes produces pink colonies often with a central dark purple dot (fish eye colonies) on EMB agar

gram-negative bacteria Proteus vulgaris and Salmonella typhimurium grow on EMB agar, but do not ferment lactose

MacConkey's Agar

-	selective for: gram-negative bacteria

growth of gram-positive bacteria (e.g.: Staphylococcus aureus in the image below) is inhibited by the crystal violet dye and bile salts in the media

-	differential for: lactose fermentation

neutral red pH indicator turns red in the presence of acid by-products of lactose fermentation

gram-negative Enterobacteria Escherichia coli and Enterobacter aerogenes ferment lactose

E. coli produces pink to red colonies often with a reddish bile precipitate surrounding colonies on MacConkey's agar

E. aerogenes produces pink to red mucoid colonies on MacConkey's agar

gram-negative bacteria Proteus vulgaris and Salmonella typhimurium grow on MacConkey's agar, but do not ferment lactose (media appears yellow to light pink in color & colonies are colorless; swarming of Proteus is inhibited)

MSA (Mannitol Salt Agar)

-	selective for: gram-positive Staphylococci bacteria

7% salt in the medium inhibits the growth of most gram-positive and gram-negative bacteria

-	differential for: mannitol fermentation

phenol red pH indicator turns yellow in the presence of acid by-products of mannitol fermentation

Staphylococcus aureus ferments mannitol

S. aureus changes the color of the medium from pink to yellow due to acid by-products of mannitol fermentation

Staphylococcus epidermidis grows on MSA, but does not ferment mannitol (media remains light pink in color & colonies are colorless

Hemolysis with Blood Agar

-	agar contains 5% sheep's blood

-	differential for: hemolysis...particularly in streptococci

-	based on the ability to break down hemoglobin or red blood cells, 3 groups of microorganisms can be described

alpha-hemolysis: a green to light-brown halo is seen around the colonies; bacteria partially break down hemoglobin leaving a green pigment (biliverdin)

beta-hemolysis: a clearing is seen around the colonies; bacteria produce a "beta-hemolysin" (streptolysin O or S), which lyses red blood cells in the medium

gamma-hemolysis (no hemolysis): no hemolysis is observed; bacteria do not produce a hemolysin