Types of Agar

MacConKey Agar

• designed to grow Gram-negative bacteria and stain them for lactose fermentation.

• contains bile salts (to inhibit most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus),

• crystal violet dye (which also inhibits certain Gram-positive bacteria),

• neutral red dye (which stains microbes fermenting lactose), lactose and peptone

MacConkey Agar

Organism Growth Color

Eschericia coli + red

Salmonella typhi + colorless

Staphylococcus faecalis

-

MacConkey Agar

Mannitol Salt

• contains high concentration of salt (7.5%) • Selects for organisms of the genus Staphylococcus,

since they can tolerate high saline levels. • contains the sugar mannitol and the pH indicator

phenol red. If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow.

• Most pathogenic staphylococci, such as Staphylococcus aureus, will ferment mannitol. Most non-pathogenic staphylococci will not ferment mannitol.

Mannitol Salt Agar

EMB Agar

• selective stain for Gram-negative bacteria.• inhibits the growth of Gram-positive bacteria

and provides a color indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).

• Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.

EMB Agar

• On EMB if E.coli is grown it will give a distinctive metallic green sheen

• It is one of the few bacteria that grow on EMB this way

EMB Agar

Blood Agar

• Contains nutrients and 5% sheep blood. It is useful for determining the hemolytic capabilities of an organism.

• Some bacteria produce enzymes that lyse red blood cells and degrade hemoglobin; these are called hemolysins.

• Bacteria can produce different types of hemolysins.

Hemolysis on Blood agar

Blood Agar

• Beta-hemolysin breaks down the red blood cells and hemoglobin completely. This leaves a clear zone around the bacterial growth. Such results are referred to as β-hemolysis (beta hemolysis).

Blood Agar

• Alpha-hemolysin partially breaks down the red blood cells and leaves a greenish color behind. This is referred to as α-hemolysis (alpha hemolysis). The greenish color is caused by the presence of biliverdin, which is a by-product of the breakdown of hemoglobin.

Blood Agar

• If the organism does not produce hemolysins and does not break down the blood cells, no clearing will occur. This is called γ-hemolysis (gamma hemolysis).