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).