Download Types of Agar

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 Grampositive 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).