Selective and Differential Media
Selective and differential media are used to isolate or identify particular organisms.
Selective media allow certain types of organisms to grow, and inhibit the growth of other organisms. The selectivity is accomplished in several ways. For example, organisms that can utilize a given sugar are easily screened by making that sugar the only carbon source in the medium. On the other hand, selective inhibition of some types of microorganisms can be achieved by adding dyes, antibiotics, salts or specific inhibitors which affect the metabolism or enzyme systems of the organisms.
For example, media containing potassium tellurite, sodium azide or thallium acetate (at concentrations of 0.1 - 0.5 g/l) will inhibit the growth of Gram-negative bacteria. Media supplemented with penicillin (5-50 units/ml) or crystal violet (2 mg/l) will inhibit the growth of Gram-positive bacteria. Tellurite agar, therefore, is used to select for Gram-positive organisms,
and nutrient agar supplemented with penicillin can be used to select for Gram negative organisms.
Differential media are used to differentiate closely related organisms or groups of organisms. Owing to the presence of certain dyes or chemicals in the media, the organisms will produce characteristic changes or growth patterns that are used for identification or differentiation. A variety of selective and differential media are used in medical, diagnostic and water pollution laboratories, and in food and dairy laboratories.
MANNITOL SALT AGAR (MSA)
Mannitol salt agar is a selective medium used for the isolation of pathogenic staphylococci. The medium contains mannitol, a phenol red indicator, and 7.5% sodium chloride. The high salt concentration inhibits the growth of most bacteria other than staphylococci. On MSA, pathogenic Staphylococcus aureus produces small colonies surrounded by yellow zones. The reason for this change in color is that S. aureus ferments the mannitol, producing an acid, which, in turn, changes the indicator from red to yellow. The growth of other types of bacteria is generally inhibited.
MacCONKEY'S AGAR
MacConkey’s agar is a selective & differential plating medium designed to grow Gram Negative bacteria and stain them for lactose fermentation. It 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. The growth of Gram-positive organisms is inhibited because of the crystal violet and bile salts in the medium. Lactose is the differential component of McConkey Agar. The bacteria that can ferment lactose changes the pH of the media to become acidic and the media will turn pink due a pH indicator in the agar. Bacteria that does not ferment lactose will grow on the plate without casing a colour change as no acid was produced.
HEKTOEN-ENTERIC AGAR (HE)
HE agar is a selective & differential agar primarily used to recover Salmonella and Shigella from patient specimens. HE agar contains 3 carbohydrate (lactose, salicin and sucrose) and two indicator dyes (Bromthymol Blue and Acid Fuchsin). The high bile salt concentration inhibits growth of all gram-positive bacteria and retards the growth of many strains of coliforms. Acids may be produced from the carbohydrates, and acid fuchsin reacting with thymol blue produces a yellow colour when the pH is lowered. Sodium thiosulfate is a sulphur source, and H2S gas is detected by ferric ammonium citrate. Rapid lactose fermenters (e.g. E.coli) are moderately inhibited and produce bright orange to salmon pink colonies. Salmonella colonies are blue-green, typically with black centres from H2S gas. Shigella appears greener than Salmonella, with the colour fading to the periphery of the colony. Proteus strains are somewhat inhibited, forming small transparent and more glistening or watery in appearance than species of Salmonella or Shigella.
BLOOD AGAR
Contains mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAP are an enriched, differential media used to isolate fastidious organisms and detect hemolytic activity. β-hemolytic activity will show complete lysis of red blood cells surrounding colony. Examples include Streptococcus haemolyticus. α-hemolysis will only partially lyse hemoglobin and will appear green. An example of this would be Streptococcus viridans. γ-hemolysis (or non-hemolytic) is the term referring to a lack of hemolytic activity.
Selective and differential media are used to isolate or identify particular organisms.
Selective media allow certain types of organisms to grow, and inhibit the growth of other organisms. The selectivity is accomplished in several ways. For example, organisms that can utilize a given sugar are easily screened by making that sugar the only carbon source in the medium. On the other hand, selective inhibition of some types of microorganisms can be achieved by adding dyes, antibiotics, salts or specific inhibitors which affect the metabolism or enzyme systems of the organisms.
For example, media containing potassium tellurite, sodium azide or thallium acetate (at concentrations of 0.1 - 0.5 g/l) will inhibit the growth of Gram-negative bacteria. Media supplemented with penicillin (5-50 units/ml) or crystal violet (2 mg/l) will inhibit the growth of Gram-positive bacteria. Tellurite agar, therefore, is used to select for Gram-positive organisms,
and nutrient agar supplemented with penicillin can be used to select for Gram negative organisms.
Differential media are used to differentiate closely related organisms or groups of organisms. Owing to the presence of certain dyes or chemicals in the media, the organisms will produce characteristic changes or growth patterns that are used for identification or differentiation. A variety of selective and differential media are used in medical, diagnostic and water pollution laboratories, and in food and dairy laboratories.
MANNITOL SALT AGAR (MSA)
Mannitol salt agar is a selective medium used for the isolation of pathogenic staphylococci. The medium contains mannitol, a phenol red indicator, and 7.5% sodium chloride. The high salt concentration inhibits the growth of most bacteria other than staphylococci. On MSA, pathogenic Staphylococcus aureus produces small colonies surrounded by yellow zones. The reason for this change in color is that S. aureus ferments the mannitol, producing an acid, which, in turn, changes the indicator from red to yellow. The growth of other types of bacteria is generally inhibited.
MacCONKEY'S AGAR
MacConkey’s agar is a selective & differential plating medium designed to grow Gram Negative bacteria and stain them for lactose fermentation. It 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. The growth of Gram-positive organisms is inhibited because of the crystal violet and bile salts in the medium. Lactose is the differential component of McConkey Agar. The bacteria that can ferment lactose changes the pH of the media to become acidic and the media will turn pink due a pH indicator in the agar. Bacteria that does not ferment lactose will grow on the plate without casing a colour change as no acid was produced.
HEKTOEN-ENTERIC AGAR (HE)
HE agar is a selective & differential agar primarily used to recover Salmonella and Shigella from patient specimens. HE agar contains 3 carbohydrate (lactose, salicin and sucrose) and two indicator dyes (Bromthymol Blue and Acid Fuchsin). The high bile salt concentration inhibits growth of all gram-positive bacteria and retards the growth of many strains of coliforms. Acids may be produced from the carbohydrates, and acid fuchsin reacting with thymol blue produces a yellow colour when the pH is lowered. Sodium thiosulfate is a sulphur source, and H2S gas is detected by ferric ammonium citrate. Rapid lactose fermenters (e.g. E.coli) are moderately inhibited and produce bright orange to salmon pink colonies. Salmonella colonies are blue-green, typically with black centres from H2S gas. Shigella appears greener than Salmonella, with the colour fading to the periphery of the colony. Proteus strains are somewhat inhibited, forming small transparent and more glistening or watery in appearance than species of Salmonella or Shigella.
BLOOD AGAR
Contains mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAP are an enriched, differential media used to isolate fastidious organisms and detect hemolytic activity. β-hemolytic activity will show complete lysis of red blood cells surrounding colony. Examples include Streptococcus haemolyticus. α-hemolysis will only partially lyse hemoglobin and will appear green. An example of this would be Streptococcus viridans. γ-hemolysis (or non-hemolytic) is the term referring to a lack of hemolytic activity.
Biochemical Test 1 -
Materials:
- 6 different bacteria ( Escherichia coli, Staphylococcus aureus, Entrobacter aeurogenes, Streptococcus faecalis, Proteus vulgaris and Micrococcus luteus ) .
- 3 plates each of Blood agar, MacCokey agar, MSC agar and HE agar.
- Inoculating loops.
- Marker pen.
Procedures:
- Divide each of the agar plates into half using the marker pen.
- Streak each bacteria on half of each four plates.
- Close the lid and inverse the petri dish.
- Incubate at 37°C for 48 hours and store at 4°C after 48 hours.
- Observe the plates and combine the results for all six microorganism on each plate.
Results:
(click to view results)
Blood Agar
