تويت
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 Gramnegative 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. Three of the more common selective and differential media
are described below and will be used in the laboratory exercise.
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.50
EOSIN METHYLENE BLUE AGAR (EMB agar)
Eosin methylene blue agar is a differential medium used for the detection
and isolation of Gram-negative intestinal pathogens. A combination of eosin and
methylene blue is used as an indicator and allows differentiation between
organisms that ferment lactose and those that do not. Saccharose is also included
in the medium because certain members of the Enterobacteria or coliform group
ferment saccharose more readily than they ferment lactose. In addition, methylene
blue acts as an inhibitor to Gram-positive organisms.
Colonies of E. coli normally have a dark center and a greenish metallic
sheen, whereas the pinkish colonies of Enterobacter aerogenes are usually mucoid
and much larger than colonies of E. coli. Other organisms, such as Salmonella
(one of the causative agents of food poisoning), do not ferment lactose or
saccharose and produce colonies that are noncolored.
MacCONKEY'S AGAR
MacConkey's agar is a differential plating medium used in the detection
and isolation of all types of dysentery, typhoid and paratyphoid organisms. It is
generally used for differentiating strains of Salmonella typhosa from members of
the coliform group; however, the medium supports the growth of all Salmonella
and Shigella strains and gives good differentiation between these enteric
pathogens and the coliform group. When grown on MacConkeyís medium,
colonies of coliform bacteria are brick-red in color and are surrounded by a zone
of precipitated bile. These reactions are due to the acid produced by the
fermentation of lactose. The acid end-products act on bile salts, and neutral red is
absorbed by the precipitated salts. Dysentery, typhoid and paratyphoid bacilli do
not ferment lactose but give an alkaline reaction when grown on the medium.
Colonies of these organisms are noncolored and transparent. The growth of Grampositive organisms is inhibited because of the crystal violet and bile salts in the
medium.
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 Gramnegative 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. Three of the more common selective and differential media
are described below and will be used in the laboratory exercise.
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.50
EOSIN METHYLENE BLUE AGAR (EMB agar)
Eosin methylene blue agar is a differential medium used for the detection
and isolation of Gram-negative intestinal pathogens. A combination of eosin and
methylene blue is used as an indicator and allows differentiation between
organisms that ferment lactose and those that do not. Saccharose is also included
in the medium because certain members of the Enterobacteria or coliform group
ferment saccharose more readily than they ferment lactose. In addition, methylene
blue acts as an inhibitor to Gram-positive organisms.
Colonies of E. coli normally have a dark center and a greenish metallic
sheen, whereas the pinkish colonies of Enterobacter aerogenes are usually mucoid
and much larger than colonies of E. coli. Other organisms, such as Salmonella
(one of the causative agents of food poisoning), do not ferment lactose or
saccharose and produce colonies that are noncolored.
MacCONKEY'S AGAR
MacConkey's agar is a differential plating medium used in the detection
and isolation of all types of dysentery, typhoid and paratyphoid organisms. It is
generally used for differentiating strains of Salmonella typhosa from members of
the coliform group; however, the medium supports the growth of all Salmonella
and Shigella strains and gives good differentiation between these enteric
pathogens and the coliform group. When grown on MacConkeyís medium,
colonies of coliform bacteria are brick-red in color and are surrounded by a zone
of precipitated bile. These reactions are due to the acid produced by the
fermentation of lactose. The acid end-products act on bile salts, and neutral red is
absorbed by the precipitated salts. Dysentery, typhoid and paratyphoid bacilli do
not ferment lactose but give an alkaline reaction when grown on the medium.
Colonies of these organisms are noncolored and transparent. The growth of Grampositive organisms is inhibited because of the crystal violet and bile salts in the
medium.