SELECTIVE, DIFFERENTIAL
AND ENRICHED MEDIA
• Selective media: used to select (isolate) specific
groups of bacteria;
• chemical substances in the media inhibit the growth
of one type of bacteria while permitting growth of
another (MSA, EMB, MacConkey)
• Differential media: distinguishes among
morphologically and biochemically related groups of
organisms;
• chemical compounds (following inoculation and
incubation) produce a characteristic change in the
appearance of bacterial growth and/or the medium
surrounding the colonies (MSA, EMB, MacConkey)
• Enriched media: supplemented with highly nutritious
materials, such as blood, serum, or yeast extracts, for
the cultivation of fastidious organisms
Mannitol Salt Agar
(MSA)
• Mannitol salt agar is both
selective and differential
• Selective: It favors
organisms capable of
tolerating high salt
concentrations
(7.5 % NaCl)
• Differential: It
distinguishes bacteria
based on their ability to
ferment mannitol
• Differentiates
Staphylococcus species,
by mannitol fermentation
• (S. aureus ferments,
S. epidermidis does not)
• Phenol red is the pH
indicator
Neutral - Basic pH
red at 7.4 to 8.4
Acidic pH
yellow below 6.8
RESULTS:
• Positive Results:
The development of “yellow halos” around the
bacterial growth means mannitol has been
fermented and acid end products have been
produced (S. aureus)
• Negative Results:
No color change in the medium is a negative
result (S. epidermidis)
• No growth on the medium indicates a Gramorganism (E. coli)
PROCEDURE:
On an MSA Plate:
S. aureus (growth, yellow)
E.coli
S.epidermidis
(No growth)
(growth, red)
Eosin Methylene Blue Agar (EMB)
• Eosin methylene blue agar is a
selective and differential
medium
• Selective: EMB contains
the dyes methylene blue
and eosin which inhibit
Gram + bacteria, thus
favoring growth of Gram –
• Differential: EMB contains
lactose, thus allowing for
the distinction between
lactose fermenters and
nonferments
Typically used for the family
Enterobacteriaceae—
enteric (gut) bacteria (Gramrods)
They may be divided into
those that produce acid from
lactose fermentation
(coliforms) and those that do
not
Coliforms include
Escherichia coli and
Enterobacter aerogenes
Results
• Large amounts of acid from lactose
fermentation cause the dyes to
precipitate on the colony surface,
producing a black center or a “green
metallic sheen” (E. coli)
• Smaller amounts of acid production
result in pink coloration of the
growth (E. aerogenes)
• Nonfermenting enterics do not
produce acid so their colonies
remain colorless or take on the color
of the media (P. vulgaris)
• No growth indicates a Gram +
organism (S.aureus)
•Large amounts of acid from lactose fermentation cause
the dyes to precipitate on the colony surface, producing
a black center or a “green metallic sheen” (E. coli)
PROCEDURE:
On an EMB Plate:
P. vulgaris
S. aureus
(growth,
(no growth)
no color)
E. aerogenes
black colonies or green
metallic sheen
E.coli
black colonies or
green metallic sheen
MacConkey Agar
(MAC)
• A selective and
differential medium
used to isolate
members of the
Enterobacteriaceae
• Contains nutrients,
including lactose, as
well as bile salts,
neutral red and
crystal violet
• Bile salts and crystal
violet inhibit growth of
G+ organisms
(selective)
• Neutral red is a pH
indicator that is
colorless, but yellow
above pH 8 and red at
pH less than 6.8
(differential)
• Acid accumulating from lactose fermentation
turns the colorless neutral red to a red color—
therefore coliforms produce a red “halo” on the
medium (E.coli, E.aerogenes)
• Lactose nonfermenters will grow, but don’t
produce acid. Therefore, the neutral red remains
colorless (P. vulgaris)
• No growth indicates a
Gram + organism
(S.aureus)
Macconkey agar with lactose(left) and non-lactose(right) fermenters
PROCEDURE:
• On a MacConkey Agar Plate:
P.vulgaris (colorless)
E.aerogenes
(Red)
S.aureus (no growth)
E.coli (Red)