Introduction
 In addition to general-purpose media, which allow the growth of
most types of bacteria, microbiologists use specialized media to
identify and/or isolate specific groups of bacteria.
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 Gramnegative 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 does not necessarily inhibit bacterial
growth, but instead makes the bacteria look
different.
Differential media works best with closely related organisms,
and the differential agent is what causes the bacteria look
different.
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.
Some media are both selective and differential, that is,
they are able to select against the growth of certain
organisms while the organisms that do grow may
exhibit some differential growth characteristics.
Three of the more common selective and differential media are
described below and will be used in the laboratory exercise.
Mannitol Salt Agar
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.
Ferment mannitol (differential)
Phenol red = pH indicator
 Selective: The high salt concentration inhibits the growth
of most bacteria other than staphylococci.
(Note: The high salt content does not kill Gram negative
bacteria, it just inhibits growth!)
 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.
 Other Staphylococcus don’t ferment mannitol don’t produce a
color change from the normal red-pink color of the medium
 The growth of other types of bacteria is generally inhibited.
 S. aureus = salt tolerant; ferments mannitol (yellow)
 S. epidermidis = salt tolerant; does NOT ferment (red)
 M. luteus = not salt tolerant; does NOT grow on mannitol
Levine's Eosin-methylene Blue Agar
 (EMB) is primarily a differential medium. However, it does
inhibit the growth of some Gram positive bacteria.
 EMB is used to differentiate between enteric lactose
fermenters (coliforms) and non-lactose fermenters as well as
specifically identifying E. coli.
 Eosin Methylene Blue agar contains peptone, lactose, and the
dyes eosin Y and methylene blue. The sugars provide
fermentable substrates to encourage growth of fecal coliforms.
The dyes inhibit growth of Gram-positive organisms and,
under acidic conditions, also produce a dark purple.
The eosin and methylene blue dyes cause lactose
fermenters to have pink colonies. E. coli incorporates
so much of the dye that the dyes precipitate in the
cells and give the colonies a metallic green sheen.
 Non-lactose fermenters will remain colorless or take on the
color of the medium such as Salmonella (one of the causative
agents of food poisoning).
 Note: This media is used to confirm the presence of E. coli in
water samples contaminated with sewage or fecal material.
 You will use this agar again with in the water sampling
experiment to differentiate between E. coli and Enterobacter.
 The dark colonies produced on EMB agar is a result of the
acid produced during lactose fermentation precipitating the
dyes in the media.
MacConkey agar
 Used to isolate Gram negative enterics (coliforms)
 Selective and differential medium.
 Selective - Gram positive bacteria are inhibited by the
presence of bile salts and crystal violet inhibitors in the
medium. Most of gram negative bacteria will grow.
 Differentiate- Between Gram negative bacteria by their ability
to ferment lactose.
 Coliform bacilli produce acid from lactose fermentation
causing the colonies to turn red from the pH indicator.
E. coli produces even greater quantities of acid causing
the surrounding medium and the colonies to turn red.
 Non-coliform bacilli do not
ferment lactose, and appear
uncolored or transparent on
Mac.
 Note: The Gram positive
bacteria do not die on this
media, their growth is just
inhibited.
END OF LECTURE