Selective media

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Selective media

• Lab 6

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Types of Bacteriological Media

• Basic nutritive media
• E.g. nutrient agar
• Enriched media
• E.g. blood agar
• Selective media
• Selective for the growth of particular bacteria
• Used in diagnostic bacteriology
• They contain inhibitory substances (e.g.
  antibacterials) that inhibit the growth of
  unwanted bacteria

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Eosin Methylene Blue (EMB)

• Constituents Nutrients, lactose, neutral red,
  eosin and methylene blue.
• Appearance A clear red medium
• A selective stain for Gram-negative bacteria.
• It is a blend of two stains, eosin and methylene
  blue in the ratio of 61.
• A common application of this stain is in the
  preparation of EMB agar, a differential
  microbiological medium, which inhibits the growth
  of Gram-positive bacteria and provides a colour
  indicator distinguishing between organisms that
  ferment lactose (e.g., E. coli) and those that do
  not (e.g., Salmonella, Shigella).
• Lactose fermentation produces acids, which lower
  the pH. This encourages dye absorption by the
  colonies, which are now coloured purple-black.
• Lactose non-fermenters may increase the pH by
  deamination of proteins. This ensures that the
  dye is not absorbed.
• On EMB if E. coli is grown it will give a
  distinctive metallic green sheen

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Eosin Methylene Blue (EMB)
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MacConkey Agar

• Constituents Nutrients, Agar, Bile Salts,
  Lactose, Neutral Red
• Appearance A clear red medium
• Purpose For the detection and counting of
  coliforms and as an aid to the identification of
  other bacteria. It inhibits many non enteric
  bacteria because of the presence of bile salts.
  Bacteria growing on the medium may ferment
  lactose to produce acid which turns the colony
  red/pink. Non lactose-fermenters may digest the
  protein in the medium which turns the colony
  yellow.
• Examples
• Bacteria inhibited include Streptococcus and
  Pasteurella multocida.
• Bacteria which grow include
• Escherichia coli (pink), lactose fermenter (LF),
• Salmonella spp. (yellow), non lactose fermenter
  (NLF)
• Staphylococcus spp small deep pink colonies (LF)

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MacConkey Agar
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Diagnostic Sensitivity Test Agar

• Constituents Nutrients, Agar, Buffers, Added
  adenine, guanine, uracil and xanthine, 7 Blood.
• Appearance An opaque red medium
  indistinguishable from blood agar unless
  labelled.
• Purpose
• For the growth of a similar range of bacteria to
  blood agar.
• To determine the sensitivity of bacterial
  cultures to antimicrobials in standard
  conditions.
• The presence of the added bases prevents folate
  antagonism to trimethoprim and sulphonamides.

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DNAase Agar (DNA)

• Constituents Nutrients, Agar, Deoxyribonucleic
  acid, Sodium chloride
• Appearance A clear medium resembling Nutrient
  Agar.
• Purpose For the detection of deoxyribonuclease
  production by bacteria such as Staphylococci.
  Point inoculations of Staphylococci digest the
  DNA in the medium surrounding them during
  incubation. The digestion is revealed by flooding
  the plate with hydrochloric acid which
  precipitates intact DNA leaving a clear zone
  around colonies which have digested the DNA.
  Pathogenic staphylococci are DNAase positive and
  most non-pathogens are DNAase negative.
• Examples Staphylococcus aureus is DNAase
  positive.
• Staphylococcus epidermidis is DNAase negative.

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DNAase Agar (DNA)
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'Chocolate' or Heated Blood Agar

• Constituents Nutrients, Agar, 7 Blood (either
  Sheep or Horse).
• Appearance An opaque chocolate brown medium
• Purpose For the growth of bacteria which require
  the presence of serum and the nutrients to be
  found in red cells, but which cannot liberate
  these nutrients from the red cells themselves.
  The nutrients available include haemin, altered
  haemoglobin and NAD. Bacteria which show
  satellitism on blood agar grow freely on this
  medium. Many other bacteria grow on 'chocolate'
  agar and produce larger colonies than on blood
  agar.
• Examples Bacteria which grow on 'chocolate' agar
  but not on blood agar include the Haemophilus
  group such as H. parasuis and H. somnus and other
  NAD-requiring bacteria such as Actinobacillus
  pleuropneumoniae and Taylorella equigenitalis.
• They may be distinguished from other bacteria
  only by their colony size and colour. Effects on
  the medium are limited to slight greening in some
  cases and adhesion to the medium.

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Chocolate Agar
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Deoxycholate Citrate Agar (DCA)

• Constituents Nutrients, Agar, Lactose, Sodium
  citrate, Sodium thiosulphate, Ferric citrate,
  Sodium desoxycholate, Neutral red
• Appearance Slightly cloudy or clear pink medium
• Purpose To select Salmonella (and Shigella) from
  pathological material directly or after
  enrichment. Gram positive and most non-enteric
  coliform organisms are inhibited by the sodium
  desoxycholate, thiosulphate and citrate.
  Thiosulphate and iron act as indicators for
  sulphide production which gives a black colour.
  Lactose and the indicators allow the
  identification of lactose fermenters. This medium
  is slightly less selective than Salmonella
  Shigella Agar.
• Examples Escherichia coli is often inhibited,
  but if it survives, it appears as a pink or red
  colony.
• Faecal streptococci do not grow.
• Salmonella will grow and form colourless
  colonies.
• Proteus and pseudomonas will also grow as
  colourless colonies and may have to be
  distinguished from salmonellae.

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Xylose-Lysine Deoxicholate (XLD)
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Salmonella Shigella Agar (SS)

• Constituents Nutrients, Agar, Lactose, Bile
  salts, Sodium citrate, Sodium thiosulphate,
  Ferric citrate, Brilliant green, Neutral red
• Appearance A clear pink medium.
• Purpose To select Salmonella (and Shigella) from
  pathological material directly or after
  enrichment. Gram positive and most coliform
  organisms such as Escherichia coli are inhibited
  by the brilliant green, bile salts, thiosulphate
  and citrate. Thiosulphate and iron act as
  indicators for sulphide production which gives a
  black colour. Lactose and the indicators allow
  the identification of lactose fermenters.
• Examples
• Escherichia coli is usually inhibited, but if it
  survives, it appears as a pink or red colony.
• Faecal streptococci do not grow.
• Salmonella will grow and form colourless
  colonies.
• Proteus and pseudomonas will also grow as
  colourless colonies and may have to be
  distinguished from salmonellae.

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Brilliant Green Agar (BGA)
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Edwards medium

• Contains crystal violet to inhibit the growth of
  other bacteria than Streptococci, and sheep blood
  to indicate the haemolysis type.
• S. agalactiae is ß haemolytic and S. dysgalactiae
  is a haemolytic. S. uberis is ? (non haemolytic)
  or may be a haemolytic.
• The presence of aesculin allows S. uberis to be
  distinguished from S. dysgalactiae because it
  splits aesculin to give a black colour. S. uberis
  colonies appear black.

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Milk Agar

• Selective for bacteria that produce caseinase
• Proteolytic bacteria will be surrounded by a
  clear zone