Selective, Differential and Enriched Media

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Selective, Differential and Enriched Media

• (PEA, EMB, Mannitol Salt, Blood Agar)
• Exp. 15

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Selective Media

• Media that suppresses the growth of unwanted
  microbes and encourages the growth of desirable
  microbes with the use of selective agents.
• A selective agent can be antibiotics, chemicals
  (NaCl) or dyes (MB, CV).
• Used for isolation of pathogenic staphlococci

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Differential Media

• Media designed to distinguish between species or
  groups of microbes
• Includes a substance that is altered or utilized
  by some organisms but not others
• Causing a visible change in the colony or the
  media

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Enriched Media

• Media has been supplemented with highly
  nutritious materials for cultivating fastidious
  organisms
• Some materials include
• Blood (Sheep)
• Serum
• Yeast extracts

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Mannitol Salt Agar Selective Differential

• Contains high concentrations of NaCl (7.5) which
  inhibits growth of most bacteria except
  staphylococci
• Contains mannitol (carb) fermentation produces an
  acid
• Contains pH indicator (phenol red) which turns
  yellow in the presence of the acid
• Thus mannitol-fermenting staphs will have a
  yellow zone around their growth

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RESULTS

• Staph--small colonies surrounded by yellow zones
• Fermentation of mannitol produces an acid as end
  product

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Blood AgarDifferential

• Contains 5 defibrinated blood
• Used for the detection and isolation of gram (-)
  intestinal organisms (coliforms) mainly E. coli
  and to a lesser extent Enterobacter aerogenes
• Used to classify streptococci into three groups
  depending on their hemolytic reaction

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3 Streptococci Hemolytic Reactions

• 1. a-hemolytic incomplete digestion of the
  hemoglobin
• a-hemolysis reduces hemoglobin (red)
• Forms a greenish zone around colony
• 2. ß-hemolytic complete digestion of the
  hemoglobin
• ß-hemolysis lyses the surrounding blood cells
  
• Causes a clear zone around colony
• Indicative virulent and pathogen strep
• 3. ?-hemolytic no digestion of the hemoglobin
• ?-hemolysis (non-hemolytic) no reaction
• Considered non-pathogenic no lysis of blood
  cells

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EMB Methylene Blue AgarSelective Differential

• Contains dyes eosin and methylene blue
• Contains lactose
• Suppresses growth of gram ()
• Differentiates between gram (-) that can ferment
  lactose producing acids like enterobacter and
  those that cannot like salmonella

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RESULTS

• Those with high acid production the dye
  precipitates and forms a green sheen
• E. coli
• Lactose fermenters form large, mucoid, deep
  purple/pinkish colonies with dark centers
• Enterobacter
• Nonfermenterscolorless
• take on color of medium

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PEA Phenyl Ethyl Alcohol Selective

• chemical suppression
• of gram (-)
• encourages growth
• of gram ()

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Procedure

• Use needle and SINGLE line streak
• Inoculate 1 plate PEA
• Inoculate 1 plate EMB
• Inoculate 1 plate Mannitol Salt
• Inoculate 1 plate Blood Agar then use loop to
  stab 3-4 times at 45 degree angle

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Fill InAddBloodAgar MannitolSalt