Microbiology lab 4 (lab 4 selective and differential media)

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Microbiology lab 4 (lab 4 selective and differential media)
2013-10-28 20:32:07
Microbiology lab selective differential media

4. Microbiology lab (lab 4 selective and differential media)
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  1. mannitol salt agar
    • -supports
    • only gram +

    •             - isolates staphylococcus bacteria
    • from other types of bacteria.


    •             This is how bacillus megaterium
    • looks like in the MSA:



    •             This is how Staphylococcus aureus looks
    • like in the MSA:





    Selective feature: high amount of salts


    Differential: mannitol and phenol red
  2. vogel johnson agar
    • -differentiates
    • between pathogenic strains of staph. Aureus.


                Selective: glycine


    • Differential: Mannitol and potassium tellurite
    • (pathogenic strains of staph. Aureus have the enzyme coagulase which acts on
    • potassium tellurite, which produces metal tellurite which gives a black
    • deposit.


    • -If the plate produces yellow and black deposits,
    • then the strain of staph aureus is pathogenic.
  3. MacConkey agar
    • Selective:
    • Bile alts and crystal violet dye


    •             Differential: lactose and neutral
    • red (which turns pink in acidic conditions)


                Differentiates between:


    • Coliforms (non-pathogenic intestinal
    • and stomach bacteria) and pathogenic
    • enteric bacteria (are capable of living in the intestine and they are pathogenic)

    •                         -Coliforms can only
    • ferment lactose.
  4. eosin methylene blue agar (EMB)
    • Selective: eosin Y (which inhibits growth of gram +
    • bacteria) and methylene blue


                Differential: lactose and sucrose


    • If bacterium produces slightly acidic conditions in
    • this plate, that part of the plate will turn purple (ex. Salmonella)


    • If bacterium produces very acidic conditions, that
    • part of the plate will turn metallic green (ex. E Coli)
  5. brilliant green agar
    -differentiates salmonella from fecal specimen


    • Specific: Brilliant green dye (inhibits all gram +
    • and most gram -)


    Differential: lactose and phenol red and sucrose


    • -phenol red àin the absence of acid is
    • red, in acidic conditions is yellow


    -E Coli produce yellow color


                -Salmonella produce pink color
  6. he agar
    • -supports
    • growth of pathogenic enteric bacteria.

    •             - differentiates b/t shigella and salmonella
    • (both of which are pathogenic)


    •             Selective: bile salts (inhibits gram
    • +)


    •             Diff. 1: lactose and bromothymol
    • blue

                -E Coli à yellow



    • Diff. 2: another differential feature is à ferric ammonium citrate and
    • sodium thiosulfate (react with H2S (which is produces by pathogenic
    • enterobacteria) to produce a black deposit
  7. blood agar
    • -Only
    • differential

    •             -supports growth of only pathogenic
    • bacteria

                -contains 5-10% sheep blood

                -RBC à contains heme and globin


    • a. β hemolysis- indicates a very pathogenic
    • bacterium. This bacterium consumes both globin and heme. Produces a clear zone
    • of inhibition around it


    • b. α hemolysis- bacteria only consumes heme protein,
    • not globin. Greenish color. Moderately pathogenic.


    • c. γ (gamma) hemolysis- least
    • pathogenic. Silverish tinge.
  8. When using selective and differential media, why is it important to use TSA as well?
    as a positive control