Microbiology Lab Exam #3

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Microbiology Lab Exam #3
2014-04-02 14:59:31
Microbiology Lab

Microbiology Lab Exam #3
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  1. Mannitol Salt Agar (MSA)
    Selective = salt inhibits most gram - & selects for gram +

    Results - Positive = growth, Negative = no growth

    Differential = mannitol fermentation causes acid waste release by bacterium resulting in a lower pH around bacteria changing color of agar from red to yellow

    Results - Positive = agar turns yellow around growth, Negative:  agar remains red around growth

    Staphylococcus species will grow & Streptococcus species will not; pathogenic Staphylococcus species (S. aureus) will be positive for mannitol fermentation
  2. Coagulase
    Tests whether bacterium produces the enzyme Coagulase which can coagulate blood plasma

    Results - Positive = visible clotting in tube (S. aureus), Negative = no clotting in tube (S. epidermidis)
  3. Blood Agar
    Enriched = agar has blood added to enrich for blood requiring bacteria

    Differential = presence of blood allows determination of the type of hemolysis utilized by bacterium caused by bacteraial production of exotoxins that lyse blood cells

    Results - Gamma = no hemolysis around colonies, Alpha = darkening in agar around colonies, Beta = complete clear halo around colonies

    Pathogenic bacteria such as Staphylococcus aureus & Streptococcus pyrogenes produce Beta hemolysis
  4. Lancefield Grouping
    Groups Streptococcus species into groups A through R based off serology (group A is most pathogenic)

    Bactracin sensitivity = disk-diffusion test; only Streptococcus pyrogenes is sensitive

    P-disc (Optochin sensitivity) = disk-diffusion test; only Streptococcus pneumonia is sensitive
  5. Bile Esculin Agar
    Selective = bile selects growth of gastrointestinal bacterial

    Differential = esculin utilization turns agar black

    Results - Positive = agar turns black around growth, Negative = agar remains tan around growth

    Lancefield Streptococcus group D & Enterococci should be positive for this test
  6. Salt Broth Test
    Tests whether bacteria can grow in presence of high salt

    Result - Positive = growth, Negative = no growth

    Lancefield Streptococcus group D should be negative & Enterococci should be positive
  7. Catalase
    Tests whether bacterium produces the enzyme catalase which can break down Hydrogen peroxide to water & oxygen

    Results - Positive = bubbling (Staphylococcus, Micrococcus), Negative = no bubbling (Streptococcus & Enterococcus)
  8. Oxidase
    Tests whether bacterium produces the enzyme Oxidase

    Results - Positive = purple color, Negative = clear color

    Nisseria should be the only positive
  9. Eosin Methylene Blue (EMB)
    Selective = dyes inhibit most gram + & selects for gram -

    Results - Positive = growth, Negative = no growth

    Differential = Lactose fermentation causes colonies to appear darker than agar

    Results - Positive = dark colored colonies, Negative = colonies remain normal or agar color

    E. coli colonies turn metallic green due to lactose fermentation
  10. MacConkey
    Same as EMB except poitive colonies turn pink instead of darker than agar color
  11. Durham Fermentation Tubes
    This media will only have one sugar source, either Glucose or Lactose.  Tests determine whether a bacterium can use a specific sugar for fermentation.  Fermentation of the sugar will lower media pH causing media to turn from red to yellow.  Test also looks at whether the bacterium produces gas as a result of fermentation

    Result - A (acid) = media is yellos, AG (acid & gas) = media is yellow and has bubbles in inverted tube, NC (no changed) = media is red & no gas
  12. Triple Sugar Iron Agar (TSA)
    Tests whether the bacteria can utilize glucose and/or lactose for fermentation & also whether the bacterium produces hydrogen sulfide

    • Result - 
    • red slant/red butt = negative for all sugars
    • red slant/yellow butt = positive for glucose
    • yellow slant/yellow butt = positive for glucose & lactose
    • blackening = positive for hydrogen sulfide
  13. Sulfide Indole Motility (SIM)
    Tests whether the bacterium produces hydrogen sulfide, whether the bacterium produces indole, & whether the bacterium is motile

    • Result - 
    • black agar = hydrogen sulfide producer
    • pink-red indicator = indole positive
    • growth away from stab = motility positive
  14. Methyl Red - Voges Proskauer (MR-VP)
    MR tests for the production of acid.  VP tests for the production of 2,3-butylen glycol

    Result - Positive = red, Negative = yellow

    E. coli should be positive for MR & negative for VP.  Enterobacter aerogenes should be positive for VP & negative for MR
  15. Citrate
    Tests whether the bacterium can utilize citrate as a carbon source

    Result - Positive = blue media, Negative = green media
  16. Urease
    Tests whether bacterium produces the enzyme Urease which can breakdown urea

    Result - Positive = pink media, Negative = clear or pale yellow media