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Microbiology Identification and Testing Methods

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  • Purpose of Mannitol Salt Agar (MSA)
    MSA is used to select and differentiate bacteria based on salt tolerance and mannitol fermentation, especially Staphylococcus and some Bacillus species.
  • Selective agent in Mannitol Salt Agar
    7.5% NaCl inhibits most bacteria except halotolerant species.
  • Differential indicator in Mannitol Salt Agar
    Phenol red changes to yellow when mannitol is fermented, indicating acid production.
  • Positive MSA result example
    Staphylococcus aureus ferments mannitol, turning the medium yellow.
  • Bile Esculin Agar purpose
    Identifies bacteria that hydrolyze esculin in the presence of bile, such as Enterococcus faecalis.
  • Positive reaction on Bile Esculin Agar
    Blackening of the medium due to esculetin reacting with ferric ions.
  • Enzymes tested by Starch Agar
    Amylase and oligo-1,6-glucosidase hydrolyze starch, detected by clear zones after iodine addition.
  • Indicator used in Starch Agar test
    Iodine solution is added to visualize starch hydrolysis by clearing around colonies.
  • Enzyme and substrate in Lipid Agar test
    Lipase hydrolyzes lipids such as tributyrin, producing clear zones around colonies.
  • Casein Agar test purpose
    Tests for casein hydrolysis by caseinase, indicated by clear zones on milk agar.
  • Gelatin Hydrolysis positive result
    Liquefaction of gelatin after incubation indicates gelatinase activity.
  • Catalase test distinguishes which genera?
    Staphylococcus (catalase-positive) from Streptococcus (catalase-negative).
  • DNase Agar positive result
    Clear zone around colonies indicates DNA hydrolysis.
  • Novobiocin susceptibility test purpose
    Differentiates Staphylococcus species by resistance or susceptibility on Mueller-Hinton agar.
  • Hemolysis types on Blood Agar
    Alpha: partial (greenish), Beta: complete (clear), Gamma: none.
  • Triple Sugar Iron (TSI) Slant tests for
    Carbohydrate fermentation, gas production, and hydrogen sulfide (H2S) production.
  • Urea Hydrolysis test indicator and positive result
    Phenol red indicator turns pink when urease produces ammonia, raising pH.
  • SIM Agar tests for
    Sulfur reduction (black precipitate), indole production (red with Kovac's reagent), and motility.
  • MRVP test distinguishes
    Fermentation pathways: Methyl Red positive (mixed acid fermentation) vs. Voges-Proskauer positive (butanediol fermentation).
  • Citrate test positive result
    Blue color indicates use of citrate as sole carbon source.
  • Nitrate Reduction test positive results
    Red color after reagents or no color after zinc indicates nitrate reduction.
  • Lysine Iron Agar (LIA) purple butt indicates
    Lysine decarboxylation under anaerobic conditions.
  • MacConkey Agar selects for and differentiates
    Selects Gram-negative bacteria; differentiates lactose fermenters (pink/red) from non-fermenters (colorless).
  • Rapid Strep Test principle
    Detects Group A Streptococcus antigens by immunochromatography; positive shows colored line.
  • Rapid Staph Test principle
    Detects Staphylococcus aureus by latex agglutination using beads coated with fibrinogen and IgG.
  • Most Probable Number (MPN) test principle
    Estimates bacterial concentration using serial dilutions and statistical probability from positive tubes.
  • ELISA positive result
    Color change after substrate addition indicates presence of antigen or antibody.