STAPHYLOCOCCI

Cards (45)

  • Procedure for investigating an unknown organism in microbiology laboratory
    1. Undergo screening test
    2. Perform presumptive test
    3. Perform confirmatory test
  • Screening test
    Screens for the presence or absence of the organism in the specimen
  • Presumptive test
    Presumes the identification of the unknown organism
  • Confirmatory test

    Confirms the presence of the organism of interest in the sample
  • Microscopic evaluation of the morphology of the organism is the first objective in the screening test
  • Screening test is always done first before presumptive and confirmatory tests
  • If the screening test is negative, it is impractical to perform the confirmatory test
  • Presumptive tests
    1. Pigment production on potato agar
    2. Catalase production on trypticase soy agar (TSA)
    3. Hemolysis and colonial morphology on blood agar (BA)
  • Confirmatory tests
    1. Mannitol fermentation test on mannitol salt agar (MSA)
    2. Coagulase test
  • Staphylococci
    • Gram positive cocci arranged in clusters
    • Appear purple under the microscope
  • Morphology alone cannot differentiate Staphylococcus aureus from Staphylococcus epidermidis
  • Catalase test

    Detects the ability of the organism to produce the enzyme catalase using hydrogen peroxide
  • Usefulness of catalase test
    Differentiates Staphylococci from Streptococci
  • Performing catalase test
    1. Grow colonies on trypticase soy agar (TSA)
    2. Treat grown colonies with hydrogen peroxide (H2O2)
    3. Appearance of gas bubbles indicates a positive test
  • Positive catalase test indicates the organism is from the Staphylococci family
  • Blood agar
    Enrichment and differential medium
  • Staphylococcus aureus
    • Produces small, circular, golden-yellow colonies
  • Staphylococcus epidermidis
    • Produces white colonies
  • Hemolysis/hemolytic pattern

    Indicates the ability of the organism to cause lysis of red blood cells in the blood agar medium
  • Agar
    Medium that particular organism has to utilize substances found inside the red blood cell (a content of blood agar)
  • Organism utilizing substance inside RBC
    1. Destroy RBC to get substance
    2. Cause destruction or lysis of RBCs in blood agar medium
  • Hemolytic patterns

    Different patterns observed in blood agar medium depending on organism's ability to cause lysis of RBCs
  • Hemolytic patterns
    • Beta-hemolysis
    • Alpha-hemolysis
    • Gamma-hemolysis
  • Beta-hemolysis
    Complete hemolysis/destruction/lysis of RBCs in blood agar medium
  • Alpha-hemolysis
    Incomplete destruction of RBCs
  • Gamma-hemolysis
    No hemolysis of red blood cells
  • Staphylococcus aureus produces beta-hemolysis, Staphylococcus epidermidis produces gamma-hemolysis
  • Mannitol salt agar

    Selective and differential medium containing mannitol and 7.5% sodium chloride
  • Selective characteristic of mannitol salt agar

    Only salt-loving organisms can survive and grow
  • Differential characteristic of mannitol salt agar
    Organism fermenting mannitol produces acid, changing agar color from light pink to yellow
  • Staphylococcus aureus is mannitol fermentation positive, Staphylococcus epidermidis is mannitol fermentation negative
  • Coagulase
    Enzyme produced by Staphylococcus aureus that can convert fibrinogen to fibrin, causing clot formation
  • Staphylococcus aureus is coagulase positive, Staphylococcus epidermidis is coagulase negative
  • Coagulase tests
    • Slide coagulase
    • Tube coagulase
  • Staphylococcus aureus produces yellow pigment (lipochrome), Staphylococcus epidermidis produces white pigment
  • Criteria for identifying Staphylococci
    • Colonial appearance on blood agar
    • Colonial appearance on MSA
    • Gram staining reaction
    • Microscopic morphology
    • Catalase & coagulase production
    • Mannitol fermentation
  • Catalase test

    Detects ability of bacteria to convert hydrogen peroxide to oxygen and water
  • Staphylococci are catalase positive, Streptococci are catalase negative
  • Coagulase test

    Detects ability of Staphylococcus aureus to produce coagulase enzyme
  • Coagulase test types
    • Slide coagulase
    • Tube coagulase