Microbiology Notes on Oxygen Relations and Bacterial Identification

Oxygen Relation in Microorganisms

  • Facultative Anaerobe

    • Definition: Does not require oxygen but can grow in its presence.
    • Example: E. Coli
    • Characteristics:
    • Can detoxify toxic by-products of oxygen.
    • Growth throughout the test tube, indicative of its ability to grow with or without oxygen.

  • Microaerophile

    • Definition: Requires a small amount of oxygen for growth.
    • Example: Micrococcus luteus
    • Characteristics:
    • Cannot grow in atmospheric oxygen.
    • Growth is in the middle of the tube, indicating survival at low oxygen concentration.

  • Anaerobe

    • Definition: Lacks the enzyme to utilize oxygen and cannot survive in its presence.
    • Example: Clostridium
    • Characteristics:
    • Cannot detoxify toxic by-products of oxygen.
    • Growth occurs at the bottom of the tube, showing a preference for anaerobic conditions.

  • Oxygen Gradient and Testing

    • Test tubes used have a gradient of oxygen, with the highest concentration at the top and near-zero at the bottom.
    • Anaerobic jars can be utilized, containing a chemical pack and water that produces hydrogen and CO2, creating anaerobic conditions.
    • Methylene blue indicator is used:
    • Blue = oxygen presence
    • White = no oxygen

Gram-Negative Bacteria

  • Enteric Coliforms

    • Lactose Fermenters:
    • E. Coli: Green metallic sheen on EMB; pink-orange on Hektoen due to lactose fermentation.
    • Enterobactor: Pink color on EMB and Hektoen due to lactose fermentation.
    • Differences:
    • E. Coli shows green sheen on EMB, which distinguishes it from Enterobactor.

  • Lactose Non-fermenters:

    • Proteus: No color on EMB; black precipitate on Hektoen due to H2S production.
    • Non-pathogenic; may be part of normal flora.
    • Salmonella: No color on EMB; black precipitate on Hektoen due to H2S production; considered a true pathogen.
    • Shigella: No color on EMB; blue-green on Hektoen, no black precipitate.

  • Differentiation Method:

    • Utilize Enterotube for species identification within Enterics.
    • Gram-positive bacteria do not grow on selective EMB and Hektoen agars due to bile salts and dyes.

Gram-Positive Bacteria

  • Staphylococcus Species:

    • Staphylococcus aureus:
    • Not normal flora; ferments mannitol, creates yellow color on agar; beta hemolysis on blood agar.
    • Confirmed using agglutination test.
    • Staphylococcus epidermidis:
    • Normal skin flora; does not ferment mannitol; produces pink on agar; gamma hemolysis.

  • Streptococcus Species:

    • Streptococcus pyogenes:
    • Not normal flora; beta hemolysis; sensitive to Bacitracin.
    • Streptococcus pneumoniae:
    • Normal nasal flora; alpha hemolysis, green coloration; sensitive to Optichin.

Agar Plate Results

  • EMB Plate:

    • E. Coli: Green sheen
    • Enterobactor: Pink color
    • Proteus: No color
    • Salmonella: No color
    • Shigella: No color

  • Hektoen Plate:

    • E. Coli: Salmon color
    • Enterobactor: Salmon color
    • Proteus: Black precipitate
    • Salmonella: Black precipitate
    • Shigella: Blue-green, no black precipitate

  • Mannitol Agar Plate:

    • Staphylococcus aureus: Yellow color
    • Staphylococcus epidermidis: Pink color

  • Blood Agar Plate:

    • Staphylococcus aureus: Beta hemolysis
    • Staphylococcus epidermidis: Gamma hemolysis
    • Streptococcus pyogenes: Beta hemolysis
    • Streptococcus pneumoniae: Alpha hemolysis

Diseases Associated with Bacteria

  • Staphylococcus aureus:

    • Causes: Impetigo, cellulitis, abscess, boils, food poisoning, Toxic Shock Syndrome, septicemia.

  • Streptococcus species:

    • Causes: Sepsis, scarlet fever, rheumatic fever, necrotizing fasciitis, Toxic Shock Syndrome, septicemia, nephritis.

Urine Samples and Findings

  • Blood Agar:

    • Alpha hemolysis: E. coli, Enterobacter
    • Beta hemolysis: Staph. aureus
    • Gamma hemolysis: Staph. epidermidis, sometimes alpha, Candida

  • EMB Agar:

    • Positive growth: E. coli (green sheen), Enterobactor (pink)

  • Sabouraud Agar:

    • Good growth: White, creamy Candida; pinpoint colonies for day-old E. coli, Enterobacter, Staph. aureus

Gram-Negative Bacilli - Stool Samples

  • EMB Results:

    • Lactose +: Pink Enterobacter, metallic green sheen E. coli
    • Lactose -: Colorless Salmonella, Shigella, Proteus

  • Hektoen Results:

    • Lactose +: Yellow-orange Enterobacter, and Salmon for E. coli
    • Lactose -: Blue-green Shigella, black for Salmonella, Proteus
    • No pathogens found; proceed to Enterotube for further examination.