Selective and Differential Media

Selective and Differential Media

Objective

  • Identify bacterial species based on their physical and metabolic differences.

Key Concepts

  • Selective Media

    • Contains chemicals that inhibit the growth of unwanted bacterial species while allowing the desired species to grow.
    • Examples of selective chemicals: antibiotics, dyes, salts.

  • Differential Media

    • Used to identify bacterial species based on differences in metabolism, often linked to the presence or absence of specific enzymes.
    • The growth allows for visual differentiation between species, typically in color.

Mannitol Salt Agar (MSA)

  • Selective Agent: 7.5% NaCl

    • Inhibits non-salt tolerant microbes, allowing for the growth of salt-tolerant Gram-positive halophilic species such as Staphylococcus species, which thrive on human skin.

  • Differential Agent: Mannitol

    • A carbohydrate energy source that, when fermented, produces acid end-products lowering the pH.
    • pH Indicator: Phenol red
    • Yellow: pH < 6.8 (acidic)
    • Red: pH 6.8-7.4
    • Pink: pH > 7.4 (alkaline)

  • Observations:

    • Most Staphylococci do not ferment mannitol (red), but Staphylococcus aureus ferments it (yellow).

MacConkey Agar (MAC)

  • Selective Agents: Bile salts and crystal violet

    • Inhibit growth of Gram-positive bacteria and allow growth of Gram-negative enteric bacteria (Enterobacteriaceae).

  • Differential Agent: Lactose

    • Fermentation produces acid end-products that lower pH, indicating fermentation.
    • pH Indicator: Neutral red
    • Pink: pH < 6.8 (acidic)
    • Clear/Yellow: pH > 6.8 (alkaline)

  • Observations:

    • Lactose fermenters (coliforms) produce acid and appear red.

Bacterial Classification

  • Domain: Bacteria, Archaea, Eukarya
  • Kingdom: Fungi, Plant, Animal, Protist
  • Phylum:
  • Class:
  • Order:
  • Family: Enterobacteriaceae
  • Genus: Escherichia
  • Species: Escherichia coli

Enterobacteriaceae Family

  • Characteristics:
    • Bacillus-shaped, Gram-negative bacteria known as enterics due to their intestinal habitat (Greek "enteron" = intestine).
    • Facultative anaerobes that typically ferment glucose.
  • Normal Microbiome Members:
    • Includes Enterobacter, Klebsiella, Proteus, and most Escherichia coli strains.
  • Pathogenic Members:
    • Diarrheal diseases: Shigella, Salmonella enterica, and pathogenic strains of E. coli (e.g., E. coli O157:H7).
    • Life-threatening conditions caused by:
    • Salmonella enterica serotype Typhi (typhoid fever).
    • Yersinia pestis (bubonic and pneumonic plague).

Coliforms

  • Enteric bacteria that ferment lactose, used as water fecal pollution indicators (e.g., E. coli, Enterobacter aerogenes).
  • Presence indicates possible fecal-borne pathogens in water.

Eosin Methylene Blue Agar (EMB)

  • Selective Agents: Eosin and methylene blue dyes

    • Inhibit most Gram-positive microbes.

  • Differential Agent: Lactose

    • Fermentation lowers pH, causing color changes due to dye interaction:
    • Dark purple/black: lactose fermentation.
    • Metallic green: E. coli lactose fermentation.
    • Light purple/pink: non-lactose fermenters.

Bile Esculin Agar (BEA)

  • Selective Agent: Bile (oxgall)

    • Selects for Group D streptococci and enterococci.

  • Differential Action: Ability to hydrolyze esculin via esculinase

    • Reaction: Esculin → Glucose + Esculetin.
    • Esculetin forms a dark brown/black precipitate with ferric citrate, darkening the medium.
    • Identifies esculinase-positive species, such as Enterococcus faecalis, Enterococcus casseliflavus, and Group D Streptococcus species.

Diagram of Bile Esculin Hydrolysis Reaction

  • Highlights the site of hydrolysis and the cleavage that results in the formation of a dark precipitate.

Conclusion

  • Selective and differential media are important in identifying and isolating bacterial species, enhancing our understanding of microbiological profiles and their implications for health and safety.