Agar Plates

Mannitol Salt Agar (MSA)

  • Purpose: Used to differentiate Staphylococcus species, particularly Staphylococcus aureus.

  • Concentration: Contains 7.5% NaCl to create a high salt environment.

  • Differential Component: Mannitol is the fermentable carbohydrate.

  • pH Indicator: Phenol Red is used to indicate pH changes.

    • Results: If growth leads to yellow coloration, this indicates a positive result for Staphylococcus aureus.

Phenethyl Alcohol Agar (PEA)

  • Ingredients: Contains soybean, casein, and sodium chloride to stabilize the environment.

  • Purpose: Isolates staphylococci and streptococci while screening out Gram-negative bacteria such as E. coli and Proteus species.

  • Growth Results:

    • Zero to Little Growth: Generally inhibits Gram-negative growth.

    • Good Growth: Indicators of growth include Staphylococcus, Streptococcus, Enterococcus, or Lactococcus.

Columbia CNA with 5% Sheep Blood Agar

  • Type: Selective medium for Gram-positive organisms.

  • Ingredients: Casein, digested animal tissue, beef, yeast extract, corn starch, sheep blood, colistin, and nalidixic acid.

  • Growth Results:

    • Zero Growth: No growth of Gram-negative organisms.

    • Good Growth with Clearing: Shows potential growth of Staphylococcus, Streptococcus, and Enterococcus.

MacConkey Agar (MAC)

  • Ingredients: Contains lactose, bile salts, crystal violet, a neutral pH indicator.

  • Purpose: Isolates and differentiates members of the family Enterobacteriaceae.

  • Growth Results:

    • Pink to Red Growth: Indicates lactose fermentation, signifying coliform bacteria presence.

Hecktoen Enteric Agar (HEA)

  • Purpose: Differentiates and identifies Salmonella and Shigella.

  • Functionality: Tests for fermentation of lactose, sucrose, or salicin, producing acid.

  • Sulfur Reduction: Indicated by a black precipitate for Salmonella.

  • Inhibition: Selectively inhibits Gram-positive bacteria.

Eosin Methylene Blue Agar (EMB)

  • Composition: Contains gelatin, lactose, eosin, and methylene blue.

  • Functionality: Dyes inhibit Gram-positive bacteria and react with lactose fermentation end products.

  • Growth Results:

    • Dark Purple or Black Colonies: Indicates acid production from lactose fermentation, used to identify coliforms.

Chromagar

  • Purpose: Used to identify a wide range of prevalent microorganisms, particularly pathogens.