Selective, Differential and Enriched Media

  • Selective media: used to select (isolate) specific groups of bacteria;

  • chemical substances in the media inhibit the growth of one type of bacteria while permitting growth of another (MSA, EMB, MacConkey)

  • Differential media: distinguishes among morphologically and biochemically related groups of organisms;

  • chemical compounds (following inoculation and incubation) produce a characteristic change in the appearance of bacterial growth and/or the medium surrounding the colonies (MSA, EMB, MacConkey)

  • Enriched media: supplemented with highly nutritious materials, such as blood, serum, or yeast extracts, for the cultivation of fastidious organisms

  • Mannitol Salt Agar

    (MSA)

  • Mannitol salt agar is both selective and differential

  • Selective: It favors organisms capable of tolerating high salt concentrations

  (7.5 % NaCl)

  • Differential: It distinguishes bacteria based on their ability to ferment mannitol

  • Differentiates Staphylococcus species, by mannitol fermentation

  • (S. aureus ferments,      S. epidermidis does not)

  • Phenol red is the pH indicator

  Neutral – Basic pH

  red  at 7.4 to 8.4

  Acidic pH

  yellow below 6.8

  • RESULTS:

  • Positive Results:

  The development of “yellow halos” around the bacterial growth means mannitol has been fermented and acid end products have been produced  (S. aureus)

  • Negative Results:

  No color change in the medium is a negative result  (S. epidermidis)

  • No growth on the medium indicates a Gram- organism (E. coli)

  • PROCEDURE:

  On an MSA Plate:   

  • Eosin methylene blue agar is a selective and differential medium

  • Selective: EMB contains the dyes methylene blue and eosin which inhibit Gram + bacteria, thus favoring growth of Gram –

  • Differential: EMB contains lactose, thus allowing for the distinction between lactose fermenters and nonferments

  • Large amounts of acid from lactose fermentation cause the dyes to precipitate on the colony surface, producing a black center or a “green metallic sheen” (E. coli)

  • Smaller amounts of acid production result in pink coloration of the growth (E. aerogenes)

  • Nonfermenting enterics do not produce acid so their colonies remain colorless or take on the color of the media (P. vulgaris)

  • No growth indicates a Gram + organism (S.aureus)

  • PROCEDURE:

  On an EMB Plate:

  • MacConkey Agar

    (MAC)

  • A selective and differential medium used to isolate members of the Enterobacteriaceae

  • Contains nutrients, including lactose, as well as bile salts, neutral red and crystal violet

  • Bile salts and crystal violet inhibit growth of G+ organisms (selective)

  • Neutral red is a pH indicator that is colorless, but yellow above pH 8 and red at pH less than 6.8

  (differential)

  • Acid accumulating from lactose fermentation turns the colorless neutral red to a red color—therefore coliforms produce a red “halo” on the medium (E.coli, E.aerogenes)

  • Lactose nonfermenters will grow, but don’t produce acid. Therefore, the neutral red remains colorless (P. vulgaris)

  • No growth indicates a

Gram + organism

(S.aureus)

  • PROCEDURE:

  • On a MacConkey Agar Plate:


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