• 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:

…