Etiket Arşivleri: MacConkey Agar

EMB Agar

EMB agar

  Only gram-negative bacteria grow on EMB agar. (Gram-positive bacteria are inhibited by the dyes eosin and methylene blue added to the agar.) Based on its rate of lactose fermentation, E. coli produces dark, blue-black colonies with a metallic green sheen on EMB agar.

Growing on EMB Agar

Nutrient Agar

Mannitol Salt Agar

Mannitol salt agar is both a selective and differential growth medium. It is used to differentiate pathogenic Staphylococcus species from nonpathogenic members of the genus Micrococcus.

Fermentation of mannitol: This medium contains an indicator, phenol red. The indicator is a pinkish-red at neutral pH, is really red at ph 7.4 and above (basic) and is yellow below pH 6.8 (acidic). An organism can either ferment (metabolize) mannitol or not and this will be indicated by a color change on the MSA plate–which makes this a differential test.

  1. S. aureus

    Salt tolerance +

    Mannitol fermentation + (yellow)

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


  • 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


  • 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)


  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)


  On an EMB Plate:

  • MacConkey Agar


  • 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


  • 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



  • On a MacConkey Agar Plate: