BISC209: Selective,and/or Differential, and/or Enriched media

Selective and/or Differential and/or Enriched media

Selective for Gram positive Organisms

Phenylethyl Alcohol Agar (PEA) PEA selects for the gwroth of Gram Positive organisms and inhibits the growth of Gram negative organisms because the Phenylethyl alcohol interfers with DNA synthesis in the Gram negatives. It is particularly useful if you think your organism is a Proteus sp.
RECIPE: 10g tryptose, 3g Beef extract, 5g Sodium Chloride, 2.5g Phenylethyl alcohol, 15g Agar to 1 liter distilled or deionized water pH 7.1-7.5.

Mannitol Salt Agar (also see Targeted Enrichment media list)

Sugar Fermentation Media

Phenol Red Agar with mannitol: 1% mannitol is added to phenol red agar base (1% peptone, 0.1% beef extract, 0.5% NaCl, 1.5% agar and 25µl/ml phenol red) to test for the fermentation of mannitol by the bacteria. A positive test is a yellow color due to the formation of acidic products of mannitol fermentation. The indicator, phenol red, turns yellow when the pH falls below 6.8. Staphylococcus aureus usually ferments mannitol within 24 to 48 hours. An inoculum should be spread on the slant and then stabbed to the bottom of the butt, a region that remains fairly anaerobic. The inoculated slants should then be incubated at 37°C for 24-48 hours. Yellow color indicates a positive (+) test.

Phenol red agar with sucrose: 1% sucrose is added to phenol red agar base to test for organisms able to utilize sucrose. Acidic products will turn the medium yellow when the pH falls below 6.8. Inoculate the slant with a loop of culture and stab the butt area as in the mannitol fermentation test. Incubate for 24-48 hours. A yellow color indicates a positive (+) test.