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)

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