Eosin Methylene Blue (EMB) Agar (Levin)

Test

• Inoculate the samples on the surface of the plates using the spreading method.
• Incubate the plates at 35 degrees Celsius for 18-24 hours. Avoid exposing the plates to light. If the culture results are negative after 24 hours, incubate the plates for another 24 hours.

Results and Interpretation

Coliforms, including lactose-fermenting gram-negative bacteria, produce a metallic green sheen on their colonies due to the acidic conditions that occur during their growth and sugar fermentation in the EMB medium, resulting in a color reaction.

The typical appearance of some of the coliforms is as follows:

Escherichia coli bacteria have large, blue-black colonies with a metallic sheen.

Enterobacter/Klebsiella bacteria produce large, mucoid colonies that are blue-black in color.

Proteus, Salmonella, and Shigella bacteria have large, colorless colonies.

Pseudomonas bacteria have irregular and colorless colonies.

Notes:

• Some strains of Salmonella and Shigella do not grow on this medium due to the presence of eosin and methylene blue in its composition.
• Some gram-positive bacteria such as Enterococcus, Staphylococcus, and yeast can grow on this medium and produce pinpoint colonies.
• Non-pathogenic, non-lactose-fermenting bacteria can also grow on this medium. Therefore, for more accurate identification of these organisms, biochemical tests should also be used in addition to the pathogenic strains.