Differential Media:
Overview of Some Common Enteric Plating Media

Page 3:  Our old explanation of MacConkey Agar, Hektoen Enteric Agar, Brilliant Green Agar and XLD Agar – oriented toward the isolation of Salmonella.

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1 = typical Salmonella,  2 = Citrobacter (as explained above),  3 = typical coliform

Only well-isolated colonies are examined for the pH reactions associated with them. Where there is confluent growth (or if the plates have been incubated too long), there can be a tendency toward an alkaline reaction. As an example, see XLD plate #3. (Think about reasons why this can happen. In some cases, the methyl red reaction of an organism can help to explain reversion to an alkaline pH from an acidic pH.)

On MacConkey, Hektoen Enteric and Brilliant Green Agars, Salmonella and Citrobacter produce alkaline colonies (and an alkaline effect in the medium) due to non-fermentation of the sugar(s) in these media. Coliforms are acidic. On Hektoen Enteric Agar, H₂S production by Citrobacter and Salmonella can be seen by the black-centered colonies.

Regarding XLD Agar: If lysine were not present in the medium, both Salmonella and Citrobacter would produce an acidic reaction (due to xylose fermentation) and look the same. As a relatively large amount of lysine is present in XLD Agar, Salmonella decarboxylates it and Citrobacter does not, so Salmonella is differentiated by a net alkaline reaction. H₂S production by Salmonella can also be seen by the black-centered colonies. H₂S production by Citrobacter is usually not seen on XLD Agar, as the black iron sulfide precipitate is broken down by the low pH.

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Enteric Plating Media.