Page 3: Our old explanation of MacConkey Agar, Hektoen Enteric Agar, Brilliant Green Agar and XLD Agar – oriented toward the isolation of Salmonella.
If you are running Netscape 4.x, see the note here regarding links to images.
The following two tables explain and illustrate differentiation of Salmonella from some possible competitors on MacConkey Agar (MAC), Hektoen Enteric Agar (HEA), Brilliant Green Agar (BGA) and XLD Agar.
The first table reviews the characteristics of typical strains of Salmonella and coliforms which are relevant to these plating media. Many strains of Citrobacter (a commonly occurring, non pathogenic enteric) can be indistinguishable from Salmonella on most enteric isolation media and are also included in the table. To assist in differentiating Salmonella more effectively, XLD Agar is useful as shown below.
|biochemical process||typical Salmonella||Citrobacter strains|
which do not
lactose or sucrose
|typical coliform||process applicable to|
(or exploited by)
|amino acid deamination||+ (alkaline)||+ (alkaline)||+ (alkaline)||X||X||X||X|
|lactose fermentation||–||–||+ (acid)||X||X||X||X|
|sucrose fermentation||–||–||+ (acid)||X||X||X|
|xylose fermentation||+ (acid)||+ (acid)||+ (acid)||X|
|salicin fermentation||–||–||+ or –||X|
|lysine decarboxylation||+ (alkaline)||–||+ or –||X|
|H2S production||+ (black)||+ (black)||–||X||X|
Based on the formulations of these media and the physiological characteristics of the organisms, a net alkaline or acidic reaction will result. Note the alkaline and acidic colonies on the plates below, and how XLD Agar will differentiate effectively between Salmonella and Citrobacter.
Click on images for larger view in separate window.
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.
Return to the main page of
|Page last modified on 3/21/04 at 8:15 PM, CST.|
John Lindquist: new homepage, complete site outline.
Department of Bacteriology, U.W.-Madison