Differential Media:
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Page 2: Continuation of how the following media work: MacConkey (the "regular" formulation and the Bact. 102 modification), EMB, Hektoen Enteric, Brilliant Green and XLD Agars.If you are running Netscape 4.x, see the note here regarding links to images. |
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MacConkey Agar differentiates between lactose fermenters (e.g., coliforms) and non-lactose-fermenters (e.g., most strains of Citrobacter and typical enteric pathogens such as Salmonella and Shigella).
| example | coliform | Salmonella Shigella Citrobacter(typical) |
| amino acids deaminated (alkaline rx.) |
+ | + |
| lactose fermented (strong acidic rx.) |
+ | – |
| net pH reaction | acidic (red colony) | alkaline (white colony) |
| Click on image for wider view in separate window. |
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The "modification" of the medium allows differentiation between organisms which reduce thiosulfate to H2S and those which do not. H2S is detected by the formation of FeS, a black precipitate. (In a separate photo, H2S production by Salmonella can be seen on the modified MacConkey Agar on the right. Regular MacConkey Agar is at left.)
| example | coliform | Shigella | Salmonella Citrobacter(typical) |
| amino acids deaminated (alkaline rx.) |
+ | + | + |
| lactose fermented (strong acidic rx.) |
+ | – | – |
| thiosulfate reduced to H2S (black color) |
– | – | + |
| net pH reaction | acidic (red colony) | alkaline (white colony) | alkaline (white colony plus black center due to H2S) |
| Click on image for wider view in separate window. |
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This medium presently has its own page here.
| example | coliform | Shigella | Salmonella Citrobacter(typical) |
| amino acids deaminated (alkaline rx.) |
+ | + | + |
| salicin fermented (weak acidic rx.) |
+ or – | – | – |
| lactose and/or sucrose fermented (strong acidic rx.) |
+ | – | – |
| thiosulfate reduced to H2S (black color) |
– | – | + |
| net pH reaction | acidic (yellow-orange) | alkaline (blue-green) | alkaline (blue-green) |
| Click on image for wider view in separate window. |
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Image not yet available |  |
This medium differentiates between those that ferment lactose and/or sucrose and those that ferment neither, such as the typical enteric pathogens (Salmonella and Shigella) and most strains of Citrobacter. There is no detection of H2S production.
| example | coliform | Salmonella Shigella Citrobacter(typical) |
| amino acids deaminated (alkaline rx.) |
+ | + |
| lactose and/or sucrose fermented (strong acidic rx.) |
+ | – |
| net pH reaction | acidic (yellow) | alkaline (red) |
| Click on image for wider view in separate window. |
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This medium includes a relatively large amount of lysine which can be decarboxylated (an anaerobic process), producing an alkaline product. Thus, Salmonella (lysine decarboxylation-positive) can be differentiated from Citrobacter (lysine decarboxylation-negative) on this medium. A net alkaline colony is more likely to be an enteric pathogen on this medium than on the other media mentioned here.
| example | coliform | Citrobacter(typical) | Salmonella | Shigella |
| amino acids deaminated (alkaline rx.) |
+ | + | + | + |
| xylose fermented (weak acidic rx.) |
+ | + | + | – |
| lactose and/or sucrose fermented (strong acidic rx.) |
+ | – | – | – |
| lysine decarboxylated (alkaline rx.) |
+ or – | – | + | – |
| thiosulfate reduced to H2S (black color) |
– | + (Low pH usually inhibits formation of the black compound, FeS.) |
+ | – |
| net pH reaction | acidic (yellow) | acidic (yellow) | alkaline (red plus black center due to H2S) |
alkaline (red) |
| Click on image for wider view in separate window. |
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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 |