Differential Media:
Overview of Some Common Enteric Plating 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.

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MacConkey Agar – Usual Formulation and the Bacteriology 102 Modification

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.

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.

EMB Agar

This medium presently has its own page here.

Hektoen Enteric Agar

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.		 Image not yet available

Brilliant Green Agar

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.

XLD Agar

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.

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

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Enteric Plating Media.