Page 4: A virtual demonstration of enteric plating media – developed to coincide with the isolation step in the Bacteriology (now Microbiology) 102 experiment on the Family Enterobacteriaceae.
First, a review of the selective agents, indicators and reactive substrates in the two media demonstrated here:
|selective agent(s)||bile salts|
|source of amino acids which|
may be deaminated1
|amino acid added for detection|
|fermentable sugar(s)2||lactose (1%)||lactose (0.75%)|
|pH indicator||neutral red:|
net acid = red
net alkaline = white
net acid = yellow
net alkaline = red
|source of H2S||sodium thiosulfate||sodium thiosulfate|
|indicator of H2S production||ferric ammonium citrate||ferric ammonium citrate|
1 Alkaline reactions.
The following table reviews the reactions of typical strains of coliforms, Citrobacter and Salmonella on various substrates included in the two media, Modified MacConkey Agar and XLD Agar. Also shown are photos of colonies showing the net pH reactions from metabolic activities on the two media. Clicking on an image will show the full view in a separate window.
The Modified MacConkey Agar shown here is used in Bacteriology 102 to represent many of the enteric plating media out in "the real world" that detect (1) fermentation from one or more sugars and also (2) hydrogen sulfide. Enabling the medium to detect hydrogen sulfide from reduction of thiosulfate is the "modification"; otherwise the only differential substrate of MacConkey Agar would be lactose.
Typical strains of Salmonella are indistinguishable from certain other enterics on most enteric isolation media (including the Modified MacConkey Agar) due to negative fermentation reactions for the sugars in these media, and the need is shown for better differentiation of Salmonella which can be of special interest when clinical and food samples are tested. Many of the colonies that appear Salmonella-like are of Citrobacter strains which ferment lactose slowly or not at all (as opposed to the less-common "coliform" strains of Citrobacter which produce acid and gas rapidly from lactose fermentation).
XLD Agar contains three sugars (lactose, sucrose and xylose) plus the amino acid lysine. The alkaline reaction from lysine decarboxylation can overneutralize the acidic reaction from xylose fermentation, a carbohydrate present in a relatively low amount which is fermented by many enterics including Salmonella and Citrobacter. However, this alkaline reaction will be overneutralized by the large amount of acid produced by organisms which can ferment lactose and/or sucrose; each of these sugars is present in XLD Agar in a relatively high amount – but are not among the sugars fermented by typical Salmonella and Citrobacter strains. So, the differentiation of Salmonella is assisted by (1) its non-fermentation of lactose and sucrose – which distinguishes it from many enterics including coliforms – and (2) its ability to decarboxylate lysine – which results in a net alkaline reaction that distinguishes it from Citrobacter.
|Biochemical Process in Modified|
MacConkey and XLD
|Amino Acid Deamination||+ (alk)||+ (alk)||+ (alk)|
|Lactose Fermentation||+ (acid)||–||–|
|Sucrose Fermentation (XLD only)||+ (acid)||–||–|
|Xylose Fermentation (XLD only)||+ (acid)||+ (acid)||+ (acid)|
|Lysine Decarboxylation (XLD only)||+ (alk) or –||+ (alk)||–|
|Net pH reaction on Modified|
MacConkey Agar 1
|acid (red)||alkaline (white)||alkaline (white)|
|Net pH reaction on XLD Agar 1||acid (yellow)||alkaline (red)||acid (yellow)|
1 H2S is detectable by black FeS precipitate, unless the FeS is dissolved by a
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|Page last modified on 7/22/09 at 5:30 PM, CDT.|
John Lindquist: new homepage, complete site outline.
Department of Bacteriology, U.W.-Madison