As an introduction or review: Many of our pH-based differential media have one or more of the features shown on the following table which should be helpful in analyzing the reactions of certain selective-differential media used in the isolation of enterics. The introductory page of the Differential Media Site addresses the concept of "programming" differential media to detect certain physiological types, and a good practical overview of the general concept can be found here.
The following two tables summarize six selective-differential plating media used in the isolation of enterics. When formulating such media, one takes advantage of the enterics' gram-negativity, and one or more agents that inhibit gram-positive bacteria are added. In the isolation of the enteric pathogens Salmonella, Shigella and Edwardsiella from food or clinical samples, an important consideration is the fact that they usually do not ferment certain sugars, especially lactose and sucrose. Therefore, one tends to pick colonies showing a net alkaline reaction on media containing the appropriate sugar(s), ignoring the acidic colonies which coliforms and various other enterics tend to produce.
In Bacteriology 102 for the enteric isolation experiment, we make use of MacConkey Agar which is modified by the addition of the H2S indicator system possessed by XLD Agar – i.e., sodium thiosulfate and ferric ammonium citrate. As far as we know, no such medium exists out there in the "real world," but we use it to illustrate some features of multipurpose selective-differential media one might encounter in a clinical laboratory.
Dr. Scott Cayley's explanation of the selective aspect of MacConkey Agar is shown here.
A study of the special differential features of XLD Agar is especially rewarding. This medium incorporates lysine which may be decarboxylated, resulting in an alkaline reaction that may or may not overneutralize any acid formed from fermentation of the various sugars. As shown below and on the following pages, incorporation of lysine in XLD Agar "tips the balance" to distinguish strains of Salmonella (which characteristically decarboxylate lysine and thus produce a net alkaline reaction on XLD Agar) from the "Salmonella-like strains" of Citrobacter. (Many other strains of Citrobacter rapidly ferment lactose with the production of acid and gas and would be termed "coliform strains" of Citrobacter, remembering the definition of coliform.) Without the lysine present, both organisms would produce a net acid reaction (from the fermentation of xylose), and the medium would be like other isolation media which render these two organisms indistinguishable from each other – making the isolation process for Salmonella a bit more tedious.
Characteristic reactions observed on these media are shown and explained on the next page.
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Department of Bacteriology, U.W.-Madison