[Microbiology] Xylose Lysine Desoxycholate Agar

Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Xylose Lysine Desoxycholate Agar

Purpose

Xylose Lysine Desoxycholate (XLD) Agar is a selective and differential medium used to isolate and identify Shigella and Providencia from stool samples.

Principle

XLD Agar is a selective and differential medium containing sodium desoxycholate, xylose, L-lysine, and ferric ammonium citrate. Desoxycholate is a Gram-positive inhibitor, xylose is a fermentable carbohydrate, L-lysine is an amino acid provided for decarboxylation (See Decarboxylation, page 67), and ferric ammonium citrate is an indicator to mark the presence of hydrogen sulfide gas (H2S) from sulfur reduction. Phenol red, which is yellow when acidic and red or pink when alkaline, is added as a pH indicator. Organisms that ferment xylose will acidify the medium and produce yellow colonies. Organisms able to remove the carboxyl group from (decarboxylate) L-lysine will release alkaline products and produce red colonies. Organisms able to reduce sulfur will produce a black precipitate in the growth due to the reaction of ferric ammonium citrate with H2S.

Shigella and Providencia, which do not ferment xylose but decarboxylate lysine, appear red on the medium. Salmonella species, which ferment xylose but then decarboxylate the lysine also appear as red colonies but with black centers due to the reduction of sulfur to H2S. Other enterics that would ordinarily revert to decarboxylationafter exhausting the sugar and alkalinize the medium are prevented from doing so by the high sugar content and the short incubation time. These organisms appear yellow on the medium (Figure 2-31). 

2-31 XYLOSE LYSINE DESOXYCHOLATE AGARXLD agar inoculated with (clockwise from top): Proteus
mirabilis (positive for sulfur reduction), Salmonella
typhimurium (atypically negative for sulfur reduction),
and Escherichia coli (positive for lactose fermentation).  

Suggested Reading

- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011