The final pH should be 7.4 +/- 0.2 at 25✬. Composition of Xylose Lysine Deoxycholate (XLD) Agar Escherichia coli, Citrobacter, and Proteus species. Organisms that ferment xylose, are lysine decarboxylase-negative and do not ferment lactose or sucrose cause an acid pH in the medium, and form yellow colonies e.g. The addition of sodium thiosulfate and ferric ammonium citrate as a sulfur source and indicator, respectively, allows hydrogen sulfide (H 2S) forming organisms to produce colonies with black centers, under alkaline conditions. Medium is an indicator due to having two indicators, one pH indicator (phenol red) and another hydrogen sulfide indicator(ferric ammonium citrate). To avoid this reversal to a Shigella reaction, lactose and sucrose are added in excess. Lysine decarboxylation reverts the pH of the medium to an alkaline condition. A second differential mechanism for Salmonella is employed by the addition of lysine. Xylose is fermented by most enterics except for Shigella species, and these colonies appear red on this medium as a result.

Xylose, lysine, lactose, and sucrose act as both differential as well as carbohydrate source ingredients. The selective agent is sodium deoxycholate, which inhibits the growth of gram-positive organisms. Xylose lysine deoxycholate (XLD) agar is a selective, differential, and indicator medium. Principle of Xylose Lysine Deoxycholate (XLD) Agar The above picture is showing the growth of Salmonella Typhi and Salmonella Paratyphi on XLD agar left to right respectively. Salmonella Paratyphi: Only red colonies no black center Salmonella Typhi: Red colonies with black center Salmonella and Shigella species colony characteristics on XLD agar – It has also been found to be an excellent medium for isolating Salmonellaspecies as well. It was specially designed to allow the growth of Shigella species and is a proven medium for the isolation of this organism. It also supports the growth of more fastidious enteric organisms. Xylose Lysine Deoxycholate (XLD) Agar is a selective, differential, and indicator medium for the isolation of enteric pathogens. Introduction of Xylose Lysine Deoxycholate (XLD) Agar

Xld agar results