X.L.D. Agar | DM230

X.L.D. Agar | DM230

An improved medium for the isolation of enteric pathogens.

Taylor1 designed X.L.D. Agar to give improved isolation of shigellae without affecting salmonellae isolation and more recent studies have demonstrated its efficacy.2-6 Unlike most enteric pathogens, shigellae and providenciae do not ferment xylose, and although Salmonella and Arizona groups do, they are made to appear as non-xylose fermenters by the presence of lysine in the medium. After fermentation of xylose is complete, lysine is decarboxylated and the colonies revert to an alkaline pH. Many other Gram negative xylose fermenters also ferment sucrose and/or lactose, so they do not revert to an alkaline pH.

Technical Formula

Directions
1. Suspend by swirling 53g of powder in 1 litre or the contents of the sachet in the stated volume of distilled or deionised water in a large flask.
2. Allow to stand for 15 minutes and bring to the boil until completely dissolved. DO NOT AUTOCLAVE.
3. Allow to cool to about 50ºC, mix well and pour plates.

In Use

The medium is inoculated with the sample and incubated for 18-24 hours. Non-xylose fermenters e.g. salmonellae and shigellae appear as red colonies. Certain salmonellae produce H2S and give black centred colonies, but some H2S producing non-pathogens do not, because of the acidity.
Commensal organisms, if they are not inhibited, ferment one of the sugars and so appear as yellow colonies. It is important that incubation is not continued beyond 24 hours as this allows reversion of pH in non-pathogens.