XLD AGAR (XYLOSE LYSINE DEOXYCHOLATE AGAR) (as per USP/BP/JP/EP)

Xylose Lysine Deoxycholate Agar is used as a selective and differential medium for the recovery of Salmonella and Shigella species. This medium is a selective as well as differential medium formulated by Taylor for the isolation and identification of enteric pathogens especially Shigellae from stool samples. Human Salmonellae infections are most commonly caused by ingestion of food, milk or water contaminated by human or animal excreta. The medium is also employed for pharmaceutical testing and non-sterile product testing for the detection of Salmonella after enrichment in Rappaport Vassialidias Salmonella Enrichment Broth in accordance with the harmonized method of USP/EP/BP/JP/IP.

Deoxycholate, ferric ammonium citrate and sodium thiosulphate are selective agents that inhibit gram-positive microorganisms. Essential nutrients, growth factors for growth of microorganism are provided by yeast extract. Xylose, sucrose and lactose are the fermentable sugars in this medium. Most enteric organisms except Shigella ferment xylose to produce acid. Salmonella also produces decarboxylate lysine which keeps the pH neutral or slightly alkaline. At this pH, Salmonella species can produce hydrogen sulphide from the reduction of thiosulphate. This is indicated by Ferric ammonium citrate producing black or black-centred colonies. Some organisms, such as Citrobacter, can also decarboxylate lysine. However, they ferment lactose and sucrose which keeps the pH too low for the production of hydrogen sulphide. Bacteria that ferment none of these sugars, e.g., Shigella, appear as red, translucent colonies. Yellow colonies indicate a rapid fermentation of lactose and acid pH, as demonstrated by Escherichia coli. Since Salmonella ferments xylose as readily as coliforms, a second differential mechanism, Xylose Lysine Deoxycholate Agar, is utilized. Those organisms that ferment xylose as well as decarboxylate lysine exhaust the xylose rapidly and the lysine reaction causes a pH reversal to the alkaline reaction similar to Shigella. Lactose and Sucrose are added in excess to prohibit this same reversion by lysine-positive coliforms. Sodium thiosulphate helps in reactivation of sulphur containing compounds and prevents the desiccation of these compounds during storage.

Xylose Lysine Deoxycholate Agar also forms the substrate for enzyme thiosulphate reductase, which breaks it to form H2S. Thiosulphate and ferric ammonium citrate are the H2S indicators in the medium. Sodium thiosulphate is also inactivator of halogens, mercurial and aldehyde and can minimize its toxicity in the testing sample, if any during microbial limit tests. Sodium chloride maintains the osmotic equilibrium in this medium. Phenol red is the pH indicator.

for selective di?erentiation and enrichment medium of Salmonella and Shigella species