Xylose Lysine Deoxycholate (XLD) Agar is a selective and differential culture medium used to isolate *Salmonella* and *Shigella* species from stool, food and environmental samples. It contains xylose, lactose, sucrose, L‑lysine, sodium thiosulfate, ferric ammonium citrate and the bile salt sodium deoxycholate, with phenol red as a pH indicator. The formulation suppresses most gram‑positive bacteria and many nonpathogenic gram‑negative enterics while allowing pathogenic enteric pathogens to be distinguished by their biochemical reactions.
Composition, reactions and interpretation
XLD agar relies on sequential metabolic events to differentiate colonies. Xylose is fermented by most enteric bacteria, producing acid and turning the medium yellow. *Shigella*, which cannot ferment xylose, forms red colonies due to the unchanged pH. *Salmonella* ferments xylose initially but then decarboxylates lysine, reversing the pH shift and returning the medium to red; these colonies often have black centers caused by hydrogen sulfide production from sodium thiosulfate reacting with ferric ions. Lactose and sucrose are included to quickly acidify the medium for nonpathogenic coliforms, which remain yellow. Sodium deoxycholate inhibits gram‑positive organisms and many commensal gram‑negative rods, increasing selectivity. Phenol red provides a clear color change between acidic (yellow) and alkaline (red) conditions. Because XLD is moderately selective, heavily contaminated specimens may require enrichment or serial plating to recover low numbers of pathogens.
Colony morphology and laboratory use
In routine clinical microbiology, XLD plates are inoculated with stool suspensions and incubated at 35‑37°C for 18‑24 hours. *Salmonella enterica* typically appears as red colonies with black centers, whereas *Shigella* species form small, translucent red colonies without blackening. *Escherichia coli* and other lactose fermenters produce yellow colonies due to sustained acid production. Quality control strains such as *Salmonella Typhimurium* and *Shigella flexneri* are used to verify performance of each batch. The medium is included in standardized protocols by the U.S. Food and Drug Administration and International Organization for Standardization for detection of enteric pathogens in foods. Variants like XLT4 agar enhance selectivity for *Salmonella* in poultry testing. Proper interpretation of colony color, size and H2S production guides subsequent confirmatory tests such as biochemical panels or serotyping.
XLD agar combines selective agents and multiple carbohydrate substrates to differentiate pathogenic *Salmonella* and *Shigella* from nonpathogenic enteric flora in stool and food cultures. Its straightforward colony color patterns and hydrogen sulfide indicator make it a staple in clinical and food microbiology laboratories.
Related Terms: Selective medium, *Salmonella*, *Shigella*, Enteric bacteria, H2S production