Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Xylose-Lysine-Deoxycholate (XLD) agar is a selective growth medium used for isolating and identifying gram-negative enteric pathogens. It differentiates bacteria based on their ability to ferment xylose, decarboxylate lysine, and reduce sulfur.
5 Must Know Facts For Your Next Test
XLD agar is primarily used to isolate Shigella and Salmonella species.
Phenol red is the pH indicator in XLD agar; it turns yellow in acidic conditions and red in alkaline conditions.
Salmonella colonies typically appear red with black centers due to hydrogen sulfide production.
Shigella colonies appear red or colorless because they do not ferment xylose or produce hydrogen sulfide.
The deoxycholate component of the medium inhibits the growth of gram-positive bacteria.
A selective and differential culture medium designed to isolate gram-negative enteric bacteria and differentiate them based on lactose fermentation.
Eosin Methylene Blue (EMB) Agar: A differential medium used for the isolation of fecal coliforms that differentiates organisms based on their ability to ferment lactose.
Hektoen Enteric (HE) Agar: A selective and differential medium primarily used to recover Salmonella and Shigella from patient specimens.