XLD-Agar

The XLD agar ( xylose -lysine - deoxycholate agar ) is a nutrient medium for the cultivation and differentiation of gram-negative bacteria first. It was developed by W. J. Taylor ( 1965) and further developed in the next two years by Taylor, B. Harris and D. Schelhart.

Operation

Sodium deoxycholate acts as an inhibitor of Gram positive bacteria. The agar was left uninoculated to a pH of about 7.4, wherein the indicator phenol red colors the pale red soil. Fermented the applied bacterium carbohydrates (such as xylose, lactose or sucrose ), the pH and the pH indicator drops shows a color change to yellow. This is for example in the coliforms, E. coli, Enterobacter and Klebsiella the case, but not in Salmonella and Shigella, so that there is here no color change of the indicator phenol red.

Also contains the XLD agar, sodium thiosulfate, which metabolize as salmonella to hydrogen sulfide; This results in a central blackening of bacterial colonies, and allows the differentiation between Salmonella and Shigella. Other enterobacteriaceae such as Proteus vulgaris can utilize one or more of the carbohydrates contained under acid form, which leads to yellowing of the medium and is also capable of forming hydrogen sulfide, so that the colonies have a black center.

The amino acid lysine as a further component of the medium is degraded by some gram- negative bacteria with decarboxylation to cadaverine, this degradation increases the pH value of the medium and can be seen around the colonies by a color change of the indicator phenol red to pink.

Typical Composition

The soil mostly consists of ( in grams per liter):

  • Yeast extract 3.0
  • Sodium chloride 5.0
  • Xylose 3.75
  • Lactose 7.5
  • Sucrose 7.5
  • Lysine 5.0
  • Sodium deoxycholate, 1.0
  • Sodium thiosulfate 6.8
  • Ammonium iron (III ) citrate 0.8
  • Phenol red 0.08
  • Agar- agar 14.5
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