XLD agar

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

Mode of action

Colonies of Serratia marcescens on XLD agar, with positive results for acid formation through carbohydrate utilization.
Colonies of Salmonella on XLD agar, with positive results for H 2 S formation.

Sodium deoxycholate acts as an inhibitor against gram-positive bacteria. Uninoculated, the nutrient medium has a pH value of around 7.4, at which the pH indicator phenol red turns the nutrient medium pale red. If the applied bacterium ferments carbohydrates ( e.g. xylose , lactose or sucrose ), the pH value drops and the pH indicator changes color to yellow. This is the case, for example, with the E. coli , Enterobacter and Klebsiella coliforms , but not with Salmonella and Shigella , so that the phenol red indicator does not change color.

The XLD agar also contains sodium thiosulphate , which, for example, metabolizes salmonella to hydrogen sulphide ; this leads to a central black coloration of the bacterial colonies and enables the differentiation between salmonella and shigella. Other Enterobacteriaceae , such as B. Proteus vulgaris can utilize one or more of the carbohydrates contained in it with acid formation, which leads to the yellowing of the medium and are also able to form hydrogen sulfide, so that the colonies have a black center.

The amino acid lysine as a further component of the medium is broken down by some gram-negative bacteria with decarboxylation to cadaverine . This breakdown increases the pH value of the medium and can be recognized around the colonies by the color change of the phenol red to pink indicator.

Typical composition

The nutrient medium usually consists of (data in grams per liter ):

See also

Individual evidence

  1. ↑ a b c Technical information XLD agar from Merck KGaA, accessed on February 11, 2013.
  2. ↑ Zajc-Satler J, Gragas AZ: Xylose lysine deoxycholate agar for the isolation of Salmonella and Shigella from clinical specimens . In: Zentralblatt für Bakteriologie [Orig A] , 1977, 237 (2-3), pp. 196-200.