XLD Agar | Principle | Preparation | Interpretation

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Ⅰ. Overview

Ⅱ. Preparation / Composition

XLD dehydrated medium

Suspend the components, dehydrated powder, in water (53 grams in 1000 ml of purified / distilled water), Heat, stirring frequently, until the medium begins to boil(DO NOT OVERHEAT and DO NOT AUTOCLAVER).

Immediately transfer to a water bath at about 50 °, continue to stir until the medium cools to about 50 ° and pour into sterile Petri dishes.

Prepared XLD medium

Melt the contents of the vial in a water bath at 100 ° C (by loosening the partially removed cap) until complete dissolution. Cool immediately to 45-50 ° C, mix well, avoiding foaming and aseptically distribute into Petri dishes.

Ⅲ. Principle and interpretation

Xylose lysine deoxycholate (XLD) agar is a selective, differential and indicator medium. The three added carbohydrate sources are present in different concentrations, xylose is limited while lactose and sucrose are considered inexhaustible during the prescribed incubation period.

◈ Selectivity : The selective agent is sodium deoxycholate, a bile salt, which inhibits the growth of gram-positive organisms.

◈ Differentiation : Is done by three indicator systems

  • Xylose, lactose and sucrose, together with phenol red, are fermentable carbohydrates. Fermentation of sugars brings the pH of the medium to an acidic state and the color of the medium turns yellow
  • The decarboxylation of lysine, in the absence of lactose and sucrose fermentation, results in a return to an alkaline pH. This alkaline pH brings back the red color of the medium.
  • Sodium thiosulfate and ferric ammonium serve as indicators of hydrogen sulfide (H2S) production under alkaline conditions

Organisms unable to utilize carbohydrates, such as Shigella and Edwardsiella, will not produce any significant changes and therefore colonies and medium will remain red after incubation

Organisms capable of fermenting xylose only, such as Salmonella, will deplete the xylose supply and start using lysine thereby changing the pH to alkaline and the color returning to red. However, the presence of Salmonella and Edwardsiella spp is differentiated from that of shigella by an indicator of hydrogen sulfide.

Other uninhibited organisms capable of fermenting lactose or sucrose will produce yellow colonies due to continued acid production, the high level of acid produced prevents the pH from returning to an alkaline value, and d 'a resulting change in pH. These organisms include Escherichia, Klebsiella, Serratia, Citrobacter koseri, Yersinia enterocolitica.

Yellow colonies are also observed for lysine-negative organisms, such as Proteus species.

The inclusion of an H2S indicator system composed of sodium thiosulphate and ferric ammonium citrate improves the differentiation capacity of the preparation: this makes it possible to visualize the production of hydrogen sulphide, which causes the formation of colonies with black centers.

Bacteria Growth Results
Salmonella H2S positive, Edwardsiella Good Red colonies with black center
Shigella spp. et Salmonella H2S negative, Providencia Good Red colonies
E. coli Partial inhibition Yellow colonies
Enterobacter / Klebsiella Partial inhibition Yellow colonies / Yellow and mucoid colonies
Proteus Red to yellow colonies, some strains of Proteus will give colonies with a black center
Pseudomonas Partial inhibition Red colonies "Shigella-like"
Enterococcus Inhibited /
Gram-positive bacteria Inhibited /


Incubation for more than 48 hours may lead to false positive results. the color disappears after 24 hours, so observations should be made between 18 and 24 hours

Prolonged incubation beyond the recommended times is not prescribed as false positives may occur due to alkaline reversion of normally acidic colonies

Some species of Shigella (1%) ferment lactose and produce atypical yellow colonies

S. Paratyphi A, S. Choleraesuis, S. Pullorum and S. Gallinarum can form red colonies without H2S, thus resembling Shigella


  1. Color Atlas of Medical Bacteriology 4ed
  2. CDC - Proteus sur gelose XLD
  3. initialement formulé par Taylor W.I. 1965
  4. Thermo Fisher Scientific- Gélose XLD
  5. BD XLD Agar (Xylose-Lysine-Desoxycholate Agar)
  6. Handbook of Culture Media for Food Microbiology- Xylose lysine deoxycholate (XLD) agar
  8. Scharlau - Xylose Lysine Deoxycholate Agar (Eur. Pharm.)
  9. Liofilchem® - XLD Agar
  10. DALYNN - XLD Agar