Milieu sélectif pour isolement et la différenciation des espèces pathogènes tel que Shigella et Providencia.

XLD Agar conforms with specifications of The United States Pharmacopeia (USP), is the complete Xylose Lysine Desoxycholate Agar, a moderately selective medium recommended for isolation and differentiation of enteric pathogens, especially Shigella species.

Identity Specifications :
Difco™ XLD Agar
Dehydrated Appearance: Pink, free-flowing, homogeneous.
Solution: 5.7% solution, soluble in purified water upon boiling. Solution is red, very slightly to slightly opalescent.
Prepared Appearance: Red, slightly opalescent.
Reaction of 5.7% (Solution at 25°C): pH 7.4 ± 0.2

Formulae
Difco™ XLD Agar - Approximate Formula* Per Liter :
Xylose : 3.75 g
L-Lysine : 5.0 g
Lactose : 7.5 g
Saccharose : 7.5 g
Sodium Chloride : 5.0 g
Yeast Extract : 3.0 g
Phenol Red : 0.08 g
Sodium Desoxycholate : 2.5 g
Sodium Thiosulfate : 6.8 g
Ferric Ammonium Citrate : 0.8 g
Agar : 15.0 g

*Adjusted and/or supplemented as required to meet performance criteria.

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Summary and Explanation
A wide variety of media have been developed to aid in the selective isolation and differentiation of enteric pathogens. Due to the large numbers of different microbial species and strains with varying nutritional requirements and chemical resistance patterns, investigators have developed various formulae to meet general as well as specific needs relative to isolation and identification of the microorganisms.
XL Agar Base was developed by Taylor1 for the nonselective isolation and differentiation of gram-negative enteric bacilli.
It is particularly recommended for obtaining counts of enteric organisms. This medium can be rendered moderately selective for enteric pathogens, particularly Shigella, by the addition of sodium desoxycholate (2.5 g/L) to make XLD Agar.

XL Agar Base can be made selective for Salmonella by adding 1.25 mL/L of 1% aqueous brilliant green to the base prior to autoclaving. Its use is recommended for Salmonella isolation after selenite or tetrathionate enrichment in food analysis; both coliforms and Shigella are inhibited.
XLD Agar was developed by Taylor in order to increase the efficiency of the isolation and identification of enteric pathogens, particularly Shigella.
The pathogens are differentiated not only from the nonpathogenic lactose fermenters but also from  many nonpathogens which do not ferment lactose or sucrose.
Additionally, the medium was formulated to increase the frequency of growth of the more fastidious pathogens,1 which in other formulations have often failed to grow due to the inclusion of excessively toxic inhibitors. The results obtained in a number of clinical evaluations have supported the claim for the relatively high efficiency of XLD Agar in the primary isolation of Shigella and Salmonella.

XLD Agar is included in the USP microbial limit test for screening specimens for the presence or absence of Salmonella and is recommended for the testing of foods, dairy products and water.

Principles of the Procedure
Xylose is incorporated into the medium since it is fermented by practically all enterics except for the shigellae, and this property enables the differentiation of Shigella species. Lysine is included to enable the Salmonella group to be differentiated from the nonpathogens since, without lysine, salmonellae rapidly would ferment the xylose and be indistinguishable from nonpathogenic species. After the salmonellae exhaust the supply of xylose, the lysine is attacked via the enzyme, lysine  decarboxylase, with reversion to an alkaline pH which mimics the Shigella reaction. To prevent similar reversion by lysinepositive coliforms, lactose and sucrose (saccharose) were added to produce acid in excess.
To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulfate and ferric ammonium citrate, is included for the visualization of the hydrogen sulfide produced, resulting in the formation of colonies with black centers. The nonpathogenic H2S producers do not decarboxylate lysine; therefore, the acid reaction produced by them prevents the blackening of the colonies.
XLD Agar is both a selective and differential medium. It utilizes sodium desoxycholate as the selective agent and, therefore, it is inhibitory to gram-positive microorganisms.

Directions for Preparation from Dehydrated Product
Difco™ XLD Agar
1. Suspend the powder in 1 L of purified water: Difco™ XLD Agar – 57 g; Mix thoroughly.
2. Heat with agitation just until the medium boils. DO NOT OVERHEAT. DO NOT AUTOCLAVE.
3. Cool to 45-50°C in a water bath and use immediately.Overheating causes precipitation.
4. Test samples of the finished product for performance using stable, typical control cultures.

Procedure
Use standard procedures to obtain isolated colonies from specimens. A nonselective medium should also be streaked to increase the chance of recovery when the population of gramnegative organisms is low and to provide an indication of other organisms present in the specimen. Incubate plates, protected from light, at 35 ± 2°C for 18-24 hours. Colonies on XLD agar may require 48 hours incubation for full color development.

Expected Results  :

Degradation of xylose, lactose and sucrose generates acid products,  causing a color change in the medium from red to yellow.

Hydrogen sulfide production under alkaline conditions causes colonies to develop black centers. This reaction is inhibited by the acid conditions that accompany carbohydrate fermentation.

Lysine decarboxylation in the absence of lactose and sucrose fermentation causes reversion to an alkaline condition and the color of the medium changes back to red.

Typical colonial morphology and reactions on XLD Agar are  as follows:
► E.coli : Large, flat, yellow; some strains may be inhibited
► Enterobacter/ Klebsiella : Mucoid, yellow
► Proteus : Red to yellow; most strains have black centers
► Salmonella : Red-yellow with black centers  Shigella, Salmonella
► H₂S-negative : Red
► Pseudomonas : Red
► Gram-positive bacteria : No growth to slight growth