Abbreviated XLD agar, Xylose Lysine Deoxycholate agar is another selective and differential growth medium mostly used in the isolation of Salmonella and Shigella species from clinical and non clinical samples. It has a pH of approximately 7.4 that gives it a bright pink or red appearance due to the indicator phenol red
It is recommended for the identification of Enterobacteriaceae and for the microbiological testing of foods, water and dairy products because it contains component that inhibit the growth of unwanted microorganism while favoring the growth of specific microorganisms.
With XLD Agar, the suspected microorganism are differentiated not only from selective inhibitory component such as lactose, xylose fermentation but also from the differential ability to produce Hydrogen sulfide from thiosulfate which leads to the production of black colonies
The growth media exhibits increased selectivity and sensitivity as compared to other culture media such as Salmonella-Shigella Agar, EMB Agar and Bismuth Sulphite Agar. The media formulation does not allow the overgrowth of other organisms over Salmonella and Shigella
Principle of XLD Agar
- It contains yeast extract, which provides nitrogen and vitamins required for growth.
- The presence of sugars such as xylose, lactose and sucrose provide sources of fermentable carbohydrates but Xylose is mainly incorporated into the medium because it is not fermented by Shigellae but practically by all enteric microorganism making Xylose a differential sugar in the medium which helps in the differentiation of Shigella species from salmonella species.
- It contains Sodium chloride, which maintains the osmotic balance of the medium while Lysine differentiates Salmonella species from the non-pathogens.
- Since Salmonella species are xylose fermenters,They rapidly exhaust the xylose supply of the medium
- Lysine is decarboxylated by an enzyme lysine decarboxylase to form amines with reversion to an alkaline pH that mimics the Shigella reaction. However, to prevent this reaction by lysine-positive coliforms, lactose and sucrose are added to produce acid in excess.
- The Degradation or fermentation of xylose, lactose and sucrose producing acid causes a change in color of the phenol red indicator to yellow.
- Bacteria that decarboxylate lysine to cadaverine (foul smelling di-amine) can be recognized by the appearance of a red colouration around the colonies due to an increase in pH. These reactions can proceed simultaneously or successively, and this may cause the pH indicator to exhibit various shades of colour or it may change its colour from yellow to red on prolonged incubation.
- To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulphate and ferric ammonium citrate, is included for the visualization of hydrogen sulfide produced, resulting in the formation of colonies with black centers. The non-pathogenic H2S producers do not decarboxylase lysine; therefore, the acid reaction produced by them prevents the blackening of the colonies
- XLD Agar is both selective and differential medium. It utilizes sodium deoxycholate as the selective agent and therefore it is inhibitory to gram-positive microorganisms.
Composition of XLD Agar
The following ingredients are used for the composition of XLD Agar.
|Ferric ammonium citrate||0.800|
|Final PH at Temperature (25°c)||7.4+/-0.2|
How to Prepare XLD Agar
The preparation of XLD Agar depends on each laboratory workload or the numbers of culture plates available. Anyway, note that a standard Petri dish is equivalent to 20ml when poured completely.
- Suspend 56.68 grams in 1000 ml distilled water.
- Heat with frequent agitation until the medium boils.
- DO NOT AUTOCLAVE OR OVERHEAT.
- Transfer immediately to a water bath at 50°C.
- After cooling, pour into sterile Petri plates.
NB: It is advisable not to prepare large volumes that will require prolonged heating, thereby producing precipitate in the process.
NB: It is also advisable to incubate a petri dish for 24hour for quality control in order to avoid wastage of resources and ease early detection of contamination after the preparation.
Appearance of Medium after preparation
Red colored clear to slightly opalescent gel forms in Petri plates
Type of Inoculums
- water samples
Plate reading of Colonies morphology on XLD Agar
|Organisms||Color of Colonies|
|Proteus vulgaris||Grey with black centers|
|Salmonella Paratyphi A||Red|
|Salmonella Paratyphi B||Red with black centers|
|Salmonella Enteritidis||Red with black centers|
|Salmonella Typhi||Red with black centers|
|Shigella dysenteriae A||Red|
|Staphylococcus aureus||No Growth (Inhibited)|
|Enterococcus faecalis A||No Growth (Inhibited)|
|Salmonella Typhimurium||Red with black centers|
|Salmonella Abony||Red with black centers|
- Some Proteus strains may give red to yellow coloration with most colonies developing black centers, giving rise to false positive reactions.
- Non-enterics like Pseudomonas and Providencia may exhibit red colonies.
- Paratyphi A, S. Choleraesuis, S. Pullorum and S. Gallinarum may form red colonies without H2S, thus resembling Shigella species
Gram-positive bacteria are inhibited in this medium due to the presence of sodium deoxycholate
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