Xylose Lysine Deoxycholate (XLD) Agar : Introduction, Principle, Preparation, Test procedure, Colony characteristics and Limitations
Introduction of Xylose Lysine Deoxycholate (XLD) Agar
Xylose Lysine Deoxycholate (XLD) Agaris a selective, differential and indicator medium for isolation of enteric pathogens. It also supports the growth of more fastidious enteric organisms. XLD agar was developed by Taylor. It was especially designed to allow the growth of Shigella species, and is a proven medium for the isolation of this organism. It has also been found to be an excellent medium for isolating Salmonellaspecies as well. Salmonella and Shigella species colony characteristics on XLD agar –
Salmonella Typhi: Red colonies with black centre
Salmonella Paratyphi: Only red colonies no black centre
Above picture is showing growth of Salmonella Typhi and Salmonella Paratyphi on XLD agar left to right respectively.
Principle of Xylose Lysine Deoxycholate (XLD) Agar
Xylose lysine deoxycholate (XLD) agar is a selective, differential and indicator medium. The selective agent is sodium deoxycholate, which inhibits the growth of gram-positive organisms. Xylose, lysine, lactose and sucrose act as both differential as well as carbohydrate source ingredients. Xylose which is fermented by most enterics except for Shigella species, and these colonies appear red on this medium as a result. A second differential mechanism for Salmonella is employed by the addition of lysine. Lysine decarboxylation reverts the pH of the medium to an alkaline condition. To avoid this reversal to a Shigella reaction, lactose and sucrose are added in excess. Medium is indicator due to having two indicators, one pH indicator (phenol red) and another hydrogen sulphide indicaor(ferric ammonium citrate).The addition of sodium thiosulfate and ferric ammonium citrate as a sulfur source and indicator, respectively, allows hydrogen sulfide (H2S) forming organisms to produce colonies with black centers, under alkaline conditions. Organisms which ferment xylose, are lysine decarboxylase-negative, and do not ferment lactose or sucrose cause an acid pH in the medium, and form yellow colonies e.g. Escherichia coli, Citrobacter andProteus species.
Composition of Xylose Lysine Deoxycholate (XLD) Agar
Sodium Thiosulfate: 6.8
Sodium Chloride: 5.0
Yeast Extract: 3.0
Sodium Deoxycholate: 2.5
Ferric Ammonium Citrate: 0.8
Deionized/distilled water: 1000 ml
Final pH should be 7.4 +/- 0.2 at 25ºC.
Preparation of Xylose Lysine Deoxycholate (XLD) Agar
Suspend 56.68 grams dehydrated powder XLD agar in 1000 ml distilled or deionized or purified water. Note: The amount of XLD agar varies manufacturer to manufacture e.g. Oxoid says 53 gm in 1 litre while Hardy Diagnostics 56.93 and Himedia 56.68.
Heat with frequent agitation until the medium boils.
Do not autoclave or over heat.
Transfer immediately to a water bath at 50°C.
After cooling, pour into sterile Petri plates.
It is advisable not to prepare large volumes that will require prolonged heating, thereby producing precipitate, thereby producing precipitate.
Storage and Shelf life of Xylose Lysine Deoxycholate (XLD) Agar
Store at 2-8ºC and away from direct light.
Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination.
Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.
Test procedure ( specimen/organism inoculation)
Allow the plates to warm at 37°C or to room temperature, and the agar surface to dry before inoculating.
Inoculate and streak the specimen as soon as possible after collection.
If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface.
Streak for isolation with a sterile loop.
Incubate plates aerobically at 35-37ºC. for 18-24 hours.
Examine colonial characteristics.
Colony Characteristics of various organisms in Xylose Lysine Deoxycholate (XLD) Agar
Salmonella: H2S positive Red colonies with black centers
Shigella species and Salmonella H2S negative: Red colonies
E. coli: Large, flat, yellow colonies
Proteus species: Red to Yellow colonies
Enterobacter and Klebsiella species: Mucoid, yellow colonies
Staphylococcus aureus: No growth
Salmonella Typhi black colonies, E. coli ( yellow colonies) and Klebsiella ( mucoid yellow colonies) on XLD agar as shown below video-
Modifications of Xylose Lysine Deoxycholate (XLD) Agar
XLD with Novobiocin (10.0 mg/mL novobiocin): XLD Agar with Novobiocin contains novobiocin, which is commonly used to inhibit the growth of Proteus species, and helps reduce the potential for false-positives from this organism.
Modified XLD (only 0.5 g/L sodium deoxycholate): Modified XLD Agar contains a reduced amount of sodium deoxycholate in order to permit the growth of a wider variety of enteric organisms normally inhibited on traditional XLD Agar.
Uses of Xylose Lysine Deoxycholate (XLD) Agar
XLD Agar is a selective, differential and indicator medium for the isolation of Gram-negative enteric pathogens from fecal specimens as well as other clinical material.
It is also recommended medium for the isolation of Salmonella and Shigella species.
It is also applicable for microbiological testing of foods, water and dairy products.
Limitations of Xylose Lysine Deoxycholate (XLD) Agar
Some Proteus strains may give red to yellow coloration with most colonies developing black centers, giving rise to false positive reactions.
Salmonella Paratyphi A, S. choleraesuis, S. pullorum and S. gallinarum may form red colonies without H2S, thus resembling Shigella species.
Some species of Salmonella may form red colonies without a black center, which resemble Shigella colonies. In addition, a few species of Shigella ferment lactose, and Salmonella that fail to decarboxylate lysine would not be detected on this medium.
Processing delays of over 2-3 hours of un-preserved stool specimens greatly jeopardizes the recovery of many enteric pathogens, as these organisms are very susceptible to the acidic changes that occur with a temperature drop of the feces.
Red, false-positive colonies may occur with Proteus, Providencia and Pseudomonas.
Incubation in excess of 48 hours may lead to false-positive results.
It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification.