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xld agar

Xylose Lysine Deoxycholate (XLD) Agar: Principle, Composition & Uses

Published: 27th Aug 2025, 16:20

You know that gnawing feeling when someone gives you a sharp look, and you just know they have figured something out about you? Can you imagine agar giving that same look to a bacterium? Sounds impossible? Not when xld agar is in question, the red plate with a sixth sense, used to identify disease-causing bacteria like Salmonella typhi and Shigella.

They say looks can be deceiving, but not here. But this medium doesn’t just support bacterial growth; it spills their secrets through colour codes, phantasmal lines, and sulphur trails. Taylor developed Xylose Lysine Deoxycholate Agar and is now widely adopted across microbiological laboratories. 

In this blog, we’ll decode what makes TM Media’s xld agar (TM 492) such a powerful tool in identifying enteric pathogens. We’ll break down its principle, composition, uses, and even the colourful drama it unfolds in your Petri dish.

What is XLD Agar?

XLD Agar is a selective and differential medium. It was designed for the isolation and enumeration of Salmonella species, such as Salmonella typhi, and Shigella, from clinical specimens, food products, dairy samples, and various water sources.

It is formulated to allow enteric pathogens to reveal their true nature and metabolic identities while keeping nosy bystanders out of the picture, serving a dual purpose: inhibiting the growth of Gram-positive bacteria (non-target organisms) while allowing specific reactions that help distinguish between distinct pathogens.

XLD Agar is exceptionally effective in the isolation and differentiation of the Enterobacteriaceae family based on their ability to ferment specific sugars, decarboxylate amino acids, as well as their ability to produce hydrogen sulphide gas (H₂S).

Principle of XLD Agar

XLD Agar works through a combination of reactions, such as decarboxylation and selective inhibition. Its function is discussed in detail below:

  • Xylose Fermentation: Most enteric bacteria, including Salmonella, initially ferment xylose by utilizing enzymes such as xylose isomerase and xylulokinase for energy production, which lowers the pH and turns the phenol red indicator yellow.
  • Lysine Decarboxylation: Salmonella spp. revert the pH of the medium to alkaline by decarboxylating lysine and give out red colonies.
  • H₂S Production: Many Salmonella strains form colonies with characteristic black centers due to the reaction with ferric ammonium citrate by reducing thiosulphate to hydrogen sulphide.
  • Sodium Deoxycholate inhibits the growth of Gram-positive bacteria.
  • Differentiation:

E. coli and other coliforms typically form yellow colonies due to the fermentation of xylose, lactose, and sucrose.

Shigella spp. do not ferment xylose or produce H₂S and form red colonies without black centers.

Composition of XLD Agar (per liter)

COMPONENTSQUANTITY FUNCTION
Yeast Extract3.000 gSource of nitrogen and growth factors
L-Lysine5.000 gIdentifies lysine decarboxylation by Salmonella
Lactose and Sucrose 7.500 gExcess fermentable sugars to distinguish coliforms
Xylose 3.500 gFermentable sugar, not fermented by Shigella
Sodium Chloride 5.000 gMaintains osmotic equilibrium
Sodium Deoxycholate 2.500 gSelective agent against Gram-positive bacteria
Sodium Thiosulphate 6.800 gSulphur source for H₂S detection
Ferric Ammonium Citrate 0.800 gDetects H₂S as black precipitate
Phenol Red0.080 gpH indicator
Agar 15.000 gSolidifying agent

Preparation and Usage

To prepare TM Media’s xld agar:

  1. Suspend 56.68 g of dehydrated medium in 1000 mL of purified or distilled water.
  2. Heat with frequent agitation until the solution just begins to boil. Do not autoclave, as overheating may damage sensitive components.
  3. Transfer the medium to a 50°C water bath and pour it into sterile Petri plates.
  4. Allow the medium to solidify, and store it in a cool, dry place until use.

Inoculate the sample on the surface of the prepared XLD plates and incubate at 35–37°C for 18–72 hours. Slight precipitation in the medium is normal and does not impact performance.

Cultural Characteristics

ORGANISMCOLONY APPEARANCEINFERENCES
Salmonella TyphimuriumRed colonies with black centersXylose fermentation, lysine +, H₂S +
Salmonella Paratyphi ARed coloniesLysine +, H₂S –
Salmonella Paratyphi BRed colonies with black centersH₂S +
Shigella dysenteriaeRed coloniesNo xylose fermentation, lysine –
E. coliYellow coloniesSugar fermenter, no H₂S
Proteus vulgarisGrey with black centersH₂S +, variable sugar fermentation
Enterobacter sppYellow coloniesLactose/sucrose fermenter
Staphylococcus aureusInhibitedGram-positive; suppressed by deoxycholate
Enterococcus faecalisInhibitedSame as above

Applications of XLD Agar

Clinical Microbiology

  • It is used in clinical diagnostic laboratories for the isolation of Salmonella spp. and Shigella spp. from medical samples.
  • It is used in the diagnosis of enteric fever and bacterial dysentery.

Food and Beverage Industry

  • It is used in the detection of enteric pathogens in raw and processed food products.
  • It is applied in Hazard Analysis and Critical Control Point programs and International food safety regulations.

Water and Environmental Testing

  • It is used in the monitoring of drinking water, surface water and wastewater for faecal contamination to detect pathogens to prevent infections through faecal-oral route.
  • It is used in public health surveillance and outbreak investigations.

Dairy Industry

  • It is used in the microbiological testing of milk and other dairy products, such as cheese and yogurt.
  • It is often employed in the detection of Salmonella and Shigella contamination in production.

Pharmaceuticals and Cosmetics

  • It is an essential tool in Quality assurance in non-sterile pharmaceutical products.
  • It is used in the screening of raw materials and environmental surfaces in cleanrooms.

Why Choose XLD Agar (TM 492) TM Media?

TM Media’s xld agar (TM 492) stands out due to its meticulously optimized formulation and proven performance in the industry. 

The formulation is aligned with global standards and has been validated through quality control testing using reference strains such as Salmonella Typhimurium (ATCC 14028) and Shigella dysenteriae (ATCC 13313).

Additional advantages include:

  • Enhanced selectivity for Salmonella and Shigella
  • Reliable colony differentiation
  • Long shelf life
  • Available in multiple packaging sizes (100 g and 500 g) for flexibility in lab usage.
  • Comprehensive technical support and documentation.

These attributes make TM Media’s xld agar a dependable choice for laboratories focused on food safety, clinical diagnostics, and environmental microbiology.

Product Specifications

FeatureSpecification
Product CodeTM 492
AppearanceLight yellow to pink, free-flowing, homogeneous powder
Prepared MediumRed-coloured, clear to slightly opalescent gel
pH (at 25°C)7.4 ± 0.2
Storage Conditions25–30°C, tightly sealed container, away from direct sunlight
Shelf Life4 years (when stored under proper conditions)

XLD Agar will continue to serve as a fundamental component in laboratory workflows for detecting critical pathogens. Its selectivity, differential capabilities, and ease of interpretation are proven efficient in laboratories worldwide.

Frequently Asked Questions (FAQs)

XLD Agar works through a combination of fermentation reactions, decarboxylation, and selective inhibition. Xylose Fermentation: Most enteric bacteria, including Salmonella, initially ferment xylose by utilizing enzymes such as xylose isomerase and xylulokinase for energy production, which lowers the pH and causes the phenol red indicator to turn yellow. Lysine Decarboxylation: Salmonella species revert the pH of the medium to alkaline by decarboxylating lysine, resulting in red colonies. H₂S Production: Many Salmonella strains form colonies with characteristic black centers due to the reaction with ferric ammonium citrate by reducing thiosulphate to hydrogen sulphide. Sodium Deoxycholate inhibits the growth of Gram-positive bacteria.
Colonies with characteristic black centers on XLD Agar are formed due to the reaction with ferric ammonium citrate by reducing thiosulphate to hydrogen sulphide.
To prepare XLD Agar: Suspend 56.68 g of dehydrated medium in 1000 mL of purified or distilled water. Heat with frequent agitation until the solution just begins to boil. Do not autoclave, as overheating may damage sensitive components. Transfer the medium to a 50°C water bath and pour it into sterile Petri plates. Allow the medium to solidify, and store it in a cool, dry place until use. Inoculate the sample on the surface of the prepared XLD plates and incubate at 35–37°C for 18–72 hours. Slight precipitation in the medium is normal and does not impact performance.
It is used in the clinical and medical laboratory for the isolation and differentiation of Salmonella typhi and Shigella from stool samples and the diagnosis of enteric fever and bacterial dysentery.

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