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.
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).
XLD Agar works through a combination of reactions, such as decarboxylation and selective inhibition. Its function is discussed in detail below:
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.
| COMPONENTS | QUANTITY | FUNCTION |
| Yeast Extract | 3.000 g | Source of nitrogen and growth factors |
| L-Lysine | 5.000 g | Identifies lysine decarboxylation by Salmonella |
| Lactose and Sucrose | 7.500 g | Excess fermentable sugars to distinguish coliforms |
| Xylose | 3.500 g | Fermentable sugar, not fermented by Shigella |
| Sodium Chloride | 5.000 g | Maintains osmotic equilibrium |
| Sodium Deoxycholate | 2.500 g | Selective agent against Gram-positive bacteria |
| Sodium Thiosulphate | 6.800 g | Sulphur source for H₂S detection |
| Ferric Ammonium Citrate | 0.800 g | Detects H₂S as black precipitate |
| Phenol Red | 0.080 g | pH indicator |
| Agar | 15.000 g | Solidifying agent |
To prepare TM Media’s xld agar:
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.
| ORGANISM | COLONY APPEARANCE | INFERENCES |
| Salmonella Typhimurium | Red colonies with black centers | Xylose fermentation, lysine +, H₂S + |
| Salmonella Paratyphi A | Red colonies | Lysine +, H₂S – |
| Salmonella Paratyphi B | Red colonies with black centers | H₂S + |
| Shigella dysenteriae | Red colonies | No xylose fermentation, lysine – |
| E. coli | Yellow colonies | Sugar fermenter, no H₂S |
| Proteus vulgaris | Grey with black centers | H₂S +, variable sugar fermentation |
| Enterobacter spp | Yellow colonies | Lactose/sucrose fermenter |
| Staphylococcus aureus | Inhibited | Gram-positive; suppressed by deoxycholate |
| Enterococcus faecalis | Inhibited | Same as above |
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:
These attributes make TM Media’s xld agar a dependable choice for laboratories focused on food safety, clinical diagnostics, and environmental microbiology.
| Feature | Specification |
| Product Code | TM 492 |
| Appearance | Light yellow to pink, free-flowing, homogeneous powder |
| Prepared Medium | Red-coloured, clear to slightly opalescent gel |
| pH (at 25°C) | 7.4 ± 0.2 |
| Storage Conditions | 25–30°C, tightly sealed container, away from direct sunlight |
| Shelf Life | 4 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.
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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