Written and maintained by CASRAI Editorial Board
Last updated
Xylose lysine deoxycholate (XLD) agar is a selective and differential culture medium used to isolate enteric gram-negative bacilli, especially Salmonella and Shigella. Salmonella typically grows as red colonies, often with black centres from hydrogen sulfide; yellow colonies indicate organisms that ferment the sugars without decarboxylating lysine. Every colour is presumptive until biochemical or serological confirmation.
What XLD agar is
XLD agar (also written xylose lysine deoxycholate agar, xylose-lysine-desoxycholate agar, or agar XLD) is a plated medium for qualitative isolation of enteric gram-negative bacilli. Remel (Thermo Fisher) describes it in its instructions for use (IFU 1980, revised July 2025) as a medium for “selective and differential isolation” of these organisms. It was developed by Taylor (Am J Clin Pathol 1965;44:471-475) for enteric pathogens, especially Shigella, and was later found satisfactory for Salmonella from clinical specimens. HiMedia’s technical data sheet M031F describes its use for isolating and enumerating Salmonella Typhi and other Salmonella in food, water and dairy testing, citing FDA BAM and APHA methods.
Selective means the formulation suppresses organisms you do not want; differential means colony colour tells groups of organisms apart. For the broader family of plate types, see what an agar plate is, and for the lactose-based sibling, MacConkey agar.
Why each ingredient is there
The Remel IFU lists the classical formula per litre as lactose 7.5 g, sucrose 7.5 g, sodium thiosulfate 6.8 g, L-lysine 5.0 g, sodium chloride 5.0 g, xylose 3.5 g, yeast extract 3.0 g, sodium desoxycholate 2.5 g, ferric ammonium citrate 0.8 g, phenol red 0.08 g and agar 13.5 g, at pH 7.4 ± 0.2 and adjusted as required to meet performance standards. The table pairs each functional component with the job that Remel and HiMedia assign to it.
| Component | What it does |
|---|---|
| Xylose | Fermented by most enteric gram-negative bacilli other than Shigella, producing acid; this separates Shigella from the rest. |
| L-lysine | Differentiates Salmonella: its lysine decarboxylase raises the pH again once the xylose is used up, giving red colonies. |
| Lactose and sucrose (in excess) | Prevent lysine-positive coliforms from reverting to alkaline, so they stay yellow instead of mimicking Salmonella. |
| Sodium thiosulfate and ferric ammonium citrate | Hydrogen sulfide indicator system: H2S production under alkaline conditions gives black-centred colonies. |
| Sodium desoxycholate (deoxycholate) | Inhibits gram-positive organisms. |
| Phenol red | pH indicator: yellow in acid, red in alkaline conditions. |
Spelling differs by vendor: Remel writes “desoxycholate”, HiMedia “deoxycholate”. Both names refer to the same bile-salt component and the same medium.
Reading colonies: Salmonella on XLD agar and its look-alikes
Remel and HiMedia both describe colonies by colour; the table below combines their descriptions. Incubation is aerobic at 33-37 °C for 18-48 hours (Remel); full colour development may need 48 hours. HiMedia’s food-testing workflow reads plates at 24 ± 2 hours at 35 °C.
| What you see | Usual meaning | Look-alikes and caveats |
|---|---|---|
| Red colonies with black centres | Typical presumptive Salmonella; HiMedia notes many cultures show glossy black centres or are almost entirely black. | Some Proteus develop black centres (false positives). |
| Red or pink colonies, no black centre | Remel: Shigella (transparent colonies that appear red from the medium) or Salmonella without a black centre. | HiMedia lists S. Paratyphi A, S. Choleraesuis, S. Pullorum and S. Gallinarum as red, H2S-negative forms that can resemble Shigella. Pseudomonas and Providencia may also be red. |
| Yellow colonies | Xylose, lactose or sucrose fermenters that are lysine-negative; acid keeps the indicator yellow. | HiMedia notes a few Salmonella cultures atypically produce yellow colonies. Some Proteus appear red-to-yellow with black centres. |
Why a black centre alone does not identify Salmonella
Non-pathogenic H2S producers do not decarboxylate lysine, so the acid they produce suppresses blackening (HiMedia). The black centre therefore needs the red, alkaline background to appear, and even then it is shared by some Proteus strains.
Quality control organisms
The Remel IFU lists CLSI-recommended control strains read at 18-24 hours at 33-37 °C:
| Control organism | Expected result |
|---|---|
| Salmonella Typhimurium ATCC 14028 | Growth; red colonies with black centres |
| Shigella flexneri ATCC 12022 | Growth; red colonies |
| Enterococcus faecalis ATCC 29212 | Inhibition |
| Escherichia coli ATCC 25922 | Partial inhibition; yellow to yellow-red colonies |
Together these four cover the positive reaction, the red-without-black reaction, the selective effect on gram-positives, and the yellow lysine-negative fermenter.
Where it matters for labs and clinics
Practical checklist
- Treat results as presumptive. Remel states that further biochemical and/or serological testing is required for definitive identification, because Pseudomonas and Proteus may resemble Salmonella or Shigella.
- Pair with other media. Remel advises inoculating both selective and nonselective media; an enrichment broth such as GN Broth may be used alongside XLD and a nonselective plate.
- Allow for slow colour. Plates read at 18-24 hours may still be developing; Remel notes up to 48 hours may be needed.
- Do not autoclave powder-prepared medium. HiMedia’s data sheet says to heat with frequent agitation to boiling, not to autoclave or overheat, then cool in a 50 °C water bath before pouring; slight precipitation is inherent and does not affect performance. The same sheet describes the medium as general purpose, which may not support fastidious organisms.
- Run the control panel in the table above for each lot or quality schedule your laboratory follows.
In food microbiology, HiMedia’s described workflow pre-enriches a 25 g sample, moves to tetrathionate broth at 43 °C, and then streaks onto XLD; see food microbiology testing. Handling of cultures that may contain enteric pathogens falls under your biosafety programme; see biosafety level 2 requirements and aseptic technique. Used plates and cultures are decontaminated by validated methods, for example autoclave cycle types.
Common misunderstandings
- “Black centre means Salmonella.” Some Proteus also form black-centred colonies.
- “Red means Shigella.” Red colonies also arise from H2S-negative Salmonella serovars and from Pseudomonas or Providencia.
- “Yellow rules out Salmonella.” A few Salmonella cultures are atypically yellow, so atypical lactose-positive, H2S-negative isolates also need confirmation.
- “XLD grows everything enteric.” It is selective: gram-positives are inhibited and E. coli is partially inhibited.
FAQ
What is XLD agar used for?
Selective and differential isolation of enteric gram-negative bacilli, especially Salmonella and Shigella, from clinical, food, water and dairy samples.
What does Salmonella look like on XLD agar?
Typically red colonies with black centres, though some are red without a black centre and some are almost entirely black. A few cultures are atypically yellow.
Why does XLD agar turn colonies yellow?
Organisms that ferment xylose, lactose or sucrose and cannot decarboxylate lysine leave the medium acidic, so phenol red stays yellow.
Is XLD agar the same as xylose lysine deoxycholate agar?
Yes. XLD agar, agar XLD, and xylose lysine deoxycholate (or desoxycholate) agar all name the same medium.
Can I identify Salmonella from an XLD plate alone?
No. Remel and HiMedia both state that biochemical and/or serological confirmation is required.
Sources
- Remel / Thermo Fisher, XLD (Xylose-Lysine-Desoxycholate) Agar, IFU 1980, revised 11 July 2025: https://documents.thermofisher.com/TFS-Assets/LSG/manuals/IFU1980.pdf (checked 29 October 2026).
- HiMedia Laboratories, XLD Agar, Technical Data M031F: https://www.himedialabs.com/media/TD/M031F.pdf (checked 29 October 2026).
- Taylor WI. Am J Clin Pathol 1965;44:471-475, as cited in the Remel IFU.
