The practical value of X.L.D. Agar Culture Media is not the plate or broth alone; it is the reproducibility of the susceptibility endpoint produced from it. The discussion below connects its documented role with preparation variables, controls, interpretation, and the limits of inference. Catalogue 610060 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
XLD (Xylose Lysine Deoxycholate) Agar is a selective medium used for the isolation and differentiation of pathogen Enterobacteriaceae, especially salmonellae and shigellae from food, environmental samples and clinical specimens. XLD Agar is formulated according to ISO 6579 and ISO 21567 for the detection of Salmonella and Shigella spp, respectively.
Method principle
Yeast extract is a source of vitamins, particularly of B-group. Sodium chloride maintains the osmotic balance of the medium. Xylose, lactose and sucrose are the fermentable carbohydrates. Lysine is the decarboxylase substrate. Sodium thiosulfate and ferric ammonium serve as indicators of hydrogen sulphide production under alkaline conditions. Phenol red is the pH indicator. Sodium deoxycholate is the selective agent inhibiting most Gram-positive bacteria. Agar is the solidifying agent.
Formula and functional interpretation
(g/l) Yeast Extract 3.0 Sodium Chloride 5.0 Xylose 3.75 Lactose 7.5 Sucrose 7.5 L-Lysine 5.0 Sodium Thiosulfate 6.8 Iron(III) Ammonium Citrate 0.8 Phenol Red 0.08 Sodium Deoxycholate 1.0 Agar 15.0 Final pH 7.4 ± 0.2 at 25°C
Review the nutritive, selective, differential, buffering, reducing, and indicator components against the expected target and background flora. Preparation errors can change recovery, inhibition, colour development, or endpoint visibility even when incubation is correct.
Preparation and execution: controlled method points
Preparation: Dehydrated medium Suspend 55.4 g of the powder in 1 liter of distilled or deionized water. Mix well. Heat to boil shaking frequently until completely dissolved. DO NOT AUTOCLAVE. Medium in bottles Melt the content of the bottle in a water bath at 100°C (loosing the cap partially removed) until completely dissolved. Immediately cool at 45-50°C, mix well avoiding foam formation and aseptically distribute into Petri dishes.
Procedure: Inoculate the plates by spread method. Incubate aerobically at 37 ± 1°C for up to 48 hours.
Translate the method into controlled worksheet fields: catalogue and lot, preparation mass or volume, water quality, pH where applicable, heating or sterilization, supplements, dispense volume, preparation date, expiry assignment, incubation atmosphere and temperature, reading window, equipment, and analyst. Record departures as deviations instead of silently normalizing them.
Quality control: what the control result protects
Plates are inoculated with the microbial strains indicated in the QC table. Inoculum for productivity: ≤100 CFU Inoculum for selectivity: >103 CFU Incubation conditions: aerobically at 37 ± 1°C for 24 ± 3 hours. QC Table. Microorganismo Growth Specification Salmonella Typhimurium WDCM 00031 Good Red colonies with black center Salmonella Enteritidis WDCM 00030 Good Red colonies with black center Shigella flexneri ATCC 12022 Good Red colonies Escherichia coli WDCM 00013 Poor Yellow colonies Enterococcus faecalis WDCM 00087 Inhibited --- WARNING AND
Challenge both sides of performance. Include a target reaction demonstrating recovery and, when relevant, a non-target or alternate reaction challenging inhibition or differentiation. Record expected and observed responses, acceptance criteria, strain and passage, lot, incubation, and disposition. A failed control blocks result interpretation until the cause is resolved.
Interpretation and confirmation pathway
After incubation observe the color of the colonies and interpret the results as indicated in the ID Table. ID Table. Microorganism
Record the raw observation before the conclusion: morphology or colour, growth pattern, endpoint, reaction intensity, reading time, and control response. Presumptive reactions remain presumptive. Route atypical, weak, mixed, or delayed reactions through the confirmation step in the current method.
Storage, stability, and pre-use inspection
The powder is very hygroscopic, store the powder at 10-30°C, in a dry environment, in its original container tightly closed. Store bottles, tubes and prepared plates at 10-25°C away from light. Do not use the product beyond its expiry date on the label or if product shows any evidence of contamination or any sign of deterioration.
Receipt and storage records should reconstruct temperature exposure, lot rotation, expiry, container integrity, and time outside controlled storage. Inspect colour, clarity, fill volume, gel integrity, contamination, dehydration, and precipitate before use.
Limitations and boundaries of use
The product does not contain hazardous substances in concentrations exceeding the limits set by current legislation and therefore is not classified as dangerous. It is nevertheless recommended to consult the safety data sheet for its correct use. The product is intended for in vitro diagnostic use and must be used only by properly trained operators.
Applicability depends on matrix, target organism, interfering flora, inoculum, validated method, and regulatory setting. Do not transfer conditions or acceptance criteria from a different matrix without verification.
Troubleshooting from evidence, not guesswork
- Poor recovery: check preparation, pH, overheating, supplement addition, inoculum, atmosphere, storage, and control performance.
- Loss of selectivity: review formulation, sterilization, expiry, inoculum burden, enrichment sequence, and incubation time.
- Weak or atypical reaction: confirm reading time and atmosphere, compare controls, document mixed growth, and follow confirmation requirements.
- Run-to-run drift: trend lot, analyst, equipment, water, preparation batch, controls, and environmental conditions.
External method context
The product should be understood as one measurement component in an antimicrobial susceptibility method. The analytical chain includes isolate purity, standardized inoculum, medium composition, antimicrobial concentration, incubation atmosphere and time, endpoint reading, and the interpretive table selected by the laboratory. A shift in any upstream variable can move an MIC or zone without a change in organism biology, which is why QC distributions and trend review are more informative than a single pass/fail control.
External laboratory-quality guidance also supports a lifecycle view: verify the method when introduced, bridge new lots against an accepted lot, trend control results, investigate nonconformities, and retain enough raw observations to reconstruct the decision. This is the practical difference between reproducing a preparation instruction and demonstrating that the resulting measurement is fit for its intended use.
FDA BAM Chapter 5: Salmonella
The current FDA BAM Salmonella workflow separates pre-enrichment, selective enrichment, selective plating, and confirmation. That sequence is scientifically important: enrichment increases target representation in a mixed matrix, selective conditions manage competing flora, and confirmation prevents a colour or colony morphology from being treated as identity. A laboratory transferring the method should verify recovery and selectivity on its own matrix classes rather than assuming that a medium performs identically across foods, water, and environmental samples. Review the authoritative source.
EUCAST disk diffusion and quality control
EUCAST requires standardized methodology, current breakpoint tables, and routine quality control; the laboratory remains responsible for the final susceptibility report. For broth dilution, inoculum density, cation content, pH, incubation atmosphere, endpoint definition, and two-fold dilution design influence MIC reproducibility. Breakpoints are interpretive rules applied after a technically valid MIC or zone measurement; they are not properties of the product alone. Review the authoritative source.
WHO Laboratory Quality Management System handbook
WHO laboratory quality guidance treats document control, equipment, purchasing and inventory, process control, records, occurrence management, and continual improvement as connected parts of reliable testing. Review the authoritative source.
Technical references
- Manufacturer product documentation for catalogue 610060.
- FDA BAM Chapter 5: Salmonella (access checked 2026-07-20).
- EUCAST disk diffusion and quality control (access checked 2026-07-20).
- WHO Laboratory Quality Management System handbook (access checked 2026-07-20).
Bench-use checklist
- Verify product identity, catalogue number, lot, expiry, and storage history.
- Document preparation and incubation exactly as controlled in the SOP.
- Define controls and acceptance criteria before testing.
- Record raw observations, control response, reading time, and confirmation decision.
- Escalate failed controls, atypical reactions, and departures through the quality system.