A susceptibility result can shift before the organism changes. That is why XLT 4 Agar Culture Media must be considered together with inoculum, atmosphere, incubation, and endpoint reading. The discussion below connects its documented role with preparation variables, controls, interpretation, and the limits of inference. Catalogue 610092 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
XLT4 (Xylose Lysine Tergitol 4) Agar Base is a medium used with supplements for the detection of non-typhi Salmonella spp from clinical specimens as well as from environmental and food samples.
Method principle
Peptone provides amino acids, nitrogen, carbon, minerals, vitamins and other nutrients necessary for organisms growth. Yeast extract isa source of vitamins, particularly of B-group. Lactose, sucrose and xylose are the fermentable carbohydrates, which together with the amino acid L-lysine serve for bacterial differentiation. Sodium chloride maintains the osmotic balance of the medium. Phenol red is the pH indicator. Sodium thiosulfate and ferric ammonium citrate allow to detect hydrogen-sulfide (H2S) producing organisms. Agar is the solidifying agent.Tergitol 4 (Niaproof) is an anionic surfactant which is added to the medium to provide selectivity. It is effective against gram-positive and many gram-negative bacteria, including Proteus spp.
Formula and functional interpretation
* (g/l)Peptone 1.6Yeast Extract 3.0Lactose 7.5Sucrose 7.5Xylose 3.75L-Lysine 5.0Sodium Chloride 5.0Phenol Red 0.08Sodium Thiosulfate 6.8Ferric Ammonium Citrate 0.8Agar18.0Final 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: Suspend 59.0 g of powder in 1 litre of deionized or distilled water. Add 4.6 ml of XLT4 Supplement (ref. 80010). Bring to boil and shake until completely dissolved. Avoid overheating. DO NOT AUTOCLAVE. Cool to 45-50°C. Dispense into Petri dishes.
Procedure: Inoculate the agar medium by subculturing from a suitable enrichment broth e.g. Tetrathionate Broth (ref. 24451). Streak for isolation. Incubate plates at 35 ± 2°C for 18-24 hours. If necessary, incubate for further 24 h.
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
specifications; Grams per litre of purified water.
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
Typical Salmonella colonies (H2S-positive) appear red with a black center. Other Salmonelle (H2S-negative) form pink-yellow colonies without a black center. Other Enterobacteriaceae (non-salmonellae), that grow on this medium, produce yellow colonies without evidence of blackening.Enterobacter aerogenes and Escherichia coli are markedly inhibited. Growth of Proteus, Pseudomonas, Providencia, Alteromonas putrefaciens, Yersinia enterocolitica and Acinetobacter calcoaceticus is markedly to completely inhibited. Shigella spp. are partially inhibited and appear as red colonies.
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 and use it before the expiry date on the label or until signs of deterioration or contamination are evident. Store the prepared medium at 2-8°C away from light.WARNING AND
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
For In Vitro Diagnostic use. For professional use only. Operators must be trained and have certain experience in the laboratory methods. Please read the instructions carefully before using this product. Reliability of assay results cannot be guaranteed if there are any deviations from the instructions in this document.Consult the Safety Data Sheet (SDS) for information regarding hazards and safe handling practices.
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 610092.
- 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.