The practical value of Urea Agar Base (CHRISTENSEN) 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 610107 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
Differential medium used for determining urease activity and for identification of organisms from clinical specimens and other samples. The medium complies with the requirements of the standards ISO 6579-1, ISO 10273, ISO 19250, ISO 21567 and ISO 11133. This medium is intended as an aid in the diagnosis, requiring further tests to complete the diagnostic results.
Method principle
Peptone provides nitrogen, carbon, and amino acids required for microbial growth. Glucose serves as an energy source. Sodium chloride maintains the osmotic balance of the medium. Monopotassium phosphate acts as a buffer. Phenol red is the pH indicator, and agar is the solidifying agent. Urea is included as a substrate for the urease enzyme. Microorganisms producing urease hydrolyze urea into ammonia and carbon dioxide. The release of ammonia increases the pH of the medium, shifting it toward alkaline conditions. This change is detected by the phenol red indicator, which turns from yellow/orange to pink/red.
Formula and functional interpretation
* (g/litre) Base medium Peptone 1.0 Glucose 1.0 Sodium Chloride 5.0 Monopotassium Phosphate 2.0 Phenol Red 0.012 Agar 15.0 Final pH 6.8 ± 0.2 *Adjusted and/or supplemented as required to meet
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 24.0 g of the powder in 1 liter of distilled or deionized water. Mix well. Heat until completely dissolved. Sterilize in autoclave at 121°C for 15 minutes. Cool to 45-50°C. Aseptically add 50 ml of Urea 40% supplement. Mix well and pour into sterile final containers. Medium in bottles Melt the content of the bottle in a water bath at 100°C (with the cap partially removed) until completely dissolved. Then screw the cap and check the homogeneity of the dissolved medium, if it is the case turning the bottle upside down. Cool at 45-50°C.
Procedure: Use a heavy inoculum from a pure 18–24-hour culture. Inoculate by streaking over the slant surface. Incubate at 37 ± 1°C for up to 24 hours under aerobic atmosphere (slightly loosen caps). Observe reactions after 6 and 24 hours. Longer periods of incubation may be necessary.
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
organisms. © Liofilchem - Christensen Urea Agar - Rev.0 / 27.03.2026 Page 2 of 17 SPECIMENS Clinical specimens should be sampled at the acute stage, before antimicrobial therapy (where possible) and examined as soon as possible after collection. Good laboratory practices for collection, transport 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
At the end of the incubation period observe the color of the medium. A pink color indicates a positive reaction (urease activity), while no color change, with the medium remaining yellow to orange, indicates a negative reaction. Proteus spp. usually complete the reaction in 4-5 hours and therefore can be easily differentiated from weaker urease- producing organisms such as Klebsiella and Enterobacter spp. Typical Salmonella cultures do not hydrolyze urea so that the color of the medium will remain unchanged. Some pathogenic Y.
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
of the clinical specimens should be applied. Refer to specific guidelines for more information about specimen collection 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
Invalid results can be caused by poor specimen quality, improper sample collection, improper transportation, improper laboratory processing, or a limitation of the testing technology. The operator should understand the principles of the procedures, including its
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 610107.
- 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.