A susceptibility result can shift before the organism changes. That is why Dichloran Glycerol (DG18) 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 10098 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
Dichloran Glycerol (DG18) Agar is a medium used for the selective isolation and enumeration of yeasts and moulds in food. This medium conforms to ISO 21527-2 for microbiological examination of products that have a water activity less than or equal to 0.95 such as dry fruits, cakes, jams, dried meat, salted fish, grains, cereal products, flour, nuts, spices and condiments, etc.
Method principle
Enzymatic digest of casein provides amino acids, carbon, nitrogen, vitamins and minerals for organisms growth. Glucose is included as energy source. Monopotassium phosphate is a buffering agent. Magnesium sulfate provides divalent cations and sulfur. Dichloran is an anti-fungal agent incorporated into the medium to reduce colony diameters of spreading fungi. Chloramphenicol inhibits the growth of accompanying bacterial flora. Inhibition of bacterial growth and restriction of spreading of more-rapidly growing moulds aids in the isolation of slow-growing fungi. Glycerol is a carbon source. Agar is the solidifying agent.
Formula and functional interpretation
(g/l) Casein Enzymatic Digest 5.0 D-Glucose 10.0 Monopotassium Phosphate 1.0 Magnesium Sulfate 0.5 Dichloran 0.002 Chloramphenicol 0.1 Glycerol 220.0 Agar 15.0 Final pH 5.6 ± 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: Medium in bottles Melt the content of the bottle in a water bath at 100°C (loosing 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, mix well avoiding foam formation and aseptically distribute into Petri dishes.
Procedure: According to ISO 21527-2, spread 0.1 ml of the the test sample or 0.1 ml of the initial suspension onto one plate of Dichloran Glycerol (DG18) Agar and 0.1 ml of the first decimal dilution over a second plate. Repeat inoculation with subsequent dilutions. Incubate plates aerobically at 25 ± 1°C for 5-7 days and up to 10 days if the presence of Xeromyces bisporus is suspected.
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: 50-100 CFU. Inoculum for selectivity: 104-106 CFU. Incubation conditions: aerobically at 25 ± 1°C for 5 days. QC Table. Microorganism Growth Saccharomyces cerevisiae WDCM 00058 Good Wallemia sebi WDCM 00182 Good Escherichia coli WDCM 00013 Inhibited Bacillus subtilis WDCM 00003 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
Select plates containing <150 colonies/propagules/germs and count after 2 days and again after 5-7 days of incubation. Report the results as CFU per g or per ml of sample allowing for the dilution factor.
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
Store bottles and prepared plates at 2-8°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 professional use only and must be used 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.
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.
FDA Bacteriological Analytical Manual
FDA BAM methods organize food microbiology around validated sampling, preparation, enrichment, isolation, and confirmation steps, with method applicability tied to the matrix and target organism. Sampling uncertainty, homogenization efficiency, inhibitory ingredients, dilution design, and countable range can dominate the result before the culture medium is ever inoculated; those variables belong in method verification and uncertainty review. 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 10098.
- EUCAST disk diffusion and quality control (access checked 2026-07-20).
- FDA Bacteriological Analytical Manual (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.