For antimicrobial testing, Chromatic E.COLI O157 sits at the point where organism growth becomes a quantitative or categorical measurement. The discussion below connects its documented role with preparation variables, controls, interpretation, and the limits of inference. Catalogue 11610 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
ChromaticTM E.coli O157 is a chromogenic medium used for the isolation and differentiation of Escherichia coli O157 in clinical specimens and food samples.
Method principle
Peptones, yeast and meat extracts are sources of amino acids, nitrogen, minerals and vitamins. Sodium chloride maintains the osmotic balance of the medium. Sodium deoxycholate inhibits growth of Gram-positive bacteria. The chromogenic mix allows the identification of the microorganisms on the basis of the color and morphology of the colonies. Agar is the solidifying agent.
Formula and functional interpretation
(g/l) Peptone 15.00 Casein Peptone 5.00 Yeast Extract 5.00 Meat Extract 1.00 Sodium Chloride 5.00 Sodium Deoxycholate 0.50 Chromogenic Mix 2.25 Agar 15.00 Final pH 7.0 ± 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 mediumSuspend 49 g of the powder in 1 liter of distilled or deionized water. Mix well. Heat to boil shacking frequently until completely dissolved. DO NOT AUTOCLAVE. Cool to 45-50°C. In aseptic conditions dispense in Petri dishes. Allow the medium to solidify with lids partially removed. 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.
Procedure: Inoculate the plates by streaking directly the sample onto the agar surface. Alternatively, plates may be inoculated from pre-enrichments. Incubate aerobically at 37°C for 18-24 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
Plates are inoculated with the microbial strains indicated in the QC table. Inoculum for productivity: 50-100 CFU. Inoculum for selectivity: 104-106 CFU. Inoculum for specificity: 103-104 CFU Incubation conditions: aerobically at 35±2°C for 18-24 h. QC Table. Microorganism Growth Specification Escherichia coli O157ATCC 35150 Good Mauve or mauve-purple colonies Escherichia coli ATCC 25922 Good Blue colonies Klebsiella pneumoniaeATCC 13883 Good Blue colonies Staphylococcus aureusATCC 25923 Inhibited --- Enterococcus faecalisATCC 19433 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
At the end of incubation, examine for typical mauve or mauve-purple colonies of Escherichia coli O157. Gram-positive organisms are completely inhibited. Gram-negative organisms, other than E. coli O157, are either inhibited or produce colorless, blue, green, or blue-green colonies. Some Salmonella spp. could give false positive results. Final identification should be confirmed by immunoserological tests. See pictures in Appendix I.
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 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. © Liofilchem - ChromaticTM E.coli O157 - Rev.1 / 27.09.2014 Page 1 of 2 Instructions For Use
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 11610.
- 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.