E. coli O157 · Selective isolation
Sorbitol MacConkey Agar – isolation of E. coli O157
Sorbitol MacConkey Agar (SMAC) is the classic medium for the isolation of E. coli O157. Most E. coli ferment sorbitol and form pink colonies, while O157:H7 does not and forms colourless colonies. With cefixime and tellurite it becomes CT-SMAC.
- O157: colourless colonies
- Most other E. coli: pink colonies
- Complete with the CT supplement for CT-SMAC
How Sorbitol MacConkey Agar works
Sorbitol replaces lactose. E. coli O157:H7 does not ferment sorbitol within 24 hours, so its colonies stay colourless. Cefixime and potassium tellurite, added as a supplement, inhibit Proteus and many other sorbitol-negative bacteria.
Typical colony appearance
| Organism | Colonies |
| E. coli O157:H7 | Colourless colonies |
| Sorbitol-fermenting E. coli | Pink colonies with bile precipitate |
Technical data
| Preparation | 55 g/L; boil, autoclave 121°C 15 min; at 50-55°C add 1.25 mg potassium tellurite per 500 mL; optional cefixime 0.01 mg/500 mL |
| Final pH | 7.2 ± 0.2 |
| Incubation | 35-37°C, 17-19 h |
| Storage | Dry, dark place; keep the container tightly closed |
| Shelf life | 3 years |
Applications and practical notes
E. coli O157 and other Shiga toxin-producing E. coli (STEC) are tested in meat, raw milk products, sprouts and vegetables, for example according to ISO 16654 and FDA BAM Chapter 4. Typical applications include:
- Raw and minced meat, especially beef
- Raw milk and raw-milk cheese
- Sprouts, leafy greens and other fresh produce
- Outbreak investigations and environmental samples
Working with Sorbitol MacConkey Agar: Confirm colourless colonies with the O157 latex test and biochemical identification, because some other bacteria are also sorbitol negative.
Good practice with dehydrated media: keep the bottle tightly closed in a cool, dry place, weigh the powder quickly to avoid moisture uptake, and use fresh distilled or deionised water. Check the pH of each new batch after preparation and before use, and label prepared media with the date. Performance of each lot can be verified with reference strains according to ISO 11133, the standard for the preparation, production, storage and performance testing of culture media.
About coliform and E. coli methods: coliforms and E. coli are counted either by the multiple-tube (MPN) method, with a presumptive broth followed by confirmation broths, or by colony counts on selective agars. FDA BAM Chapter 4 describes both approaches for food and water, and ISO 16649-2 gives the colony-count method for β-glucuronidase-positive E. coli on TBX Agar. E. coli O157 and other STEC need dedicated media, serology and PCR. COLO.Science offers media, supplements, rapid tests, identification kits and QC strains for every step.
Complete sets from one supplier: COLO.Science supplies culture media, supplements, rapid tests, identification kits and QC strains as complete sets for the method you follow. Send us your sample types and standards, and we will prepare an offer with the matching products and documentation.
Sorbitol MacConkey Agar is supplied by COLO.Science in the pack sizes listed under Ordering information. For larger quantities, regular deliveries or method-specific sets that include Sorbitol MacConkey Agar, please send us a request. With every delivery of Sorbitol MacConkey Agar you receive the lot number and expiry date, and on request we send the available documentation for Sorbitol MacConkey Agar.
Where Sorbitol MacConkey Agar fits in E. coli O157 testing
- Presumptive test – Lauryl Sulfate Tryptose Broth (MPN) or VRBL / chromogenic agar (colony count)
- Confirmation – BGLB Broth for coliforms; EC or EC-MUG Broth at 44.5 °C for E. coli
- Isolation – EMB, MacConkey, TBX or chromogenic E. coli agar
- Identification – IMViC kit; for O157 and STEC: SMAC or chromogenic O157 agar, latex and PCR
Ordering information
| Product | COLO code | Pack |
|---|---|---|
| Sorbitol MacConkey Agar Base (SMAC) | PRO7505 | 500 g |
Product notice: preparation, storage conditions and shelf life are given on the label. Technical data on this page are for orientation; the label and the lot documentation apply. B2B prices are quoted net, excluding VAT.
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Need the complete set for your method?
Send the sample type, target organism and the standard you follow. COLO.Science will put together the media, supplements, QC strains and confirmation tests for the whole procedure.
