Coliforms and E. coli · Isolation of Gram-negative bacteria
MacConkey Agar – lactose fermenters versus non-fermenters
MacConkey Agar is a selective and differential medium for Gram-negative enteric bacteria. Lactose fermenters such as E. coli form pink to red colonies, while non-fermenters such as Salmonella and Shigella remain colourless.
- Pink-red colonies = lactose fermenters
- Bile salts and crystal violet inhibit Gram-positives
- Used in food, water and pharmaceutical tests
How MacConkey Agar works
Bile salts and crystal violet inhibit Gram-positive bacteria. Lactose fermentation lowers the pH around the colony, neutral red turns it pink to red, and bile may precipitate as a hazy zone. Non-fermenters use peptone and stay colourless.
Typical colony appearance
| Organism | Colonies |
| E. coli | Pink to red colonies, may have bile precipitate |
| Salmonella, Shigella | Colourless, translucent colonies |
Technical data
| Preparation | 55 g/L; boil to dissolve, dispense, autoclave 121 °C 15 min |
| Final pH | 7.2 ± 0.2 |
| Incubation | 35-37 °C, 24 h |
| Storage | Dry, dark place; keep the container tightly closed |
| Shelf life | 3 years |
Applications and practical notes
Plate count and isolation media for coliforms and E. coli are used in food, feed, water and hygiene testing, for example according to ISO 4832 and ISO 16649-2. Typical applications include:
- Coliform and E. coli colony counts in food and feed
- Isolation of lactose-fermenting Gram-negative bacteria
- Process hygiene criteria in dairy, meat and vegetable production
- Teaching and routine quality-control laboratories
Working with MacConkey Agar: Pour plates to an even depth, dry the surface before streaking and read colonies after 18–24 hours.
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.
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 MacConkey Agar, please send us a request. With every delivery of MacConkey Agar you receive the lot number and expiry date, and on request we send the available documentation for MacConkey Agar.
Where MacConkey Agar fits in coliform and E. coli 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 |
|---|---|---|
| MacConkey Agar | PRO7442 | 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.
Sales and technical support
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Quotations and technical support for culture media and microbiology products across our international sales territory. B2B prices are 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.
