Coliforms and E. coli · Coliform colony count
Violet Red Bile Lactose Agar – colony count of coliforms
Violet Red Bile Lactose Agar (VRBL) is the classic medium for the colony count of coliforms in food, feed and water. Coliforms form purple-red colonies surrounded by a zone of precipitated bile. The medium is boiled, not autoclaved.
- Pour-plate count with overlay
- Purple-red colonies with bile precipitate
- No autoclaving – boil to dissolve
How Violet Red Bile Lactose Agar works
Crystal violet and bile salts inhibit Gram-positive bacteria. Lactose fermentation lowers the pH, neutral red turns the colonies purple-red and bile salts precipitate around them. A thin overlay layer gives semi-anaerobic conditions that improve the specificity of the count.
Typical colony appearance
| Organism | Colonies |
| Coliforms | Purple-red colonies, 0.5 mm or more, with red bile precipitate |
| Non-lactose fermenters | Colourless colonies |
Technical data
| Preparation | 41.5 g/L; boil to dissolve, no autoclaving; cool to 46-50°C before use |
| Final pH | 7.4 ± 0.2 |
| Incubation | 30-35°C, 24 h |
| Storage | Dry, dark place; keep the container tightly closed |
| Shelf life | 3 years unopened |
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 Violet Red Bile Lactose Agar: Use the medium on the day of preparation, keep it at 46–50 °C before pouring and do not reheat it repeatedly.
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.
Violet Red Bile Lactose 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 Violet Red Bile Lactose Agar, please send us a request. With every delivery of Violet Red Bile Lactose Agar you receive the lot number and expiry date, and on request we send the available documentation for Violet Red Bile Lactose Agar.
Where Violet Red Bile Lactose 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 |
|---|---|---|
| Violet Red Bile Lactose Agar (VRBL) | PRO7473 | 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.
