Colilert

What Colilert is

Colilert is a proprietary defined-substrate reagent from IDEXX Laboratories used in environmental and drinking-water microbiology labs to detect and quantify total coliform bacteria and E. coli from a single water sample in one test, rather than running two separate assays. It is one of the most widely used methods in North American drinking-water compliance monitoring because it is fast, requires no confirmatory subculturing, and is backed by a specific U.S. EPA method approval and a published Standard Methods procedure — both of which matter directly to a lab that has to defend its results to a regulator, not just get a result.

How the test works

The sample is added to a vial or bottle of Colilert reagent containing ONPG and MUG as nutrient indicators, then incubated (24 hours for standard Colilert, 18 hours for Colilert-18) and dispensed into a Quanti-Tray or Quanti-Tray/2000 for sealing. A color change to yellow indicates coliform bacteria are present (they metabolize ONPG); fluorescence under 365 nm long-wave UV light indicates E. coli is present (it metabolizes MUG). Counting the number of positive/fluorescent wells against the Quanti-Tray’s published statistical table converts the result into a quantitative Most Probable Number (MPN) per 100 mL, rather than a simple presence/absence call.

Regulatory status — why this matters before you buy

Colilert is standardized as Standard Methods 9223B (Enzyme Substrate Coliform Test) and appears on the U.S. EPA’s list of approved methods for Safe Drinking Water Act compliance monitoring under 40 CFR Part 141. For a lab whose results feed regulatory reporting (a public water system’s Total Coliform Rule / Revised Total Coliform Rule monitoring, an NPDES discharge permit, a state ambient-water program), the method has to be on the applicable approved-methods list before a result is defensible — this is the first thing a procurement decision should confirm, not an afterthought. A comparable international standard exists in ISO 9308-2 (detection and enumeration of E. coli and coliform bacteria by defined substrate methodology), relevant for labs operating under ISO/IEC 17025 accreditation outside U.S. regulatory frameworks.

Running Colilert for compliance reporting purposes also generally requires the laboratory itself, not just the reagent, to be accredited for that specific method — in the U.S. this typically means NELAP/TNI accreditation covering Standard Methods 9223B, obtained through a state or ANAB-recognized accreditation body. Buying the reagent does not substitute for having (or obtaining) that accreditation; procurement and accreditation scope should be planned together, especially for a lab adding this method for the first time.

Format options to evaluate

• Colilert (24-hour) vs. Colilert-18 (18-hour): the shorter Colilert-18 incubation matters where a regulatory or contractual holding time is tight (e.g., recreational water programs); confirm which variant your applicable method approval actually covers before standardizing on one.
• Quanti-Tray vs. Quanti-Tray/2000: the standard Quanti-Tray covers a lower MPN range (up to roughly 200 MPN/100 mL); the Quanti-Tray/2000 extends the quantifiable range (to roughly 2,419 MPN/100 mL), which matters for source water, raw water, or wastewater-adjacent samples likely to run higher than finished drinking water.
• Reagent pack size and shelf life: reagent is sold in single-test snap packs; confirm shelf life against your lab’s real sample throughput so reagent isn’t wasted, and confirm storage conditions (temperature-controlled, away from light) are achievable in your existing storage space.
• Sealer and incubator compatibility: Quanti-Trays require a specific heat sealer, and incubation requires a calibrated incubator holding the method’s specified temperature; factor this capital equipment into a first-time procurement, not just the reagent’s per-test cost.

Comparing Colilert to alternative methods

Colilert is not the only EPA-approved or Standard-Methods-listed option for total coliform/E. coli detection. Membrane filtration (Standard Methods 9222) and multiple-tube fermentation are older, still-approved culture-based alternatives that use different media, labor, and turnaround profiles. Other commercial defined-substrate products (for example, IDEXX’s own Colisure, or competing products such as Readycult Coliforms 100 or Chromocult Coliform Agar) target the same organisms through a similar substrate-color-change principle but are chemically and procedurally distinct products with their own separate method approvals — a lab cannot substitute one for another under a method approval that names a specific product without re-validating or re-applying for approval. When evaluating options, compare on the dimensions that are actually verifiable: which specific method number and product name is named on your applicable approved-methods list, published detection limits and MPN range, holding time, staff hands-on time, and total cost including required capital equipment — not on vendor marketing claims.

Sourcing and vendor evaluation

Because Colilert is a single-manufacturer (IDEXX) proprietary reagent, it is typically purchased through IDEXX directly or through authorized laboratory-supply distributors that carry IDEXX water-testing consumables alongside other lab supplies — distributors such as LAC Health list Colilert reagent kits and Quanti-Trays among their water-microbiology product lines, as one example of this distribution channel. When comparing distributors for a recurring consumable like this, the criteria that matter for a compliance-monitoring lab are lead time and reliability of supply (a stockout can mean a missed regulatory sampling deadline), cold-chain/temperature-controlled shipping for a reagent with real storage requirements, lot-traceability documentation for audit purposes, and whether the distributor can support your accreditation paperwork — not simply unit price.

Frequently asked questions

Is Colilert EPA-approved?

Yes, as Standard Methods 9223B, it is on the EPA’s list of approved analytical methods for Safe Drinking Water Act compliance monitoring under 40 CFR Part 141 — confirm the current listing and any state-specific restrictions directly against your state primacy agency’s approved-methods list before relying on it for reporting.

How long does a Colilert test take?

24 hours for standard Colilert, 18 hours for Colilert-18, from sample addition to final read.

Does Colilert tell you which coliform species is present?

No. It reports total coliform presence/quantity (via the ONPG color reaction) and E. coli presence/quantity specifically (via the MUG fluorescence reaction) as a defined pair of results; it does not identify or speciate other individual coliform organisms.

Do I need special lab accreditation to run Colilert for compliance samples?

For results used in regulatory compliance reporting, generally yes — the laboratory needs accreditation (in the U.S., typically NELAP/TNI) covering the specific method (Standard Methods 9223B), separate from simply purchasing and running the reagent correctly.