ISO 16649-3:2015

ISO 16649-3:2015 - Overview

ISO 16649-3:2015 defines a horizontal microbiological method for the detection and enumeration of β‑glucuronidase‑positive Escherichia coli (E. coli) using a liquid‑medium culture technique and the Most Probable Number (MPN) calculation. The method uses the chromogenic substrate 5‑bromo‑4‑chloro‑3‑indolyl‑β‑D‑glucuronide (BCIG) and a two‑stage incubation (first at 37 ± 1 °C, then at 44 ± 1 °C) to identify typical blue/blue‑green colonies on tryptone bile glucuronide agar. ISO 16649‑3 applies to food, animal feed and environmental samples in food production and handling areas.

Key technical topics and requirements

• Scope and suitability
  • Applicable to products for human consumption and animal feed, plus environmental sampling in food production zones.
  • Suitable for enumeration of E. coli cells stressed by dehydration, freezing, saline (marine) exposure or chlorine disinfection.
• Detection and enumeration principle
  • Liquid selective enrichment (e.g., minerals modified glutamate medium) inoculated with test portion → incubate 37 °C → screen for lactose fermentation → subculture to tryptone bile glucuronide agar and incubate 44 °C → examine for β‑glucuronidase‑positive colonies and calculate MPN.
• Media and quality assurance
  • Defines dilution fluids, selective enrichment medium and tryptone bile glucuronide agar.
  • Calls for performance testing and QA of culture media per ISO TS 11133.
• Limitations and precision
  • Method subject to relatively high variability; results must be interpreted in light of Clause 11 (precision and variability).
  • Not fully evaluated for all matrices (e.g., milk and milk products); ISO 7251 is recommended for milk.
  • Warning: β‑glucuronidase‑negative strains (including some pathogenic strains such as E. coli O157) and strains that do not grow at 44 °C may not be detected.

Practical applications and users

• Food microbiology and quality control: routine testing of processed foods, raw materials and environmental swabs in production facilities.
• Regulatory and inspection laboratories: compliance testing where enumeration of β‑glucuronidase‑positive E. coli is required.
• Seafood and marine products testing: suitable for organisms exposed to saline environments.
• Laboratories addressing stressed organisms (frozen, dried or chlorine‑exposed samples) where recovery sensitivity matters.

Related standards and references

• ISO 6887 series - sample preparation and dilutions
• ISO 7218 - general rules for microbiological examination
• ISO TS 11133‑1/2 - culture media preparation and performance testing
• ISO 7251 - MPN technique for presumptive E. coli (recommended for milk and milk products)

This summary highlights the practical value of ISO 16649‑3:2015 for E. coli detection and MPN enumeration in food and environmental testing while noting method limitations and complementary standards for specific matrices.