ISO 4831:2006 - Coliform detection and enumeration (MPN technique)

Overview

ISO 4831:2006, "Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of coliforms - Most probable number technique," provides a horizontal laboratory method for detecting and enumerating coliforms in foods, animal feed and environmental samples related to food production and handling. Enumeration uses the Most Probable Number (MPN) calculation after incubation in liquid media at 30 °C or 37 °C. The method is intended for situations where coliform levels are expected to be in the range 1 to 100 per ml or per g.

Key topics and technical requirements

• Scope: Applies to products for human consumption, animal feed and related environmental samples; incubation temperature is agreed between parties (milk products typically incubated at 30 °C).
• Detection vs. enumeration: Provides both a presence/absence detection procedure and an MPN-based enumeration procedure.
• Incubation times: Tubes are incubated for 24 h (or 48 h where specified) with examination for gas production or opacity.
• Culture media:
  • Selective enrichment: Lauryl sulfate tryptose broth (double- and single-strength formulations).
  • Confirmation medium: Brilliant green lactose bile broth.
• Test format:
  • Use of double-strength tubes for larger inocula and single-strength tubes for decimal dilutions.
  • Durham tubes to detect gas production.
  • Typical series of three tubes per dilution (other series sizes possible; MPN tables in ISO 7218 referenced for calculations).
• Interpretation and calculation: Positive tubes confirmed by transfer to confirmation medium; MPN is calculated from the number of confirmed positive tubes using standard MPN tables.
• Limitations:
  • Method detects lactose-fermenting, gas-producing strains under specified conditions - anaerogenic coliform strains may be missed.
  • MPN is inherently more variable and less precise than some plate-count methods (e.g., ISO 4832), but allows examination of larger test portions.

Applications

• Routine microbiological monitoring in food production, dairy, beverage, animal feed testing, and environmental hygiene in food-handling areas.
• Useful where low-level coliform detection is required and when sampling plans call for analysis of larger volumes or portions.
• Employed by quality assurance (QA) laboratories, contract testing labs, regulatory agencies and food safety auditors.

Who should use this standard

• Food microbiologists, QA/QC laboratory personnel, regulatory laboratories, and consultants involved in food safety testing and microbial monitoring of production environments.

Related standards

• ISO 7218 - General requirements and guidance for microbiological examinations.
• ISO 6887 / ISO 8261 - Sample preparation and dilutions.
• ISO/TS 11133 - Guidelines on preparation and quality assurance of culture media.
• ISO 4832 - Alternative (more precise) methods for coliform enumeration - referenced for method selection.

Keywords: ISO 4831:2006, coliforms, MPN, most probable number, lauryl sulfate tryptose broth, brilliant green lactose bile broth, food microbiology, detection and enumeration.