ISO 7251:2005

Overview

ISO 7251:2005 - "Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of presumptive Escherichia coli - Most probable number technique" - provides laboratory guidelines for detecting and enumerating presumptive Escherichia coli using a liquid-medium culture method and the Most Probable Number (MPN) calculation. The method uses selective enrichment, incubation at 37 °C followed by 44 °C, gas production and an indole test to report either presence/absence or an MPN per gram or millilitre. It applies to products for human and animal consumption and to environmental samples in food production and handling areas.

Key topics and technical requirements

• Principle: liquid enrichment, subculture to EC broth and indole-free peptone water, assessment of gas and indole production to identify presumptive E. coli.
• Incubation regime: initial incubation at 37 °C (up to 48 h), followed by selective incubation at 44 °C ± 1 °C (up to 48 h).
• Culture media: lauryl sulfate broth (double- and single-strength), EC broth, indole-free peptone water; media preparation, pH and sterilization requirements are specified.
• Apparatus: Durham tubes for gas capture, appropriate test tubes, incubators capable of 37 °C ±1 °C and 44 °C ±1 °C, autoclave, pH meter.
• Enumeration: use of series of tubes (double- and single-strength) and MPN tables (Annex A) to calculate the most probable number; MPN calculation refers to ISO 7218 for reporting and variability considerations.
• Limitations and warnings: susceptibility to variability; some pathogenic E. coli strains may not grow at 44 °C - interpret results with caution.

Applications

• Routine microbiological testing in food safety and quality control: raw materials, processed foods, dairy, meat, fish, feeds.
• Environmental monitoring in food production and food handling facilities (surfaces, water, processing environments).
• Regulatory/official testing and industry compliance where a horizontal, standardized MPN-based method for presumptive E. coli is required.
• Use cases: presence/absence screening and quantitative estimation (MPN) for hygiene assessment, process control and risk management.

Who should use this standard

• Food and feed microbiology laboratories
• Quality control and assurance teams in food manufacturing
• Regulatory and inspection laboratories
• Laboratories performing environmental monitoring in food chains

Related standards

• ISO 7218 - General rules for microbiological examinations (MPN calculation/reporting)
• ISO 6887 series - Sample preparation and dilutions for microbiological examination
• ISO 8261 - Milk and milk products sample preparation
• ISO/TS 11133-1 and -2 - Guidelines for preparation, production and performance testing of culture media

Keywords: ISO 7251:2005, Escherichia coli detection, MPN technique, lauryl sulfate broth, EC broth, indole test, food microbiology, food safety testing.