Overview
EN ISO 21871:2006 (ISO 21871:2006) defines a horizontal microbiological method for the detection and enumeration of low numbers of presumptive Bacillus cereus in food, animal feeding stuffs and environmental samples from food production and handling. The standard uses the Most Probable Number (MPN) technique combined with selective enrichment and plating for detection or quantification of viable cells. It is intended for laboratories performing food microbiology testing, quality control and regulatory compliance.
Key topics and technical requirements
- Scope and purpose: Detection or enumeration of low counts of viable presumptive B. cereus in products for human consumption, animal feed and environmental samples.
- Principle: Use of liquid selective enrichment (Tryptone Soya Polymyxin Broth, TSPB) followed by plating onto selective solid media and confirmation of presumptive colonies. Results are expressed as MPN per gram or millilitre, or as presence/absence for the detection method.
- Enrichment media: TSPB specified in double‑strength and single‑strength formulations (composition for enzymatic digests, NaCl, glucose and K2HPO4 is provided in the standard).
- Selective solid media: Polymyxin pyruvate egg yolk mannitol bromothymol blue agar (PEMBA) and Mannitol egg yolk polymyxin agar (MYP) are specified for isolation of presumptive colonies.
- Incubation conditions:
- Liquid enrichment: incubate 48 h at 30 °C.
- Solid media: incubate 18–48 h (PEMBA typically at 37 °C; MYP at 30 °C) and examine for characteristic colonies.
- Confirmation: Suspect colonies are confirmed by haemolysis testing or microscopic examination. Note: the standard defines “presumptive Bacillus cereus” and clarifies the confirmatory tests may not distinguish closely related Bacillus species.
- Calculation: Enumeration uses MPN tables based on positive/negative results from replicate enrichment tubes and dilutions; detection gives presence/absence per unit.
Applications and users
- Who uses it: Food and feed microbiology laboratories, quality assurance/quality control teams, third‑party testing labs, regulatory agencies and environmental monitoring groups in the food chain.
- Practical uses:
- Routine screening of raw materials, finished food products and animal feeds for low-level contamination by B. cereus.
- Environmental hygiene monitoring in food production and handling facilities.
- Supporting product release decisions, shelf-life studies and contamination investigations.
- Benefits: Harmonizes methodology across laboratories, provides a validated approach for low-count detection using the MPN technique, and integrates selective media and confirmation steps for reproducible results.
Related standards
Keywords: EN ISO 21871:2006, ISO 21871:2006, Bacillus cereus, most probable number, MPN technique, TSPB, PEMBA, MYP, food microbiology, detection method, enumeration, feed microbiology.
