Overview
ISO 20837:2006 - "Microbiology of food and animal feeding stuffs - PCR for the detection of food‑borne pathogens - Requirements for sample preparation for qualitative detection" - defines criteria and practical examples for preparing samples and extracting nucleic acids suitable for PCR-based qualitative detection of food‑borne pathogens. The standard focuses on producing PCR‑compatible samples or nucleic acids of adequate quality and quantity, describes general principles, and includes informative annexes on enrichment standards (Annex A) and a detailed DNA extraction example for Gram‑negative bacteria (Annex B).
Key topics and technical requirements
- Objective: obtain nucleic acids with chemical purity, integrity and fragment size sufficient for amplification of target PCR products.
- Enrichment & sample treatment: enrichment may precede treatment; selection of enrichment media should consider PCR inhibitors. Annex A cross‑references standards for common pathogens (e.g., Salmonella, Listeria, E. coli O157).
- Nucleic acid extraction principles: release DNA from cells and remove PCR inhibitors using physical, chemical or biochemical methods while preserving nucleic acid integrity. Examples include dilution, centrifugation, protein digestion, filtration, immunomagnetic separation, boiling, chelating agents and organic extraction.
- Annex B (Gram‑negative DNA extraction): practical protocol using centrifugation of enriched broth, wash with saline or PBS, thermal lysis (e.g., 95–100 °C for 15–20 min), cooling, centrifugation and use of supernatant for PCR. Typical centrifugation speeds/times (e.g., 10 000 g for 10 min) and wash buffer compositions are provided.
- DNA quality & quantification: assess fragment size and purity by gel electrophoresis, fluorometry or spectrophotometry; ensure methods recover DNA fragments at least as large as the PCR target. Avoid repeated freeze‑thaw and use appropriate plasticware to prevent nucleic acid binding.
- Controls & validation: follow control requirements in ISO 22174:2005; validate alternative enrichment media or extraction kits to demonstrate comparable performance.
Applications and intended users
ISO 20837:2006 is intended for laboratories performing qualitative PCR detection of food‑borne pathogens, including:
- Food microbiology and quality control labs
- Public health and regulatory testing laboratories
- Diagnostic and contract testing providers
- Method developers and kit manufacturers
- Laboratories adapting methods to feed, agricultural or environmental matrices (with suitable modifications)
Practical applications include routine screening of food samples, validation of sample preparation workflows, and guidance for reducing PCR inhibition in complex food matrices.
Related standards
Keywords: ISO 20837, PCR sample preparation, food‑borne pathogens, nucleic acid extraction, PCR inhibitors, food microbiology, Annex B, Gram‑negative DNA extraction.