Overview
ISO 17601:2016 - Soil quality specifies a standardized workflow to estimate the abundance of selected microbial gene sequences using quantitative real-time PCR (qPCR) on DNA directly extracted from soil. The standard describes the critical steps from amplicon design and qPCR standard preparation to soil DNA preparation, inhibition testing, qPCR assay execution (SYBR Green® and TaqMan® options) and validation. It emphasises that quantified gene copy numbers (e.g., 16S rRNA sequences) are proxies for microbial groups and are not necessarily equivalent to live organism counts due to variable gene copy numbers and DNA from dead cells.
Key topics and technical requirements
- Method principle: Four main tasks - (1) qPCR standard preparation and calibration, (2) preparation of soil DNA template and inhibition testing, (3) qPCR assay performance, (4) validation and result calculation.
- Amplicon design: Selection of target gene sequences and primer/probe design tailored for selected microbial groups or functional genes.
- Standards and calibration: Preparation of cloned amplicons or plasmid standards, calibration curves, and limit-of-detection considerations.
- Soil DNA handling: Use of DNA extracted according to ISO 11063 and sample handling guidance (e.g., ISO 10381-6); checks for qPCR inhibitors common in soil extracts.
- qPCR chemistries: Procedures for SYBR Green and TaqMan assays are provided in informative annexes.
- Instrumentation & reagents: Requirements include a real-time qPCR instrument (single-copy detection sensitivity), spectrophotometer/spectrofluorimeter for DNA quantification, polymerases, plasmid/cloning vectors, and common molecular-biology reagents.
- Validation & reporting: Criteria for assay validation, calculation of gene copy numbers from Ct values, and recommended test report elements. An international ring test (Annex A) is referenced for reproducibility assessment.
- Limitations: Explicit note that gene copy numbers do not directly equate to organism numbers or viability.
Applications
ISO 17601:2016 is applicable to:
- Environmental and soil-quality laboratories performing molecular microbial monitoring.
- Agricultural research and agronomy for soil microbiome assessment.
- Environmental impact assessments and remediation monitoring.
- Academic and industrial research studying microbial community structure or specific functional genes.
- Regulatory bodies and standard-setting organizations needing reproducible qPCR-based microbial indicators.
Related standards
- ISO 11063 - Method to directly extract DNA from soil samples (referenced for soil DNA extraction).
- ISO 10381-6 - Guidance on collection, handling and storage of soil for microbiological assessment.
Keywords: ISO 17601:2016, soil quality, qPCR, quantitative PCR, soil DNA, microbial gene sequences, 16S rRNA, SYBR Green, TaqMan, ISO 11063, soil microbiome, environmental monitoring.