Overview
ISO 6461-1:1986 is an internationally recognized standard established by the International Organization for Standardization (ISO) for assessing water quality through microbiological analysis. This standard specifies a method for the detection and enumeration of spores of sulfite-reducing anaerobes, specifically clostridia, using enrichment in a liquid medium. The method is applicable to all types of water, including turbid and potable water, and is particularly valuable due to the resilience of clostridia spores in various environmental conditions.
Clostridia spores serve as important indicators of fecal contamination and waterborne pollution, as they can persist longer in water environments compared to other bacteria. This method focuses on selectivity by applying a heat treatment to eliminate vegetative bacteria, followed by enrichment incubation under anaerobic conditions to promote the growth of sulfite-reducing anaerobic spores.
Key Topics
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Scope and Application
- Applicable to all water types including drinking and turbid waters.
- Used for detecting spore-forming anaerobic bacteria indicating water contamination.
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Principle of the Method
- Heat treatment at 75 ± 5 °C for 15 minutes to destroy non-spore vegetative bacteria.
- Enrichment by inoculating measured water volumes into specially prepared liquid culture media.
- Incubation at 37 ± 1 °C under anaerobic conditions for 44 ± 4 hours.
- Observation for sulfite reduction indicated by blackening due to sulfide precipitation.
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Media Preparation
- Use of double-strength and single-strength broth media enriched with meat peptone, yeast extract, glucose, sodium sulfite, and iron citrate to support clostridia growth.
- Strict control of pH, sterilization, and storage conditions for reproducible results.
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Laboratory Requirements
- Use of sterile glassware like screw-capped flasks in specified volumes (25 ml, 50 ml, 100 ml).
- Accurate temperature-controlled water baths and incubators.
- pH meters and filtration equipment for medium preparation.
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Result Interpretation
- Positive tests show blackening of the culture medium from sulfite reduction and iron sulfide formation.
- Results are expressed as the Most Probable Number (MPN) of sulfite-reducing anaerobe spores per volume of the sample.
Applications
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Water Quality Monitoring
- Routine microbiological analysis of drinking, surface, ground, and wastewater.
- Detection of fecal contamination and assessment of disinfection efficacy.
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Environmental Studies
- Tracking pollution from agricultural or urban runoff.
- Evaluating resistance of spores to water treatment processes including chlorination.
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Public Health and Safety
- Ensuring compliance with regulatory standards for potable water supplies.
- Supporting risk assessment and preventive measures in waterborne disease outbreaks.
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Research and Development
- Studying the behavior and survival of sulfite-reducing anaerobic spores in diverse aquatic environments.
- Development of new water treatment or sanitation technologies.
Related Standards
- ISO 8199 - General guidelines for microbiological water analysis; provides foundational principles for enumeration and handling of samples.
- ISO 5667 (Part 2 and 3) - Guidelines on water sampling techniques and sample preservation relevant to microbiological testing.
- ISO 3696 - Specifications for laboratory water used in preparation of culture media, ensuring purity and consistency.
By adhering to ISO 6461-1:1986, water laboratories can reliably detect and enumerate clostridia spores, facilitating accurate water quality assessments and contributing to public health protection. This method offers a standardized, reproducible approach essential for regulatory compliance, environmental monitoring, and microbiological research concerning waterborne anaerobic microorganisms.
Open preview (PDF)Water quality — Detection and enumeration of the spores of sulfite-reducing anaerobes (clostridia) — Part 1: Method by enrichment in a liquid medium