Overview
EN 26461-1:1993 (identical to ISO 6461‑1:1986) defines a microbiological method for the detection and enumeration of spores of sulfite‑reducing anaerobes (clostridia) in water by enrichment in a liquid medium. The procedure is applicable to all types of water, including turbid water, and is designed to detect hardy spore forms that can indicate remote or intermittent faecal contamination and resistance to common disinfectants.
Key topics and requirements
- Scope: Detection and enumeration of sulfite‑reducing anaerobic spores (Clostridium spp.) by liquid enrichment.
- Selection (heat shock): Samples are heated to 75 ± 5 °C for 15 minutes to destroy vegetative cells and select spores.
- Enrichment & incubation:
- Inoculate specified volumes into liquid enrichment media (single and double strength variants).
- Incubate under anaerobic conditions at 37 ± 1 °C for 44 ± 4 hours.
- Culture media & reagents:
- Use defined diluent, differential reinforced clostridial medium (DRCM) and a complete medium prepared with sodium sulfite and iron(III) citrate.
- Manufacturers’ instructions and analytical‑grade chemicals are recommended; dehydrated media improve reproducibility.
- Detection (indicator reaction): Positive tubes show blackening from reduction of sulfite and precipitation of iron(II) sulfide; these are recorded as positive for sulfite‑reducing anaerobes.
- Result expression & reporting:
- Results are expressed according to ISO 8199 procedures (most probable number, MPN).
- Test reports must state the method used, MPN per sample volume, and any deviations or incidents affecting results.
- Practical notes: Hermetic sealing and large culture volumes can generate gas; precautions for anaerobiosis (e.g., heated iron wire) are suggested.
Applications and users
This standard is used where reliable detection of spore‑forming anaerobes is required:
- Drinking water and bottled water testing
- Wastewater and environmental water monitoring
- Water utilities and treatment plant control
- Public health and regulatory laboratories
- Environmental consultants and research labs
Benefits include ability to detect residual or historical contamination and to assess treatment/disinfection efficacy against resistant spores.
Related standards
- ISO 6461‑2 / EN 26461‑2: Method by membrane filtration (companion method)
- ISO 3696: Water for laboratory use - Specifications
- ISO 5667 (sampling guidance)
- ISO 8199: Microbiological examination - result expression and enumeration guidance
Keywords: EN 26461‑1:1993, ISO 6461‑1, water quality, sulfite‑reducing anaerobes, clostridia, spore detection, liquid enrichment, MPN, microbiological analysis.
