ISO 7899-3:2025
Water quality — Enumeration of intestinal enterococci — Part 3: Most probable number method
Water quality — Enumeration of intestinal enterococci — Part 3: Most probable number method
- Статус документа:
- Действующий
- Формат:
- Электронный (PDF)
- Количество страниц:
- 10
- Дата публикации:
- 17 июля 2025 г.
- Издание:
- ISO IS 7899 edition 1 version 1
- ICS:
- 07.100.20
This document specifies a method for the enumeration of intestinal enterococci in water, including Enterococcus faecalis, Enterococcus faecium, Enterococcus durans, Enterococcus avium, Enterococcus gallinarum, Enterococcus hirae, Enterococcus casselifavus. The method is based on the growth of target organisms in a liquid medium and calculation of the “most probable number” (MPN) of microorganisms by reference to MPN tables or using suitable MPN informatic programs. This method can be applied to drinking water and bathing water (fresh or marine), together with other similar water types including those containing an appreciable amount of suspended matter, and allows the detection of enterococci at 1 colony-forming unit (CFU) per 100 ml with definitive results within (26 ± 2) h in the presence of heterotrophic bacteria in numbers as high as 1 × 106 per 100 ml of sample. For bathing waters, fresh and marine, enterococci are best enumerated when samples are diluted 1:10. The test specified in this document relies upon the detection of intestinal enterococci based upon expression of the enzyme ß-D-glucosidase and provides a confirmed result in 24 h without further testing of positive wells. This document does not apply to bottled waters, for which the method has not been validated and therefore is outside the scope of this document, unless appropriate validation of performance of this method has been undertaken by the laboratory prior to use.
Abstract
Overview - ISO 7899-3:2025 (Most Probable Number method)
ISO 7899-3:2025 specifies a validated Most Probable Number (MPN) method for the enumeration of intestinal enterococci in water. The standard targets Enterococcus species commonly used as faecal indicator organisms (including E. faecalis, E. faecium, E. durans, E. avium, E. gallinarum, E. hirae and E. casselifavus). The method uses a defined substrate medium that detects expression of the enzyme ß-D-glucosidase (ortho‑nitrophenyl‑ß‑D‑glucoside) producing a blue-to‑green colour change. Results are calculated as an MPN per 100 ml using MPN tables or suitable software, with a detection limit down to 1 CFU per 100 ml.
Key topics and technical requirements
- Method principle: growth of enterococci in a liquid defined substrate; enzymatic colour change indicates positive wells.
- MPN formats: multiple‑well trays (51‑well for up to 200 MPN/100 ml; 97‑well for up to 2 419 MPN/100 ml) or validated multiple‑tube MPN formats.
- Incubation: (41 ± 0.5) °C for (26 ± 2) h; positive wells can be confirmed within 24 h without additional testing.
- Sample handling: transport and store at (5 ± 3) °C; analysis on day of collection or within 18 h (up to 24 h in exceptional cases as referenced).
- Interferences: validated to work in the presence of heterotrophic bacteria up to 1 × 10^6 per 100 ml and in samples with suspended matter.
- Bathing water guidance: fresh and marine bathing water samples are best enumerated after 1:10 dilution prior to testing.
- Exclusions: method is not validated for bottled water unless a laboratory validates performance prior to use.
- Validated products: standard is based on validated defined‑substrate technologies (e.g., Enterolert‑DW / Quanti‑Tray performance characteristics are addressed in annexes).
Applications and who uses this standard
ISO 7899-3:2025 is intended for:
- Public health and environmental laboratories performing routine water quality surveillance
- Drinking water suppliers and distribution system integrity monitoring
- Recreational water (bathing water) monitoring programs
- Regulatory agencies and contracted testing labs enforcing microbial standards
Practical benefits include sensitive detection of faecal contamination, relatively rapid turnaround (≈24–26 h), and compatibility with MPN automation and software for reporting.
Related standards
- ISO 8199 - general microbiological lab requirements for culture methods
- ISO 19458 - sampling for microbiological analysis of water
- ISO 11133 - preparation and performance testing of culture media
- ISO 13843 - validation guidance for MPN-type methods
Keywords: ISO 7899-3:2025, intestinal enterococci, MPN method, water quality, Enterococcus, ß-D-glucosidase, drinking water, bathing water, MPN trays.
Технические детали
- Технический комитет
- ISO/TC 147/SC 4 - Microbiological methods
- SKU
- ISO 7899-3:2025
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