ISO 9308-1:2014

Overview

ISO 9308-1:2014 - Water quality - Enumeration of Escherichia coli and coliform bacteria - Part 1: Membrane filtration method for waters with low bacterial background flora - specifies a laboratory method to enumerate Escherichia coli (E. coli) and coliform bacteria in water using membrane filtration and chromogenic coliform agar (CCA). The method is intended for waters with low bacterial background (e.g., drinking water, disinfected pool water, finished water from treatment plants) where colony growth on CCA remains low. The standard emphasizes correct sampling, incubation, colony differentiation and quality assurance measures.

Key topics and technical requirements

• Principle: Filter a defined sample volume through a membrane (cellulose esters, nominal pore 0.45 µm), place the membrane on CCA, incubate and count target colonies.
• Incubation: (36 ± 2) °C for (21 ± 3) h (as specified in the document).
• Colony interpretation on CCA:
  • ß‑glucuronidase + ß‑galactosidase (dark-blue to violet) = presumptive E. coli.
  • ß‑galactosidase positive only (salmon to red) = presumptive coliforms (non‑E. coli); oxidase test required to exclude oxidase‑positive non‑coliforms (e.g., Aeromonas).
• Limitations:
  • Not suitable for surface waters or shallow wells with high background flora (background growth can interfere).
  • Some E. coli strains (e.g., ß‑D‑glucuronidase negative strains such as O157) may not appear as E. coli on CCA and will register as coliforms.
  • The draft references thresholds of about 80–100 total colonies on CCA as the upper range for routine applicability.
• Apparatus & media: autoclave, incubator, membrane filtration equipment, forceps, pH meter, CCA and oxidase reagents. Media and reagents should meet ISO 11133 and CCA performance must be verified per ISO 11133/ISO 7704.
• Results & reporting: Calculate colony counts per 100 ml (or filtered volume) following ISO 8199; reports must include method reference, sample ID, results, and any deviations.

Practical applications and users

• Intended users: drinking‑water laboratories, water utilities, pool operators, public health agencies, regulatory bodies, and accredited microbiological testing labs.
• Typical applications: routine monitoring of drinking water quality, verification of treatment plant effluent, compliance testing for disinfected recreational waters and finished water quality assurance.
• Useful where low bacterial background enables reliable chromogenic discrimination and accurate enumeration.

Related standards

• ISO 9308 (other parts): Part 2 (Most Probable Number), Part 3 (miniaturized MPN)
• ISO 11133 (culture media), ISO 7704 (membrane filter validation), ISO 8199 (enumeration guidance), ISO 19458 / ISO 5667 (sampling), ISO 3696 (water for laboratory use)

Keywords: ISO 9308-1:2014, water quality, E. coli, coliforms, membrane filtration, chromogenic coliform agar, enumeration, drinking water microbiology, water testing standard.