Overview
EN ISO 11731:2017 - Water quality - Enumeration of Legionella specifies validated culture methods for isolating and estimating Legionella in water samples. Issued by CEN as the European adoption of ISO 11731:2017, this standard applies to all kinds of water - potable, industrial, waste and natural - and to related matrices such as biofilms, sediments and sludges. It supersedes EN ISO 11731-2:2008 and is intended for competent laboratory personnel performing Legionella testing. Note: not all Legionella species are culturable, so culture-based methods do not detect every species.
Key topics and technical requirements
- Scope and principle: culture-based enumeration and isolation of Legionella from diverse water matrices.
- Sampling and sample handling: guidance on collection and preparation for analysis.
- Concentration techniques: membrane filtration, washing procedures and centrifugation options for low-concentration samples.
- Pre‑treatments: selective treatments (e.g., heat and acid) to reduce interfering microorganisms prior to culture.
- Culture media and incubation: use of selective and non-selective media (e.g., BCYE agar variants) and defined incubation/examination practices.
- Confirmation: procedures to confirm presumptive Legionella colonies (culture confirmation methods referenced).
- Result expression and reporting: how to calculate and report Legionella counts and interpret results.
- Quality assurance: media performance testing, preparation of working cultures, performance data and decision matrices.
- Annexes: informative and normative annexes covering species lists, media recipes, diluents, acid solution, sampling techniques (scraping, centrifugation), immunofluorescent confirmation and a decision matrix.
Practical applications and users
EN ISO 11731:2017 is used for:
- Routine water monitoring of building hot- and cold-water systems, cooling towers, spa pools and healthcare facilities.
- Outbreak investigations and source identification in cases of legionellosis.
- Verification and validation of control measures (thermal disinfection, chemical biocide programs).
- Regulatory compliance and laboratory accreditation where culture-based Legionella enumeration is required.
Typical users:
- Environmental and clinical microbiology laboratories
- Public health authorities and epidemiologists
- Building/facility managers, HVAC and water treatment consultants
- Risk assessors and occupational health teams
Related standards and context
- Developed by ISO/TC 147 (Water quality) and endorsed by CEN.
- Supersedes EN ISO 11731-2:2008; part of a wider suite of water microbiology standards and guidance used for Legionella control and surveillance.
For accurate implementation, laboratories should follow the full text, maintain appropriate biosafety practices and ensure staff competency when conducting EN ISO 11731:2017 culture methods.
