Legionella detection and enumeration in water samples by ISO 11731-2017: which method is the most sensitive?

Legionnaires' disease is a severe pneumonia caused by inhalation of Legionella-contaminated aerosols. The prevention of Legionella relies on water disinfection and on accurate water testing to assess the effectiveness of these interventions. The norm ISO 11731:2017 provides multiple culture-based methods for detecting Legionella in water and recommends taking into account the most unfavorable result. This study evaluated six ISO-standardized techniques, including variations in media, concentration, and pre-treatments (heat or acid), applied to 276 hospital hot water samples. Legionella was detected in 120 samples, with substantial variability across methods. The most sensitive single technique (76%) was concentration and elution with membrane filter on GVPC medium after heat treatment. Combinations of three techniques yielded a higher detection rate (98%), while using all six techniques detected all positive samples. Techniques involving concentration and elution proved superior to direct plating or simple filtration. Heat treatment enhanced detection more effectively than acid treatment, especially in samples with interfering flora. GVPC medium consistently outperformed BCYE. The study supports using a strategic combination of methods, particularly those involving membrane filtration and GVPC medium with heat treatment, to optimize Legionella detection in healthcare water systems.

This study is of critical importance as it provides a comprehensive evaluation of the sensitivity of various ISO 11731:2017-standardized methods for detecting Legionella in hospital water systems-a key concern in infection prevention. Given the high variability in recovery rates across different techniques and the limitations posed by adjacent flora, the study offers valuable insights into optimizing laboratory protocols. By analyzing 276 water samples using six techniques and their combinations, the researchers highlight the superiority of methods involving concentration, membrane filtration, and heat treatment-particularly those using GVPC medium. These findings not only support evidence-based recommendations for routine environmental surveillance in healthcare settings but also help laboratories balance accuracy, cost, and resource allocation. The study's implications are especially relevant for institutions housing vulnerable populations, where early and reliable Legionella detection is essential to prevent potentially fatal outbreaks of Legionnaires' disease.