Detection in Water Networks as per ISO 11731:2017: Can Different Filter Pore Sizes and Direct Placement on Culture Media Influence Laboratory Results?

Determination of concentrations in water networks is useful for predicting legionellosis risks. The standard culture technique using concentration with membranes filters is the most commonly used method for environmental surveillance of . The aim of this study was to verify whether filtration with different filter pore sizes (0.2 and 0.45 µm) according to (ISO) 11731:2017, followed by directly placing them on culture media, can influence detection. Three laboratories participated in an experimental study that tested a known suspension of () serogroup 1 (ATCC 33152) (approximate final cell density of 15 CFU/mL). (ATCC 11775) and (ATCC 25668) were included as control tests. The average (95% CI) percentage of recovery of was 65% using 0.45-µm filters and 15% using 0.2-µm filters ( < 0.0001). For control tests, the average (95% CI) percentage of recovery was higher with 0.45 vs. 0.2 µm filters: 97% vs. 64% for ( < 0.00001) and 105% vs. 97% ( = 0.0244) for . Our results showed that the 0.45-µm filters provided the greatest detection of . Because the current national guidelines leave the choice of membrane porosity to the operator, experimental studies are important for directing operators towards a conscious choice to standardize environmental surveillance methods.