Concordance between IDEXX Legiolert® (liquid-culture assay) and plate-culture (ISO 11731:2017), for the detection and quantification of Legionella pneumophila in water samples.

BACKGROUND: Legionella pneumophila is a water-borne bacteria that can cause Legionnaires' disease. Monitoring of this organism in healthcare settings is paramount in keeping vulnerable populations protected. Legiolert® (IDEXX, USA) is a low-labour liquid-culture assay for the detection and enumeration of L. pneumophila (SG1-15) from water, requiring minimal operator training and providing confirmed results within 7 days.

AIMS: Hospital water samples (n=100) were processed using both Legiolert® and ISO 11731:2017 plate-culture method (membrane-filtration and culture on selective agars) to analyse concordance between the procedures.

METHODS: Incubation was at 39°C and 36°C respectively for seven days, followed by most-probable enumeration for Legiolert® and sub-culturing and serogrouping of suspected Legionella colonies, with plate-culture.

FINDINGS: L. pneumophila (SG1-15) was isolated from 25/100 samples when using Legiolert® or plate-culture. Fourteen additional Legiolert® samples tested positive for L. pneumophila; analysis of the same samples by plate-culture were negative (12/14) or yielded only Legionella rubrilucens (2/14; confirmed via MALDI-ToF-MS). L. pneumophila was not captured from Quanti-Tray/Legiolert® pouch wells of these positive samples, after subculture of puncture-aliquots on BCYE agar. Both methods in concordance, did not detect L. pneumophila in 61/100 samples.

CONCLUSION: Legiolert® and plate-culture are both satisfactory methods to detect L. pneumophila from water samples, and both isolated L. pneumophila in 25% of the sample population. Legiolert® provides a faster time to result, is less resource-demanding and labour-intensive. Although, there may be a low risk of cross-reactivity with other organisms. Both methods are suitable for the analysis of water in healthcare settings, where the monitoring of L. pneumophila is imperative in preventing cases of Legionnaires' disease.