Legiolert: fast, accurate detection of Legionella pneumophila

The test is significantly faster than the spread-plate culture method, providing a confirmed result for Legionella pneumophila in seven days

Legionnaire’s disease is a life-threatening infection caused by the waterborne Legionella bacteria. More than 95 percent of cases are caused by the Legionella pneumophila species of the bacterium,1 and in practice, Legionella pneumophila should be considered the primary concern because these bacteria are most likely to cause disease.2

The traditional accepted technique for detecting and quantifying Legionella in water samples involves spread-plate culture, which is a method that dates back over 30 years. The testing protocol has multiple steps, and results can take up to two weeks to confirm. The method relies on a number of subjective decisions that need to be made by the microbiologist carrying out the testing, and the results can lack sensitivity and reproducibility, making it far from ideal.

The inconsistency and potential unreliability of these spread-plate tests can lead to difficulties in monitoring water quality accurately over time and making informed water management decisions. In situations where Legionella contamination is suspected, having access to fast, reliable results is crucial so that public health is not put at risk and remedial action can be taken quickly, and monitored accurately to ensure its effectiveness.

In 2016, IDEXX introduced the Legiolert test, a new method that uses a liquid culture. The test is significantly faster than the spread-plate culture method, providing a confirmed result for Legionella pneumophila in seven days. Peer-reviewed studies also show it to be more accurate and more sensitive, and, importantly, studies have shown the incidence of false negative results is lower, so dangerous bacteria are less likely to be missed.3 Legiolert was designed to identify all the serogroups of Legionella pneumophila, not just the most common serogroup 1.

The Legiolert liquid culture method removes the need for many of the laborious steps involved in the plate culture method and much of the associated variability. The agar plate is replaced by a liquid culture medium, which is much closer to the bacterium’s natural water habitat. The Legiolert reagent is added to a 100mL sample of water and includes the nutrients for the bacteria to grow, plus additives to suppress the growth of non-Legionella pneumophila bacteria. The reagent also contains an indicator which enables the target bacteria to be easily identified after incubation by a colour change, reducing the subjectivity of results.

Legiolert samples take between two and four minutes to prepare before they are added to an IDEXX Quanti-Tray, where the liquid is distributed between six large and 90 small reservoirs, sealed and incubated for seven days. The Legiolert colour indicator used for enumeration is extremely specific for the target Legionella pneumophila bacteria, thus there is no requirement for further confirmatory testing.

In 2019, Legiolert received NF Validation by AFNOR certification.4 This is a rigorous multi-step process that includes review by a committee of experts from regulatory authorities, public, private and industrial labs, as well as manufacturers. During this process the performance of the Legiolert method was judged equivalent to standards ISO 11731 and NF T90-431, the culture method used in France, with trial results from 14 laboratories demonstrating strong reproducibility of the Legiolert method.

1. Public Health England: Legionnaires’ disease in residents of England and Wales: 2016 Official Statistics

2. World Health Organization, Revision of Annex I of the Council Directive on the Quality of Water Intended for Human Consumption (Drinking Water Directive) - Background paper on microbiologically safe water and microbiological parameters, version 15 September 2016, page 27,. https://www.iso.org/standard/61782.html

3. Spies, K. Pleischl, B., Lange B., Longer, B., Hübner, J., Jurzki, L., Luden, K., Exner, M. International Journal of Hygiene and Environmental Health 221 (2018) 1047–1053. Full text at https://www.sciencedirect.com/science/article/pii/S1438463917306818

4. https://nf-validation.afnor.org/en/wp-content/uploads/sites/2/2019/08/IDX-33-06-06-19_en.pdf