**Performance of Legiolert Test vs. ISO 11731 to Confirm** *Legionella pneumophila* **Contamination in Potable Water Samples**

**Maria Scaturro 1, Matteo Bu** ff**oni 1, Antonietta Girolamo 1, Sandra Cristino 2, Luna Girolamini 2, Marta Mazzotta 2, Maria Antonietta Bucci Sabattini 3, Cristina Maria Zaccaro 3, Leonarda Chetti 3, Microbiology Arpa Novara Laboratory 4, Antonino Bella 1, Maria Cristina Rota 1 and Maria Luisa Ricci 1,\***


Received: 31 July 2020; Accepted: 21 August 2020; Published: 23 August 2020

**Abstract:** Detection and enumeration of *Legionella* in water samples is of grea<sup>t</sup> importance for risk assessment analysis. The plate culture method is the gold standard, but has received several well-known criticisms, which have induced researchers to develop alternative methods. The purpose of this study was to compare *Legionella* counts obtained by the analysis of potable water samples through the plate culture method and through the IDEXX liquid culture Legiolert method. *Legionella* plate culture, according to ISO 11731:1998, was performed using 1 L of water. Legiolert was performed using both the 10 mL and 100 mL Legiolert protocols. Overall, 123 potable water samples were analyzed. Thirty-seven (30%) of them, positive for *L. pneumophila*, serogroups 1 or 2–14 by plate culture, were used for comparison with the Legiolert results. The Legiolert 10 mL test detected 34 positive samples (27.6%) and the Legiolert 100 mL test detected 37 positive samples, 27.6% and 30% respectively, out of the total samples analyzed. No significant di fference was found between either the Legiolert 10 mL and Legiolert 100 mL vs. the plate culture (*p* = 0.9 and *p* = 0.3, respectively) or between the Legiolert 10 mL and Legiolert 100 mL tests (*p* = 0.83). This study confirms the reliability of the IDEXX Legiolert test for *Legionella pneumophila* detection and enumeration, as already shown in similar studies. Like the plate culture method, the Legiolert assay is also suitable for obtaining isolates for typing purposes, relevant for epidemiological investigations.

**Keywords:** *Legionella*; Legiolert; ISO 11731; plate culture; potable water samples
