Culture-Independent Quantification of Legionella pneumophila in Evaporative Cooling Systems Using Immunomagnetic Separation Coupled with Flow Cytometry

Legionella pneumophila are pathogenic bacteria that repeatedly occur in high concentrations in the process water of evaporative cooling systems (ECS). When released into the environment, the resulting bioaerosols can cause outbreaks with fatal consequences. The official, internationally accepted detection method for Legionella spp. in water samples is based on cultivation. However, cultivation is time-consuming and may underestimate the total count of viable L. pneumophila in ECS. Therefore, culture-independent methods are receiving attention for rapid monitoring. Cartridge-based immunomagnetic separation (IMS) coupled with flow cytometry (FCM) is an innovative, antibody-based method for the culture-independent quantification of L. pneumophila, using a panel of antibodies against serogroup (Sg) 1–15. We characterized the IMS-FCM method as a quantitative rapid test by general analytical procedures. Viable cryopreserved L. pneumophila standards were used in calibration experiments for the method. We achieved detection limits for Sg 1, Sg 4, and Sg 6 of 100, 105 and 88 viable cells per 100 mL, respectively. Furthermore, we demonstrated the practical applicability of IMS-FCM with real ECS samples and compared the performance against cultivation. Cultivation showed here no positive results, but IMS-FCM evidenced L. pneumophila in a range of 0–80,000 viable cells per 100 mL. This work demonstrates that IMS-FCM is a suitable, culture-independent, quantitative method for rapidly monitoring L. pneumophila.