The relevance of molecular genotyping to allocate cases in a suspected outbreak of Legionella pneumonia in patients with prolonged immunosuppressive therapy.

Three cases of pneumonia caused by Legionella pneumophila serogroup 1 (Lp1) in immunosuppressed patients with repeated hospitalization were suspected as a healthcare-associated cluster. The environmental investigation did not reveal the presence of legionellae in the hospital patient rooms. Water samples collected from the homes of two patients were also negative for Legionella spp. In the absence of environmental strains potentially involved in the infections, we proceeded to genotype environmental Lp1 strains isolated in the hospital during routine water sampling during the decade 2009-2019 and recovered after long-term storage at -20°C. These 'historical' strains exhibited a high grade of similarity and stability over time, regardless of the disinfection systems. The different molecular profiles shown among the clinical and environmental strains excluded a nosocomial outbreak. The study suggests that the application of molecular typing may be a useful tool to discriminate hospital vs community-acquired cases, mostly for severely immunosuppressed patients in whom the symptomatology could be insidious and the incubation period could be prolonged. Moreover, the genotyping allowed us to exclude any link between the cases.