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Euro Surveill. 2019 Jan;24(4). doi: 10.2807/1560-7917.ES.2019.24.4.1800192.

Air-conditioner cooling towers as complex reservoirs and continuous source of Legionella pneumophila infection evidenced by a genomic analysis study in 2017, Switzerland.

Author information

Swiss Institute of Bioinformatics, Basel, Switzerland.
Applied Microbiology Research, Department of Biomedicine, University of Basel, Basel, Switzerland.
Division of Clinical Microbiology, University Hospital Basel, Basel, Switzerland.
State Laboratory Basel-City, Basel, Switzerland.
Department of Health, Medical Services, Canton of Basel-Stadt, Basel, Switzerland.
Viollier, Allschwil, Switzerland.
National Reference Center for Legionella, Department of Laboratory medicine, Ente Ospedaliero Cantonale, Bellinzona, Switzerland.
Public Health Services, Ministry of Health, Jerusalem, Israel.
Department of Health Systems Management, School of Public Health, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel.
Division of Emergency Medicine, University Hospital Basel, Basel, Switzerland.
Division of Infectious Diseases and Hospital Epidemiology, University Hospital Basel, Basel, Switzerland.
Division of Internal Medicine, University Hospital Basel, Basel, Switzerland.
Department of Health, Medical Services, Canton of Basel-Country, Liestal, Switzerland.


IntroductionWater supply and air-conditioner cooling towers (ACCT) are potential sources of Legionella pneumophila infection in people. During outbreaks, traditional typing methods cannot sufficiently segregate L. pneumophila strains to reliably trace back transmissions to these artificial water systems. Moreover, because multiple L. pneumophila strains may be present within these systems, methods to adequately distinguish strains are needed. Whole genome sequencing (WGS) and core genome multilocus sequence typing (cgMLST), with their higher resolution are helpful in this respect. In summer 2017, the health administration of the city of Basel detected an increase of L. pneumophila infections compared with previous months, signalling an outbreak.AimWe aimed to identify L. pneumophila strains populating suspected environmental sources of the outbreak, and to assess the relations between these strains and clinical outbreak strains.MethodsAn epidemiological and WGS-based microbiological investigation was performed, involving isolates from the local water supply and two ACCTs (n = 60), clinical outbreak and non-outbreak related isolates from 2017 (n = 8) and historic isolates from 2003-2016 (n = 26).ResultsIn both ACCTs, multiple strains were found. Phylogenetic analysis of the ACCT isolates showed a diversity of a few hundred allelic differences in cgMLST. Furthermore, two isolates from one ACCT showed no allelic differences to three clinical isolates from 2017. Five clinical isolates collected in the Basel area in the last decade were also identical in cgMLST to recent isolates from the two ACCTs.ConclusionCurrent outbreak-related and historic isolates were linked to ACCTs, which form a complex environmental habitat where strains are conserved over years.


L. pneumophila; Legionella pneumophila; Legionnaires’ disease; Switzerland; WGS; cooling tower; outbreak; whole genome sequencing

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