Molecular typing of a Legionella pneumophila outbreak in Ontario, Canada

Abstract

An outbreak of Legionnaires' disease at a long-term care facility in Ontario, Canada from September to October 2005 resulted in the death of 23 residents and the illness of 112 other people. In response, molecular methods were developed to detect Legionella pneumophila in clinical lung samples and to subtype isolates from clinical and environmental samples. The targeted genetic loci included Legionella-specific virulence determinants (mip, icmO, sidA and lidA) and core bacterial determinants (ftsZ, trpS and dnaX). An established amplified fragment length polymorphism typing method provided the first indication of genetic relatedness between strains recovered from clinical samples and strains cultured from environmental samples taken from the outbreak site. These associations were verified using the European Working Group for Legionella Infections sequence-based typing protocol targeting the flaA, pilE, asd, mip, mompS and proA loci. These molecular typing methods confirmed the outbreak source as a contaminated air conditioning cooling tower.

Fig. 1.

AFLP analysis of outbreak-associated (A, B) and adjacent site (C) isolates. Molecular mass markers are indicated.

Fig. 2.

Genetic relatedness of outbreak, environmental and reference L. pneumophila strains. Phylogeny is based upon a neighbour-joining tree of the concatenated segments of core bacterial loci (trpS, ftsZ and trpS) and Legionella-specific virulence determinants (lidA, icmO, sidA, total 4243 nucleotides per entry). GenBank accession numbers for previously sequenced loci are presented in the text, and strains from the current study are abbreviated from the form 05-L-0XX to LXX. The source is indicated in parentheses. Bar, scale of the distance score.