Legionella pneumophila and Other Legionella Species Isolated from Legionellosis Patients in Japan between 2008 and 2016

Abstract

The Legionella Reference Center in Japan collected 427 Legionella clinical isolates between 2008 and 2016, including 7 representative isolates from corresponding outbreaks. The collection included 419 Legionella pneumophila isolates, of which 372 belonged to serogroup 1 (SG1) (87%) and the others belonged to SG2 to SG15 except for SG7 and SG11, and 8 isolates of other Legionella species (Legionella bozemanae, Legionella dumoffii, Legionella feeleii, Legionella longbeachae, Legionella londiniensis, and Legionella rubrilucens). L. pneumophila isolates were genotyped by sequence-based typing (SBT) and represented 187 sequence types (STs), of which 126 occurred in a single isolate (index of discrimination of 0.984). These STs were analyzed using minimum spanning tree analysis, resulting in the formation of 18 groups. The pattern of overall ST distribution among L. pneumophila isolates was diverse. In particular, some STs were frequently isolated and were suggested to be related to the infection sources. The major STs were ST23 (35 isolates), ST120 (20 isolates), and ST138 (16 isolates). ST23 was the most prevalent and most causative ST for outbreaks in Japan and Europe. ST138 has been observed only in Japan, where it has caused small-scale outbreaks; 81% of those strains (13 isolates) were suspected or confirmed to infect humans through bath water sources. On the other hand, 11 ST23 strains (31%) and 5 ST120 strains (25%) were suspected or confirmed to infect humans through bath water. These findings suggest that some ST strains frequently cause legionellosis in Japan and are found under different environmental conditions.IMPORTANCELegionella pneumophila serogroup 1 (SG1) is the most frequent cause of legionellosis. Our previous genetic analysis indicated that SG1 environmental isolates represented 8 major clonal complexes, consisting of 3 B groups, 2 C groups, and 3 S groups, which included major environmental isolates derived from bath water, cooling towers, and soil and puddles, respectively. Here, we surveyed clinical isolates collected from patients with legionellosis in Japan between 2008 and 2016. Most strains belonging to the B group were isolated from patients for whom bath water was the suspected or confirmed source of infection. Among the isolates derived from patients whose suspected infection source was soil or dust, most belonged to the S1 group and none belonged to the B or C groups. Additionally, the U group was discovered as a new group, which mainly included clinical isolates with unknown infection sources.

Keywords: Legionella pneumophila; Legionella species; genetic variability; legionellosis; sequence-based typing.

FIG 1

Annual incidence of legionellosis cases recorded by the National Epidemiological Surveillance of Infectious Diseases and the numbers of isolates submitted to the Legionella Reference Center in Japan between 2008 and 2016.

FIG 2

MST analysis of 419 L. pneumophila isolates collected in Japan between 2008 and 2016. The major ST numbers are shown beside the circles. The size of each circle indicates the number of isolates. Short thick branches connect single-locus variants, thin branches connect double- or triple-locus variants, dashed branches connect 4-locus variants, and thinner branches connect 5- or 6-locus variants. The numbers of locus variants are proportional to the lengths of the branches. Groups generated with single- and double-locus variants are indicated by shaded backgrounds. The group names corresponding to groups of environmental isolates are shown. Isolates other than SG1, shown by SG numbers in Roman numerals, are enclosed by thick lines.