Multilocus sequence typing (MLST) and whole-genome MLST of Campylobacter jejuni isolates from human infections in three districts during a seasonal peak in Finland

Abstract

A total of 95 human Campylobacter jejuni isolates acquired from domestic infections and collected from three districts in Finland during the seasonal peak (June to September) in 2012 were analyzed by PCR-based multilocus sequence typing (MLST) and by whole-genome sequencing (WGS). Four predominant sequence types (STs) were detected among the isolates: ST-45 (21%) and ST-230 (14%, ST-45 clonal complex [CC]), ST-267 (21%, ST-283 CC), and ST-677 (19%, ST-677 CC). In districts 1 and 3, most of the infections occurred from early July to the middle of August, with a peak at weeks 29 to 31, but in district 2, the infections were dispersed more evenly throughout 3 months (June to August). WGS data were used for further whole-genome MLST (wgMLST) analyses of the isolates representing the four common STs. Shared loci of the isolates within each ST were analyzed as distance matrices of allelic profiles by the neighbor-net algorithm. The highest allelic variations (>400 different alleles) were detected between different clusters of ST-45 isolates (1,121 shared loci), while ST-230 (1,264 shared loci), ST-677 (1,169 shared loci), and ST-267 isolates (1,217 shared loci) were less diverse with the clusters differing by <40 alleles. Closely related isolates showing no allelic variation (subclusters) were detected among all four major STs. In some cases, they originated from different districts, suggesting that isolates can be epidemiologically connected and may have the same infection source despite being originally identified as sporadic infections.

FIG 1

Frequencies (%) of the clonal complexes (CCs) among Finnish human C. jejuni isolates in three districts (columns 1, 2, and 3) and among human isolates (n = 454) collected from the Helsinki metropolitan area in Finland from 1996 to 2006 (column 4) (10). *CCs in which the difference in frequency in the district relative to the metropolitan area is statistically significant (P < 0.05).

FIG 2

Distribution of the infections caused by different C. jejuni STs from June to September (weeks 22 to 38) in three districts (according to the sampling date).

FIG 3

The neighbor-net networks of the C. jejuni isolates of the four common STs (ST-45, ST-230, ST-267, and ST-677). Allelic differences are shown with Arabic numerals. Networks were drawn from the shared loci of each set of isolates: ST-45 (1,121 loci), ST-230 (1,264 loci), ST-267 (1,217 loci), and ST-677 (1,169 loci). Isolates clustering together are indicated with ellipses (clusters 1 to 8), and isolates with identical alleles are indicated with capital letters (subclusters A to I).