Genetic diversity and antimicrobial susceptibility of Campylobacter jejuni isolates associated with sheep abortion in the United States and Great Britain

Abstract

Campylobacter infection is a leading cause of ovine abortion worldwide. Historically, genetically diverse Campylobacter fetus and Campylobacter jejuni strains have been implicated in such infections, but since 2003 a highly pathogenic, tetracycline-resistant C. jejuni clone (named SA) has become the predominant cause of sheep abortions in the United States. Whether clone SA was present in earlier U.S. abortion isolates (before 2000) and is associated with sheep abortions outside the United States are unknown. Here, we analyzed 54 C. jejuni isolates collected from U.S. sheep abortions at different time periods and compared them with 42 C. jejuni isolates associated with sheep abortion during 2002 to 2008 in Great Britain, using multilocus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE), and array-based comparative genomic hybridization (CGH). Although clone SA (ST-8) was present in the early U.S. isolates, it was not as tetracycline resistant (19% versus 100%) or predominant (66% versus 91%) as it was in the late U.S isolates. In contrast, C. jejuni isolates from Great Britain were genetically diverse, comprising 19 STs and lacking ST-8. PFGE and CGH analyses of representative strains further confirmed the population structure of the abortion isolates. Notably, the Great Britain isolates were essentially susceptible to most tested antibiotics, including tetracycline, while the late U.S. isolates were universally resistant to this antibiotic, which could be explained by the common use of tetracyclines for control of sheep abortions in the United States but not in Great Britain. These results suggest that the dominance of clone SA in sheep abortions is unique to the United States, and the use of tetracyclines may have facilitated selection of this highly pathogenic clone.

FIG 1

ClonalFrame phylogenetic tree for the 7-gene MLST data set, displaying the clonal relationship among the STs of 188 C. jejuni isolates from sheep abortions. The x axis is time in coalescent units. STs found among the Great Britain (GB) collection, those of the U.S. collection, and STs found among both collections are shown. Singletons (STs that could not be assigned to a CC) are denoted by UA. The number of isolates within each ST is shown in parentheses.

FIG 2

Genotype composition of C. jejuni isolates from sheep abortions, as determined by MLST. The distribution of each ST is shown within the early U.S. collection (2000 and before) of 21 isolates (A), the late U.S. collection (2003 and beyond) of 125 isolates (B), and the Great Britain (GB) collection (2002 to 2008) of 42 isolates (C). Percentages of ST-8 (clone SA) in the U.S. collections are indicated.

FIG 3

Dendrogram based on CGH analysis results for 18 C. jejuni isolates from sheep abortions. Divergent or conserved genes were translated to a binary code and analyzed with Cluster/TreeView. For each strain (shown on the top), a black line indicates the divergence of the gene in comparison to the one in IA3902. Although the pVir plasmid carries 53 genes, it is represented here as one unit. The large variable regions (RV1 to -12) are shown on the left. The conservation rate curve is shown on the right.

FIG 4

Gene enrichment analysis of the divergent genes (identified via CGH) among 18 C. jejuni isolates from sheep abortions. The fraction of genes in each COG category is shown on the x axis. COG categories that were significantly enriched (P < 0.0001, Z-test) are indicated by an asterisk.