A Genomic Island Defines Subspecies-Specific Virulence Features of the Host-Adapted Pathogen Campylobacter fetus subsp. venerealis

The pathogen Campylobacter fetus comprises two subspecies, C. fetus subsp. fetus and C. fetus subsp. venerealis. Although these taxa are highly related on the genome level, they are adapted to distinct hosts and tissues. C. fetus subsp. fetus infects a diversity of hosts, including humans, and colonizes the gastrointestinal tract. In contrast, C. fetus subsp. venerealis is largely restricted to the bovine genital tract, causing epidemic abortion in these animals. In light of their close genetic relatedness, the specific niche preferences make the C. fetus subspecies an ideal model system to investigate the molecular basis of host adaptation. In this study, a subtractive-hybridization approach was applied to the genomes of the subspecies to identify different genes potentially underlying this specificity. The comparison revealed a genomic island uniquely present in C. fetus subsp. venerealis that harbors several genes indicative of horizontal transfer and that encodes the core components necessary for bacterial type IV secretion. Macromolecular transporters of this type deliver effector molecules to host cells, thereby contributing to virulence in various pathogens. Mutational inactivation of the putative secretion system confirmed its involvement in the pathogenicity of C. fetus subsp. venerealis.Campylobacter species are Gram-negative epsilonproteobacteria highly adapted to mucosal surfaces. The majority are human and/or animal pathogens (19, 61). The 18 species comprising the genus Campylobacter display a high degree of host and tissue specificity, which makes them excellent models to study host-pathogen relationships (25). The most prominent member, Campylobacter jejuni, is a commensal of the chicken intestine and the major cause of human bacterial diarrhea (74). Comparative analysis of Campylobacter genomes has revealed a process of genome decay—supported by a small genome size (about 1.5 Mb) and the loss of metabolic genes—consistent with successful adaptation to a specific niche (41). Campylobacter genomes are among the densest bacterial genomes known, with about 95% coding sequence. Despite this evidence of reduction, plasticity in genetic composition remains evident, as strain-specific genes comprise a substantial proportion of the entire repertoire of 1,500 to 1,800 genes (16, 23, 25, 56).This study focuses on the species Campylobacter fetus, which is represented by the two subspecies C. fetus subsp. fetus and C. fetus subsp. venerealis. Although the two taxa are genetically closely related, they exhibit striking tissue and host specificity. C. fetus subsp. fetus is a human, as well as animal, pathogen. Human infection results in serious systemic disease, especially in immunocompromised people. C. fetus subsp. fetus is the Campylobacter species most often isolated from human blood (75), and it is considered an emerging pathogen (9). The infection mode shares similarities with that of Salmonella enterica serovar Typhi. Orally acquired C. fetus subsp. fetus penetrates the intestinal mucosa, leading to bacteremia, and subsequent excretion via the biliary tract leads to secondary colonization of the intestine (9). Colonization of reproductive organs induces abortion in sheep and to a lesser extent in cattle, and very rarely in humans (11). C. fetus subsp. fetus can also be isolated from the intestinal tracts of birds and reptiles (78, 80). In contrast, C. fetus subsp. venerealis is host restricted. It is isolated primarily from the bovine genital tract and causes the epidemic disease bovine venereal campylobacteriosis (BVC). The reservoir of C. fetus subsp. venerealis is the penile prepuce of the bull. Transmission to cows occurs at coitus or during artificial insemination, and infection leads to endometritis, abortion, and infertility (28). Since BVC is a worldwide problem with substantial economic consequences, diagnosed cases must be registered (75) and import and export of bovine semen and embryos for cattle breeding requires statutory preclusion of C. fetus infection (2). Despite the distinct niche preferences of the C. fetus subspecies, they show high genetic relatedness, complicating the task of correct subspecies identification (46, 62, 81). Their population structure is clonal, and C. fetus subsp. venerealis is thought to represent a bovine clone of C. fetus (81).In this study, we employed the C. fetus subspecies to investigate the genetic basis for their host and tissue specificities. A genomic subtractive-hybridization approach was taken to identify subspecies-specific genomic fragments. This led to the discovery of a genomic island exclusively present on the chromosome of the host-adapted subspecies C. fetus subsp. venerealis. This island harbors a type IV secretion system (T4SS), as well as mobility genes (insertion sequence IS] transposases and phage integrases) and shares substantial homology and similar structure with resistance plasmids found in other Campylobacter species. These features are indicative of a horizontally acquired genetic element. Finally, mutational analysis of genes within the island substantiates its involvement in C. fetus subsp. venerealis virulence.