Complement resistance of seabird-associated Borrelia garinii

Borrelia (B.) garinii, a causative agent of Lyme borreliosis, is maintained by diverse avian hosts in the marine and terrestrial transmission cycles. Data regarding survivability of B. garinii towards complement of individual seabird species is scarce. In this study, we aimed to gain insight into host association of B. garinii by examining the susceptibility to sera of marine and terrestrial birds. Four B. garinii strains isolated from seabird-associated Ixodes uriae, the Ixodes ricinus isolate B. garinii 20047 and the human isolates B. garinii G1, B. bavariensis PBi, and B. afzelii FEM1 were assessed. Serum was collected from gull (Ichthyaetus audouinii, Larus michahellis) and shearwater juveniles (Calonectris borealis) at Berlenga Island and Deserta Island (Algrave) in Portugal. Serum from common blackbirds (Turdus merula), Japanese quail (Coturnix japonica), and human donors were investigated as well. All B. garinii isolates, independent of their biological origin resisted complement-mediated killing by the bird sera analyzed whereas B. afzelii FEM1 was efficiently killed. Of note, B. bavariensis PBi remained unaffected by Japanese quail serum while being killed by all other avian sera, suggesting a broader host range for this particular Borrelia species. Following incubation with serum from Japanese quail, complement deposition assessment revealed no binding of complement C3 to the surface of B. bavariensis PBi. In contrast, B. afzelii cells were completely covered with C3 after serum challenge. Our data expand previous findings and provide further insight into the role of host innate immunity in shaping reservoir host association of B. garinii.