Borrelia burgdorferi-related isolates were tested for their sensitivity to normal human serum (NHS) and their ability to activate complement. By dark-field microscopy, electron microscopy, and subsurface plating, it was shown that exposure of a Borrelia garinii isolate to 10% or more NHS resulted in immobilization, blebbing, and killing of the spirochetes. These effects were mediated by complement, since they were not seen after heat treatment of NHS, in the presence of EDTA, or in an agammaglobulinemic serum. All seven B. garinii type 5 or 6 and all four VS116/M19 strains were serum sensitive, whereas all eight Borrelia afzelii, five of eight B. garinii type 4, and three of seven B. burgdorferi sensu stricto isolates were serum resistant. The other isolates were partially serum sensitive. Four serum-sensitive B. garinii isolates had been isolated from human cerebrospinal fluid. Most likely, activation of both the alternative pathway and the classical pathway of complement was involved, since bactericidal activity was diminished in properdin-deficient sera as well as in a C1q-depleted serum and in a C4-deficient serum. Bactericidal activity could be restored when a serum lacking C1q or C4 was mixed with a properdin-deficient serum. Isolates with various genetic backgrounds were equally able to activate C3 as measured by enzyme-linked immunosorbent assay. In the presence of Mg-EGTA, C3 was activated by all isolates after exposure to >10% NHS. This study shows that B. burgdorferi-related spirochetes can be either serum sensitive or serum resistant in vitro and that this characteristic is associated with their genetic background. Lyme borreliosis (LB) is a multisystem disorder caused by the tick-borne spirochetes Borrelia burgdorferi sensu stricto, Borrelia garinii, and Borrelia afzelii. Erythema migrans (EM), a skin lesion surrounding the site of the initial tick bite, is often the presenting symptom of LB (54). Subsequently, spirochetes can disseminate to other skin sites, resulting in multiple EM lesions and finally acrodermatitis chronica atrophicans (ACA), or to various organs, such as the heart, the joints, the eyes, or the central nervous system (54). Dissemination of Borrelia from the skin to distant sites is generally considered to occur hematogenously. After needle or tick-borne inoculation of mice with B. burgdorferi, all inoculated mice had positive blood cultures up to 6 weeks postinoculation (p.i.), while after 1 year only 1 of 10 mice was positive (6). Spirochetes have been cultured in low frequency from blood from rabbits 2 weeks p.i. (26) and from rhesus monkeys 1 to 6 weeks p.i. (41), although in another study blood cultures from such animals were negative (37). Culture of spirochetes from human blood has been successful in a few cases (7, 34, 58) but is usually negative (35). These findings suggest that spirochetes are present in the circulation in the early stage of infection but that they disappear in later stages of the disease. Whether the humoral immune system plays a role in the prevention o...
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