Mechanisms and elimination of Borrelia burgdorferi persistence in vitro

Borrelia burgdorferi sensu lato complex are species of pleomorphic spirochete bacteria causing Lyme borreliosis (LB), the most common tick-borne disease in the Northern hemisphere. Although antibiotics usually kill the bacteria, some post-treatment patients suffer from long-term sequalae of LB, the development of which is currently unclear, although immune evasion and persistence of the bacteria are thought to play major roles. This study investigated the long-term infectivity and persistence of B. burgdorferi sensu stricto (henceforth referred to as B. burgdorferi) in human non-phagocytic cells in vitro, as well as, characterised borrelial outer membrane vesicles (BbOMVs), and assessed the cytotoxic effect of BbOMVs on these human cells. In addition, the bactericidal effects of two herbal compounds against B. burgdorferi pleomorphic forms were examined. The aim was to identify the mechanisms associated with B. burgdorferi persistence in human cells in vitro, and to provide future tools for eradicating such persistence. The results demonstrated that B. burgdorferi invaded and persisted in synovial and skin cells for nine days, without killing the host cells. Moreover, B. burgdorferi was observed in a variety of shapes, while simultaneously avoiding lysosomal colocalisation during the nine days of coculture. BbOMVs were on average 33 nm in diameter and contained antigenic proteins OspA, OspC, p39 and peptidoglycan, as well as double stranded DNA. However, the vesicles did not induce cell death after 72-h of coculture with the human cells. The analysis of two herbal compounds demonstrated effective eliminating of B. burgdorferi spirochetes, round bodies, and biofilm-like aggregates. In summary, this study demonstrated that B. burgdorferi can invade human cells with the possibility to evade host immune system, and with the help of different pleomorphic forms, persist inside a host. Furthermore, BbOMVs could be utilised as decoys and repositories for persisting antigens during a borrelial infection, and plant-based antimicrobials may offer a potential for therapeutics against long-term LB.