BorreliaOuter Membrane Surface Proteins and Transmission Through the Tick

Pathogens of the bacterial genus Borrelia differ from fellow spirochetes, Treponema and Leptospira , in their use of arthropod vectors for transmission between warm-blooded hosts. The agents of relapsing fever, Borrelia recurrentis and Borrelia hermsii , are transmitted by lice and fast-feeding soft ticks, respectively, whereas the Lyme disease spirochete, Borrelia burgdorferi ( Bb ), is transmitted to human hosts via the hard-shelled tick, Ixodes (1, 2). The evolutionary adaptation to both arthropod and warm-blooded hosts involved the invention of repertoires of outer membrane surface proteins, largely lipoproteins, conferring the ability to adhere, recognize, and respond to mammalian and arthropod tissues. As described in this issue by Yang et al., the most robust model to date for Bb lipoprotein function are two plasmid-encoded genes, paralog siblings OspA and OspB , encoding prominently expressed outer envelope lipoproteins (3), which function during the passage of borreliae within the tick vector midgut (4). In turn, a third gene encoding the lipoprotein OspC is essential for egress to the tick salivary glands (5). Here I will give a brief introduction to Borrelia biology, describe the tremendous capacity and complexity of borrelial lipoprotein repertoires, and then focus on new studies and methodologies being brought to bear on understanding Bb Osp lipoprotein function during tick transmission.