Interactions betweenBorrelia burgdorferiand ticks

Borrelia burgdorferihas a complex life cycle with several different hosts, causing Lyme disease when it infects humans. In this Review, Fikrig and colleagues discuss howB. burgdorferiinfects and interacts with its tick vector to ensure onward transmission. Borrelia burgdorferiis the causative agent of Lyme disease and is transmitted to vertebrate hosts byIxodesspp. ticks. The spirochaete relies heavily on its arthropod host for basic metabolic functions and has developed complex interactions with ticks to successfully colonize, persist and, at the optimal time, exit the tick. For example, proteins shield spirochaetes from immune factors in the bloodmeal and facilitate the transition between vertebrate and arthropod environments. On infection,B. burgdorferiinduces selected tick proteins that modulate the vector gut microbiota towards an environment that favours colonization by the spirochaete. Additionally, the recent sequencing of theIxodes scapularisgenome and characterization of tick immune defence pathways, such as the JAK-STAT, immune deficiency and cross-species interferon-gamma pathways, have advanced our understanding of factors that are important forB. burgdorferipersistence in the tick. In this Review, we summarize interactions betweenB. burgdorferiandI. scapularisduring infection, as well as interactions with tick gut and salivary gland proteins important for establishing infection and transmission to the vertebrate host.