Antigenic and genetic heterogeneity of Borrelia burgdorferi populations transmitted by ticks

The genome of Borrelia burgdorferi encodes a large number of lipoproteins, many of which are expressed only at certain stages of the spirochete's life cycle. In the current study we describe the B, burgdorferi population stru cture with respect to the production of two lipoproteins [outer surface pro tein A (OspA) and outer surface protein C (OspC)] during transmission from the tick vector to the mammalian host. Before the blood meal, the bacteria in the tick were a homogeneous population that mainly produced OspA only, D uring the blood meal, the population became more heterogeneous; many bacter ia produced both OspA and OspC whereas others produced only a single Osp an d a few produced neither Osp. From the heterogeneous spirochetal population in the gut, a subset depleted of OspA entered the salivary glands and stab ly infected the host at time points >53 hr into the blood meal. We also exa mined genetic heterogeneity at the B. burgdorferi vlsE locus before and dur ing the blood meal. In unfed ticks, the vlsE locus was stable and one predo minant and two minor alleles were detected, During the blood meal, multiple vlsE alleles were observed in the tick. Tick feeding may increase recombin ation at the vlsE locus or selectively amplify rare vlsE alleles present in unfed ticks, On the basis of our data we propose a model, which is differe nt from the established model for B. burgdorferi transmission, Implicit in our model is the concept that tick transmission converts a homogeneous spir ochete population into a heterogeneous population that is poised to infect the mammalian host.