A call to order at the spirochaetal host-pathogen interface.

As the Lyme disease spirochaete Borrelia burgdorferi shuttles back and forth between arthropod vector and vertebrate host, it encounters vastly different and hostile environments. Major mechanisms contributing to the success of this pathogen throughout this complex transmission cycle are phase and antigenic variation of abundant and serotype-defining surface lipoproteins. These peripherally membrane-anchored virulence factors mediate niche-specific interactions with vector/host factors and protect the spirochaete from the perils of the mammalian immune response. In this issue of Molecular Microbiology, Tilly, Bestor and Rosa redefine the roles of two lipoproteins, OspC and VlsE, during mammalian infection. Using a variety of promoter fusions in combination with a sensitive in vivo 'use it or lose it' gene complementation assay, the authors demonstrate that proper sequential expression of OspC followed by VlsE indeed matters. A previously suggested general functional redundancy between these and other lipoproteins is shown to be limited and dependent on an immunodeficient experimental setting that is arguably of diminished ecological relevance. These data reinforce the notion that OspC plays a unique role during initial infection while the antigenically variant VlsE proteins allow for persistence in the mammalian host.