A 9-year-old boy presents with a migratory rash with central clearing on the back of his neck. The child had recently been on vacation with his family in Oregon and had gone hiking. The child's pediatrician observes the rash and suspects an infection with B. burgdorferi. Which of the following is thought to be a virulence factor of this organism

Differential expression of outer surface proteins is thought to be involved with virulence; answers A, B and C are incorrect: (A) intracellular growth in leukocytes is a virulence factor of Ehrlichia; (B) endotoxins are characteristic of gram-negative organisms, not Borrelia; (C) localization in reticuloendothelial cells occurs in infections with Francisella tularensis.