Spirochete burden in infected nymphs fed on naïve and infected wild-type mice.

<p>Cohorts of nymphs infected as larvae with <i>B</i>. <i>burgdorferi</i> strain B31 were fed to repletion on groups of naïve, heterologously-infected (PKo) or homologously-infected (B31) mice, as identified at the bottom of the graph. Fed nymphs were collected at drop-off, pooled for individual mouse/tick cohorts, and crushed. The number of viable spirochetes per tick was estimated by plating an aliquot of each pooled homogenate for colony forming units (CFU). Each point on the graph represents the average spirochete load per tick for each cohort of 5–12 infected nymphs fed upon individual mice, with a total of 6–8 animals per group. (**)P = 0.02 naïve versus homologous; (*)P = 0.1 heterologous versus homologous; P = 0.8 naïve versus heterologous; P values calculated using non-parametric rank order test.