Spirochete burden in infected nymphs fed on naïve and infected Rag1 KO mice.

Cohorts of nymphs infected as larvae with B. burgdorferi strain B31 were fed to repletion on groups of naïve, heterologously infected (PKo) or homologously infected (B31) Rag1 KO 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 2 animals per group. The number of viable spirochetes per tick was estimated by plating an aliquot of the pooled homogenate of crushed ticks from each mouse/tick cohort for colony forming units (CFU). Naïve versus homologous P = 0.3; heterologous versus homologous P = 0.3; naïve versus heterologous P = 0.6; calculated using non-parametric rank order test.