Experimental assessment of infectious phenotype of spirochetes in fed nymphs.

<p><i>Ixodes scapularis</i> nymphs (infected as larvae with <i>B</i>. <i>burgdorferi</i> strain B31) were fed on naïve mice, on mice infected with the homologous strain (B31), or on mice infected with a heterologous strain (PKo) and collected after feeding to repletion. Blood-meal hosts representing different strains of wild-type and immune-deficient laboratory mice were used in separate experiments. <b>A.)</b> OspC+ spirochetes in the midguts of a subset of fed ticks from each mouse/tick cohort were visualized by immunofluorescence assay (IFA) with a monoclonal antibody specific for OspC and a polyclonal anti-<i>B</i>. <i>burgdorferi</i> serum to counter-stain midgut spirochetes <b>B.)</b> Viable spirochetes in homogenates prepared from the remaining fed ticks (pooled) in each mouse/tick cohort were quantified by plating an aliquot for colony forming units (CFU). <b>C.)</b> Mice were injected with serial dilutions containing defined numbers of viable spirochetes from fed tick homogenates to assess the relative infectivity of each experimental group.