Schematic representation of the preparation of <i>E. chaffeensis</i> transposon mutants and <i>in vivo</i> screening to identify genes important for the pathogen's growth in deer and tick hosts.

<p>The protocol involves recovering <i>in vitro</i> culture-derived <i>E. chaffeensis</i> organisms, subjecting them to transposon mutagenesis, reinfecting to naïve host cells, selecting the mutants in culture resisting to antibiotic clearance, infecting a natural host (white-tailed deer), acquisition feding of ticks on the deer and finally assessing the mutants survived in deer and ticks.