Restriction and expansion of Ehrlichia strain diversity.
Ehrlichia are tick-borne gram negative, obligately intracellular bacteria. The 16S rRNA gene DNA sequences are highly conserved among strains of each Ehrlichia species. The 28-kDa/Map-1 outer membrane protein genes are highly diversified among strains of Ehrlichia chaffeensis and E. ruminantium, but are highly conserved among E. canis isolates. The diversity of the immunodominant proteins of E. chaffeensis and E. ruminantium in contrast with the conservation of the immunodominant proteins of E. canis suggests that E. chaffeensis and E. ruminantium face more host immune pressure than E. canis or that E. chaffeensis and E. ruminantium evolved earlier than E. canis and have diverged.