ULTRASTRUCTURAL DIFFERENTIATION OF THE GENOGROUPS IN THE GENUS EHRLICHIA

Ultrastructural characteristics of 15 strains and isolates of ehrlichi ae belonging to three genogroups, or clades of genetically related org anisms united in the genera Ehrlichia, Cowdria, Anaplasma, Neoricketts ia and a strain of Wolbachia pipientis which represents a fourth genog roup in this cluster of species, were studied in continuous cell cultu re or in vivo: E. canis (Oklahoma strain and VHE isolate), E. muris (A S 145), E. chaffeensis (Arkansas, 91HE17 and Sapulpa), human granulocy tic ehrlichiae (HGE)(BDS, 96HE27, 96HE37, #54, #55 and #72), E. equi ( MRK), E. sennetsu (Miyayama), E. risticii (HRC-IL). Wolbachia pipienti s was studied in the naturally infected Aedes albopictus mosquito cell line Aa23. All organisms were similar in the normal ultrastructure of individual cells and in the ability to form abnormal, pathological eh rlichial cells of the same type irrespective of the species. Normally all ehrlichiae studied in cell culture existed in two morphological fo rms - reticulate and dense-cored cells, both of which could divide by binary fission. Most alterations were related to their membranes, espe cially the cell wall. Differences in the structure of intravacuolar mi crocolonies (morulae) of ehrlichiae and their inter-relations with the host cells allowed differentiation of the genogroups: the E. canis-E. chaffeensis-E. muris genogroup formed large morulae, with many ehrlic hiae, often suspended in a fibrillar matrix, and the host cell mitocho ndria and endoplasmic reticulum usually aggregated near the morulae an d were in contact with the morula membrane; the E. phagocytophila-E. e qui-HGE group morulae had no fibrillar matrix, no contacts with host c ell mitochodria, and they did not aggregate around the morulae; E. sen netsu-E. risticii group usually developed in small individual vacuoles that did not fuse with each other and divided along with the ehrlichi ae.