The genera Anaplasma, Ehrlichia, Cowdria, Neorickettsia and Wolbachia encompass a group of obligate intracellular bacteria that reside in vacuoles of eukaryotic cells and were previously placed in taxa based upon morphological, ecological, epidemiological and clinical characteristics. Recent genetic analyses of 16S rRNA genes, groESL and surface protein genes have indicated that the existing taxa designations are flawed. All 16S rRNA gene and groESL sequences deposited in GenBank prior to 2000 and selected sequences deposited thereafter were aligned and phylogenetic trees and bootstrap values were calculated using the neighbour-joining method and compared with trees generated with maximum-probability, maximum-likelihood, majority-rule consensus and parsimony methods. Supported by bootstrap probabilities of at least 54 %, 16S rRNA gene comparisons consistently clustered to yield four distinct clades characterized roughly as Anaplasma (including the Ehrlichia phagocytophila group, Ehrlichia platys and Ehrlichia bovis) with a minimum of 96 1 % similarity, Ehrlichia (including Cowdria ruminantium) with a minimum of 97 7 % similarity, Wolbachia with a minimum of 95 6 % similarity and Neorickettsia (including Ehrlichia sennetsu and Ehrlichia risticii ) with a minimum of 94 9 % similarity. Maximum similarity between clades ranged from 87 1 to 94 9 %. Insufficient differences existed among E. phagocytophila, Ehrlichia equi and the human granulocytic ehrlichiosis (HGE) agent to support separate species designations, and this group was at least 98 2 % similar to any Anaplasma species. These 16S rRNA gene analyses are strongly supported by similar groESL clades, as well as biological and antigenic characteristics. It is proposed that all members of the tribes Ehrlichieae and Wolbachieae be transferred to the family Anaplasmataceae and that the tribe structure of the family Rickettsiaceae be eliminated. The genus Anaplasma should be emended to include Anaplasma (Ehrlichia) phagocytophila comb. nov. (which also encompasses the former E. equi and the HGE agent), Anaplasma (Ehrlichia) bovis comb. nov. and Anaplasma (Ehrlichia) platys comb. nov., the genus Ehrlichia should be emended to include Ehrlichia (Cowdria) ruminantium comb. nov. and the genus Neorickettsia should be emended to include Neorickettsia (Ehrlichia) risticii comb. nov. and Neorickettsia (Ehrlichia) sennetsu comb. nov. Keywords : J. S. Dumler and others INTRODUCTIONRecent improvements in molecular technologies have significantly advanced our abilities to conduct genetic analyses and, for the first time, clearly indicated the proper phylogenetic positions of most of the fastidious bacterial species in the families Rickettsiaceae, Bartonellaceae and Anaplasmataceae in the order Rickettsiales (Woese et al., 1990 ;Weisburg et al., 1989 ; Brenner et al., 1993 ; Birtles et al., 1995). By 16S rRNA sequencing, Weisburg et al. (1989) demonstrated that Coxiella burnetii and Wolbachia persica belonged to the γ-Proteobacteria, while the remaining members of the order Ri...
The rickettsia Anaplasma marginale is the most prevalent tick-borne livestock pathogen worldwide and is a severe constraint to animal health. A. marginale establishes lifelong persistence in infected ruminants and these animals serve as a reservoir for ticks to acquire and transmit the pathogen. Within the mammalian host, A. marginale generates antigenic variants by changing a surface coat composed of numerous proteins. By sequencing and annotating the complete 1,197,687-bp genome of the St. Maries strain of A. marginale, we show that this surface coat is dominated by two families containing immunodominant proteins: the msp2 superfamily and the msp1 superfamily. Of the 949 annotated coding sequences, just 62 are predicted to be outer membrane proteins, and of these, 49 belong to one of these two superfamilies. The genome contains unusual functional pseudogenes that belong to the msp2 superfamily and play an integral role in surface coat antigenic variation, and are thus distinctly different from pseudogenes described as byproducts of reductive evolution in other Rickettsiales.rickettsiales ͉ bacterial artificial chromosome ͉ St. Maries strain
Ehrlichiae are responsible for important tick-transmitted diseases, including anaplasmosis, the most prevalent tick-borne infection of livestock worldwide, and the emerging human diseases monocytic and granulocytic ehrlichiosis. Antigenic variation of major surface proteins is a key feature of these pathogens that allows persistence in the mammalian host, a requisite for subsequent tick transmission. In Anaplasma marginale pseudogenes for two antigenically variable gene families, msp 2 and msp 3, appear in concert. These pseudogenes can be recombined into the functional expression site to generate new antigenic variants. Coordinated control of the recombination of these genes would allow these two gene families to act synergistically to evade the host immune response.
Anaplasma marginale is a tick-borne pathogen, one of several closely related ehrlichial organisms that cause disease in animals and humans. These Ehrlichia species have complex life cycles that require, in addition to replication and development within the tick vector, evasion of the immune system in order to persist in the mammalian reservoir host. This complexity requires efficient use of the small ehrlichial genome. A. marginale and related ehrlichiae express immunoprotective, variable outer membrane proteins that have similar structures and are encoded by polymorphic multigene families. We show here that the major outer membrane protein of A. marginale, MSP2, is encoded on a polycistronic mRNA. The genomic expression site for this mRNA is polymorphic and encodes numerous amino acid sequence variants in bloodstream populations of A. marginale. A potential mechanism for persistence is segmental gene conversion of the expression site to link hypervariable msp2 sequences to the promoter and polycistron.Ehrlichiae are major causes of tick-borne diseases, including the recently emergent human monocytic and two granulocytic ehrlichioses and the most prevalent tick-borne infection in cattle worldwide, anaplasmosis (6). These pathogens, classified in genogroups I and II of the tribe Ehrlichieae, have a complex life cycle characterized by acute and persistent infection in the mammalian host and several replicative and developmental stages within the tick vector (10, 17). Notably, this complexity is achieved using a small genome, only 0.8 to 1.5 Mb in a single chromosome (1,25). Persistence of Anaplasma marginale in cattle, which is fundamental for continued transmission, reflects sequential expression of antigenically variant outer membrane proteins that are encoded by the msp2 multigene family (21). The outer membrane proteins of different ehrlichial organisms are significantly similar to one another in amino acid sequence, are all encoded by multigene families, and possess one to four variable regions (8, 13, 18-20, 22, 26, 29, 32). In A. marginale-infected cattle, three to six MSP2 variants are expressed in each sequential rickettsemic cycle, which recur every 4 to 8 weeks during persistent infection (8,9,15,16). Thus, over the 7-year period in which A. marginale has been shown to persist, over 500 variants may be expressed. Although the cyclic emergence and immune control of A. marginale is similar to that occurring in African trypanosomiasis, the mechanisms used by the organism to persist in mammalian hosts are unknown. We show here that variation of msp2 in erythrocyte stages of A. marginale proceeds through the formation of different sequence mosaics in a polycistronic expression site. MATERIALS AND METHODSIsolation and sequencing of A. marginale genomic DNA. Florida and South Idaho strains of A. marginale were maintained as liquid nitrogen-cryopreserved stabilates of infected bovine erythrocytes in dimethyl sulfoxide-phosphate-buffered saline that were then used to infect cattle (20). A. marginale genomic DNA ...
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