Phylogenetic Evidence for the Existence of Multiple Strains of Rickettsia parkeri in the New World

Fernanda A Nieri-Bastos, Arlei Marcili, Rita De Sousa, Christopher D Paddock, Marcelo B Labruna
Applied and Environmental Microbiology 2018 April 15, 84 (8)
The bacterium Rickettsia parkeri has been reported to infect ticks of the " Amblyomma maculatum species complex" in the New World, where it causes spotted fever illness in humans. In South America, three additional rickettsial strains, namely, Atlantic rainforest, NOD, and Parvitarsum, have been isolated from the ticks Amblyomma ovale , Amblyomma nodosum , and Amblyomma parvitarsum , respectively. These three strains are phylogenetically closely related to R. parkeri , Rickettsia africae , and Rickettsia sibirica Herein, we performed a robust phylogenetic analysis encompassing 5 genes ( gltA , ompA , virB4 , dnaA , and dnaK ) and 3 intergenic spacers ( mppE-pur , rrl-rrf -ITS, and rpmE -tRNAfMet ) from 41 rickettsial isolates, including different isolates of R. parkeri , R. africae , R. sibirica , Rickettsia conorii , and strains Atlantic rainforest, NOD, and Parvitarsum. In our phylogenetic analyses, all New World isolates grouped in a major clade distinct from the Old World Rickettsia species ( R. conorii , R. sibirica , and R. africae ). This New World clade was subdivided into the following 4 clades: the R. parkeri sensu stricto clade, comprising the type strain Maculatum 20 and all other isolates of R. parkeri from North and South America, associated with ticks of the A. maculatum species complex; the strain NOD clade, comprising two South American isolates from A. nodosum ticks; the Parvitarsum clade, comprising two South American isolates from A. parvitarsum ticks; and the strain Atlantic rainforest clade, comprising six South American isolates from the A. ovale species complex ( A. ovale or Amblyomma aureolatum ). Under such evidences, we propose that strains Atlantic rainforest, NOD, and Parvitarsum are South American strains of R. parkeri IMPORTANCE Since the description of Rickettsia parkeri infecting ticks of the " Amblyomma maculatum species complex" and humans in the New World, three novel phylogenetic close-related rickettsial isolates were reported in South America. Herein, we provide genetic evidence that these novel isolates, namely, strains Atlantic rainforest, NOD, and Parvitarsum, are South American strains of R. parkeri. Interestingly, each of these R. parkeri strains seems to be primarily associated with a tick species group, namely, R. parkeri sensu stricto with the " Amblyomma maculatum species group," R. parkeri strain NOD with Amblyomma nodosum , R. parkeri strain Parvitarsum with Amblyomma parvitarsum , and R. parkeri strain Atlantic rainforest with the " Amblyomma ovale species group." Such rickettsial strain-tick species specificity suggests a coevolution of each tick-strain association. Finally, because R. parkeri sensu stricto and R. parkeri strain Atlantic rainforest are human pathogens, the potential of R. parkeri strains NOD and Parvitarsum to be human pathogens cannot be discarded.

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