Plasmids and Rickettsial Evolution: Insight from Rickettsia felis

Abstract

BackgroundThe genome sequence of Rickettsia felis revealed a number of rickettsial genetic anomalies that likely contribute not only to a large genome size relative to other rickettsiae, but also to phenotypic oddities that have confounded the categorization of R. felis as either typhus group (TG) or spotted fever group (SFG) rickettsiae. Most intriguing was the first report from rickettsiae of a conjugative plasmid (pRF) that contains 68 putative open reading frames, several of which are predicted to encode proteins with high similarity to conjugative machinery in other plasmid-containing bacteria.Methodology/Principal FindingsUsing phylogeny estimation, we determined the mode of inheritance of pRF genes relative to conserved rickettsial chromosomal genes. Phylogenies of chromosomal genes were in agreement with other published rickettsial trees. However, phylogenies including pRF genes yielded different topologies and suggest a close relationship between pRF and ancestral group (AG) rickettsiae, including the recently completed genome of R. bellii str. RML369-C. This relatedness is further supported by the distribution of pRF genes across other rickettsiae, as 10 pRF genes (or inactive derivatives) also occur in AG (but not SFG) rickettsiae, with five of these genes characteristic of typical plasmids. Detailed characterization of pRF genes resulted in two novel findings: the identification of oriV and replication termination regions, and the likelihood that a second proposed plasmid, pRFδ, is an artifact of the original genome assembly.Conclusion/SignificanceAltogether, we propose a new rickettsial classification scheme with the addition of a fourth lineage, transitional group (TRG) rickettsiae, that is unique from TG and SFG rickettsiae and harbors genes from possible exchanges with AG rickettsiae via conjugation. We offer insight into the evolution of a plastic plasmid system in rickettsiae, including the role plasmids may have played in the acquirement of virulence traits in pathogenic strains, and the likely origin of plasmids within the rickettsial tree.

National Surveillance of Spotted Fever Group Rickettsioses in the United States, 2008–2012

“…The separation of TRG and SFG Rickettsia species, based on vector and phylogenetic age, is a relatively recent development (2007) and much of the past and current literature still refers to members of both groups collectively as SFG Rickettsia species. 14 This report uses the term SFG rickettsiosis to refer to human infections of both SFG and TRG Rickettsia represented in passive surveillance data reported under the category "spotted fever rickettsiosis. "…”

Seroprevalence of Rickettsia bellii and Rickettsia felis in dogs, São José dos Pinhais, State of Paraná, Brazil

“…are obligate intracellular bacteria, members of the Rickettsiales order and Rickettsiaceae family (RAOULT; ROUX, 1997). The genus Rickettsia was recently divided into four distinct groups: the spotted fever group (SFG), the typhus group (TG), the transitional group (TRG), and the ancestral group (AG) (GILLESPIE et al, 2007). In Brazil at least seven species of Rickettsia have been described: Rickettsia rickettsii, Rickettsia parkeri, Rickettsia rhipicephali, and Rickettsia amblyommii, all of them from the SFG transmitted through ticks; Rickettsia felis, of TRG, and Rickettsia typhi, of TG, both transmitted through fleas; and Rickettsia bellii, of AG, transmitted through ticks (LABRUNA, 2009).…”

Survey of Ticks (Acari: Ixodidae) and Their <I>Rickettsia</I> in an Atlantic Rain Forest Reserve in the State of São Paulo, Brazil

“…Pathogenic Rickettsia species have been classically classiÞed into the spotted fever group (SFG) and the typhus group (Fournier and Raoult 2007). More recently, part of the SFG species was split into a new group, designated as the transitional group (Gillespie et al 2007). Currently, all SFG species are primarily associated with ticks (Weinert et al 2009).…”