Rickettsiaeare Gram-negative obligately intracellular coccobacilli belonging to the family Rickettsiaceaeand order Rickettsialesin the alpha subdivision of the class Proteobacteria. They can be found in the cytoplasm or nucleus of eukaryotic host cells .
The term “rickettsia” historically denoted small intracellular bacteria, which could not be identified by cultivation in axenic media due to their obligate intracellular nature . The order Rickettsialescontains the families Rickettsiaceae, Bartonellaceae, and Anaplasmataceae. The family Rickettsiaceaecontains the tribes Rickettsiae, Ehrlichia, and Wolbachia; the tribe Rickettsiaeincludes the genera Coxiella, Rickettsia, and Rochalimaea. The advent of molecular taxonomic methods including 16S rRNAgene analysis resulted in reclassification of rickettsial taxa, and several genera (e.g., Coxiella, Bartonella, and Rochalimaea) have been removed from the order Rickettsiales[10, 11]. Currently, the family Rickettsiaceaecontains genera Rickettsiaand Orientia. The genus Rickettsiatraditionally contained two groups of pathogenic Rickettsiae: the typhus group and the spotted fever group (SFG). The latter included approximately 20 species, mostly transmitted by ticks. Over several years, a remarkable diversity of Rickettsiaein arthropods has been found, which led to a new description of the ancestral group and includes Rickettsia belliiand Rickettsia canadensis, which are clearly distinct from other Rickettsiae. Subsequently, a transitional group containing Rickettsia felisand Rickettsia akariwas established, since these species share molecular features with both the typhus group and the SFG . A recent phylogenetic study, based on whole-genome data, provided a single tree topology that well describes the evolutionary history of the core genome and is, in general, consistent with previous studies .
2.4 Rickettsiaeassociated with ticks
Interestingly, hard tick (Ixodidae) hosts are found across the phylogeny of Rickettsiae, and related rickettsial species tend to share related tick host species. This suggests a tick was the most plausible ancestral host for rickettsial species associated with arthropods .
Of the approximately 900 known tick species, 81 nonrandomly selected species were tested for the presence of bacterial endosymbionts and 55.6% harbored Rickettsiae. The most prevalent endosymbiont in arthropods is Wolbachiaand in ticks Coxiella-LE (Coxiella-like endosymbiont) with 52.0 and 60.5% of the known species being infected, respectively. However, these results may be biased by uneven sample collections, e.g., in the study by Weinert et al. , the vectors of rickettsial diseases were highly overrepresented, and in the study by Duron et al. , the available tick species varied widely.
2.4.1 Perpetuation of Rickettsiaein nature
There are two types of Rickettsiaetransmission in ticks—vertical and horizontal.
Vertical transmissiontakes place from female to offspring via egg cytoplasm or from one arthropod stage to another after molting (i.e., from larva to nymph, from nymph to adult) [7, 16]. Rickettsiaecapable of invading ovarian tissues during oogenesis develop in the interstitial cells of tick ovaries and within oogonia and oocytes. Other tissues of rickettsial endosymbionts of ticks are rarely infected, as reported for Rickettsia peacockiiand Rickettsia buchneri. It has been documented in several pathogenic Rickettsiaethat bacteria can negatively interfere with tick reproduction. Species reported to use transovarial transmission are shown in Table 1.
|Species||Pathogenicity||Host species||TOT reported||Ref.|
|R. aeschlimannii||Spotted fever||A. variegatum, Rhipicephalusspp., Hyalommaspp., Hae. punctata||No|||
|R. africae||African tick bite fever||Amblyommaspp., Rhipicephalusspp., Hyalommaspp.||Yes|||
|R. amblyommatis||Unknown||Amblyommaspp., Rhipicephalusspp., D. nitens||Yes|||
|R. argasii||Unknown||A. dewae||No|||
|R. asembonensis||Unknown||Rh. sanguineus(mostly associated fleas)||No|||
|R. australis||Queensland tick typhus||Ixodesspp.||No|||
|R. bellii||Unknown||Amblyommaspp., Dermacentorspp., Haemaphysalisspp., I. loricatus, O. concanensis, C. capensis||Yes|||
|R. buchneri*||Unknown||I. scapularis||Yes|||
|R. canadensis||Unknown||Hae. leporispalustris||No|||
|R. conoriisubsp. caspia||Astrakhan fever||Rhipicephalusspp.||No|||
|R. conoriisubsp. conorii||Mediterranean spotted fever||Rhipicephalusspp., Haemaphysalisspp.||Yes|||
|R. conoriisubsp. indica||Indian tick typhus||Rh. sanguineus||No|||
|R. conoriisubsp. israelensis||Israeli spotted fever||Rh. sanguineus||Yes|||
|R. felis||Flea-borne spotted fever||Hae. flava, Rh. sanguineus, I. ovatus, C. capensis(mostly associated with fleas)||Yes|||
|R. gravesii||Unknown||A. triguttatum||No|||
|R. heilongjiangensis||Far Eastern spotted fever||Haemaphysalisspp., D. silvarum||No|||
|R. helvetica||Unnamed rickettsiosis||Ixodesspp.||Yes|||
|R. honei||Flinders Island spotted fever, Australian spotted fever (str. marmionii)||B. hydrosauri, Ixodesspp. (str. RB), Hae. novaeguineae (str. marmionii)||Yes|||
|R. hoogstraalii||Unknown||Haemaphysalisspp., Cariosspp., Arg. persicus||No|||
|R. japonica||Japanese spotted fever||Haemaphysalisspp., I. ovatus, D. taiwanensis||Yes|||
|R. massiliae||Unnamed rickettsiosis||Rhipicephalusspp., I. ricinus, Hae. paraleachi||Yes|||
|R. monacensis||Spotted fever||Ixodesspp.||Yes|||
|R. montanensis||Unknown||Dermacentorspp., A. americanum||Yes|||
|R. monteiroi||Unknown||A. incisum||No|||
|R. parkeri||Mild rickettsiosis||Amblyommaspp., D. variabilis||Yes|||
|R. peacockii*||Unknown||D. andersoni||Yes|||
|R. raoultii||SENLAT||Dermacentorspp., I. ricinus, Haemaphysalisspp., A. testudinarium||Yes|||
|R. rhipicephali||Unknown||Rhipicephalusspp., Dermacentorspp., Hae. juxtakochi||Yes|||
|R. rickettsii||Rocky Mountain spotted fever (str. Iowa avirulent)||Dermacentorspp., Rh. sanguineus, Amblyommaspp., Hae. leporispalustris||Yes|||
|R. sibiricasubsp. mongolitimoniae||Lymphangitis-associated rickettsiosis||Hyalommaspp., Rh. pusillus||No|||
|R. sibiricasubsp. sibirica||Siberian tick typhus||Dermacentorspp., Haemaphysalisspp., I. persulcatus||Yes?|||
|R. tamurae||Spotted fever||A. testudinarium||No|||
Valid and published Rickettsialspecies associated with ticks .
Abbreviations: A., Amblyomma; Arg., Argas; B., Bothriocroton; D., Dermacentor; H., Hyalomma; Hae., Haemaphysalis; I., Ixodes; O., Ornithodoros; R., Rickettsia; Ref., reference; Rh., Rhipicephalus; SENLAT, scalp eschar and neck lymphadenopathy after a tick bite; spp., species (plural); str., strain; TOT, transovarial transmission.
Horizontal transmission, i.e., transfer among host individuals, may involve several mechanisms. Co-feeding (i.e., several ticks feeding close to each other on the same host individual) seems to be one mode of accidental horizontal transmission of tick rickettsial endosymbionts . Sexual transmission (via copulation) has been reported but probably does not play a significant role in perpetuation of Rickettsiaein tick populations.
For successful horizontal transfer of Rickettsiaefrom a vertebrate host under natural conditions, a host must develop rickettsemia with sufficient levels of bacteria in the blood and for a sufficient duration. Since some Rickettsiaenegatively impact the health of their tick hosts (which is more evident for pathogenic species), a vertebrate host must maintain such Rickettsiaein nature (e.g., capybara for Rickettsia rickettsiiin South America ). However, the role of vertebrates in perpetuation of tick-borne Rickettsiaeremains largely unknown .
Rickettsial endosymbionts of ticks are mainly transmitted vertically, while pathogenic Rickettsiae are typically transmitted horizontally. Occasional horizontal transfer allows symbionts to disperse beyond their primary host species, which leads to limited phylogenetic congruence between tick hosts and rickettsial symbionts [14, 20].
2.4.2 Infection of Rickettsiae-free ticks
The initially infected site of a Rickettsia-free tick may be the gut when feeding on a Rickettsiae-infected vertebrate host . The first interaction with tick cells after Rickettsiaeingestion occurs in the midgut, the storage organ . Rickettsiaepass through the midgut barrier and escape the ticks’ immune response by entering hemocytes present in the hemolymph, then enter the epithelial cells, and replicate. After that, bacteria invade tissues and organs, where they replicate and persist .
2.4.3 Strict blood diet of ticks and rickettsial endosymbionts
For decades, it was not fully understood why ticks harbor rickettsial endosymbionts. It was previously suggested that some endosymbionts may manipulate reproduction or enable survival in changing environments ; however, specific reasons remained unclear until recently.
Some of arthropod endosymbionts became obligate mutualists that adapted to host specialization to a restricted diet, e.g., blood or plant sap . It had been found that the rickettsial endosymbiont of Ixodes scapularis, R. buchneri, was presented only in females of this tick species. As males do not feed with blood, a possible relationship of the rickettsial endosymbiont and the tick blood diet had been suggested .
This hypothesis has been confirmed by metabolic reconstructions derived from rickettsial endosymbiont genomes of R. buchneriand Rickettsiaspecies phylotype G021, which showed that they contain all the genes required for folate (vitamin B9) biosynthesis . This is in accordance with the expected nutritional compounds required for strict hematophagy . Vitamin B9 is not present in a restricted blood diet in sufficient amounts. Moreover, Rickettsiaspecies phylotype G021 was shown to massively proliferate after a tick blood meal in all stages .
2.4.4 Insights into rickettsial genomes
In the last decade, whole-genome sequences of several rickettsial species (including obligate endosymbionts) were published, which allows detailed analyses of their evolution and host associations .
The recurrent biphasic model described in parasitic and symbiotic organisms is characterized by longer phases of genome reduction and simplification, interrupted by shorter phases of episodic expansion . Rickettsial chromosomes and plasmids are in progressive degradation and size reduction and contain numerous laterally acquired genes that display evidence of horizontal transfer between Rickettsia, other Rickettsiaeand bacterial endosymbionts (such as Cardinium), and even eukaryotes [9, 13, 28]. For instance, rickettsial plasmids have gained novel metabolic functions that are missing in rickettsial chromosomes and which may fill host-metabolic gaps .
A convergent reductive pattern has led to relatively small rickettsial genomes, ranging from 1.1 Mb for pathogenic Rickettsia prowazekiiand Rickettsia typhito 2.1 Mb for the obligate endosymbiotic R. buchneri.
2.4.5 Are pathogenic and endosymbiotic Rickettsiaetwo separate groups?
The phylogenetic position does not define the pathogenicity since tick rickettsial endosymbiotic and vertebrate pathogen species are dispersed along the phylogeny . In the most recent review on tick-borne rickettsioses, it was stated that every member of the SFG should be considered a potential pathogen . Numerous pathogenic tick-borne Rickettsiaeare vertically transmitted ; hence, transovarial transmission is not a sign of nonpathogenicity. The ability of Rickettsiaeto invade tick host cells seems to be the crucial feature that was lost by endosymbionts. R. peacockii, in Dermacentor andersoni, is not able to enter hemocytes and salivary gland tissues, which establishes its obligate endosymbiotic nature and prevents infection of vertebrates . The borderline between pathogens and endosymbionts is not sharp since there are avirulent strains of pathogenic Rickettsiaethat retain the ability to persist in ticks and can be transmitted transovarially, such as R. rickettsiistrain Iowa .
The pathogenic and endosymbiotic lifestyle could probably evolve via various scenarios: First, loss of pathogenicity, as described for strictly endosymbiotic R. peacockii, which is closely related to the most clinically severe R. rickettsii. The genome of R. peacockiicontains various deletions and mutations caused by a recombination of transposon copies that extinguished its ability to cause cytopathic effects [32, 33]; a similar situation exists with nonpathogenic R. buchneri, which is closely related to pathogenic Rickettsia monacensis. However, since rickettsial phylogeny shows repeated occurrences of horizontal transfer, this may lead to the appearance of novel bacterial phenotypes as described in Q fever cases caused by Coxiella burnetii, which probably originated from a Coxiella-LE that infected vertebrate cells . Rickettsia vini, an obligate endosymbiont of ornithophilic Ixodes arboricolaand Ixodes lividusticks, has repeatedly been detected in Ixodes ricinusticks, which may illustrate horizontal transmission of endosymbiotic Rickettsiaevia co-feeding [34, 35, 36]. Since this species is a member of the SFG and was successfully isolated in vertebrate Vero cells, it may represent a potential candidate for a vertebrate pathogen .
2.4.6 Rickettsial endosymbionts in relationship to other maternally inherited bacteria within ticks
Ten distinct genera of maternally inherited bacteria have been recently described in ticks (e.g., [23, 38, 39]). Based on a recent study by Duron, the most prevalent bacterial genera in ticks are Coxiella-LE (60.5%) and Rickettsia(55.6%), both of which have been identified in more tick species than any other genera . While 43.2% of tested tick species harbored one bacterium, 56.3% were infected with two or more bacterial genera. Rickettsiahas also been found to nonrandomly aggregate with Midichloria. Such fixed multiple endosymbiotic associations may imply that, collectively, the bacteria can synthesize all the components needed for certain essential pathways and hence are interdependent .
Only 2 out of 81 tick species (2.5%) did not harbor any maternally inherited bacteria . In some filarial nematodes, symbiont genes acquired from bacteria via lateral gene transfer have been found in the host chromosome . This could explain why Duron did not detect any bacterial endosymbiont in two tick species. However, such horizontal gene transfer has yet to be reported in ticks .
Infection frequencies vary among different geographical populations of a given tick species . Combining maternal inheritance with horizontal transfer allows unrelated bacteria to coinfect one individual host and to form an endosymbiotic community with complex interactions resulting in phenotypic differentiation within tick populations . Recent studies have revealed that relationships among bacterial communities within ticks are more complex than had been previously assumed .
2.4.7 Interaction of nonpathogenic rickettsial endosymbionts and pathogenic bacteria
Ticks are exposed to various Rickettsiaewhile feeding on multiple hosts . However, typically only one rickettsial species is observed per individual tick . Transovarial transmission of more than one rickettsial species from the SFG has not been proven. It is believed that infection of tick ovaries could induce a specific molecular response that results in a second infection being blocking . However, the coexistence of R. bellii, which belongs to the ancestral group, with SFG Rickettsiaehas been described . Additionally, interactions of Rickettsiaewith other pathogens have been reported. The occurrence of R. belliiin D. andersoniticks precludes infection of Anaplasma marginale. Males of I. scapularisinfected by R. buchneriwere significantly protected against infection by Borrelia burgdorfericompared to R. buchneri-free males .