Humans parasitized by the hard tick Ixodes ricinus are seropositive to Midichloria mitochondrii: is Midichloria a novel pathogen, or just a marker of tick bite?

Infection by Tickborne Bacterium Candidatus Midichloria Associated with First Trimester Pregnancy Loss, Tennessee, USA

A previously healthy 26-year-old woman in middle Tennessee, USA, experienced a first trimester pregnancy loss after multiple tick bites. Histopathology, 16S rRNA sequencing, and electron microscopy examination of the products of conception revealed an infection by a bacterium within the Candidatus Midichloria genus.

Metatranscriptomics provide insights into the role of the symbiont Midichloria mitochondrii in Ixodes ticks

Ticks are important vectors of bacterial, viral, and protozoan pathogens of humans and animals worldwide. Candidatus Midichloria mitochondrii is a highly abundant bacterial endosymbiont found in many tick species, including two medically important ticks respectively found in Europe and Australia, Ixodes ricinus and Ixodes holocyclus. The present study aimed to determine the symbiont's biological role by identifying lateral gene transfer (LGT) events, characterizing the transcriptome, and performing differential expression analyses. Metatranscriptomic data revealed that M. mitochondrii species in I. ricinus and I. holocyclus were equipped with the metabolic potential and were actively transcribing the genes for several important roles including heme, biotin and folate synthesis, oxidative stress response, osmotic regulation, and ATP production in microaerobic conditions. Differential expression analyses additionally showed an upregulation in stringent response and DNA repair genes in M. mitochondrii of I. holocyclus nymphs compared to adults. Low rates of differential expression suggest the symbiont may lack global gene regulation, as observed in other endosymbionts. Moreover, the identification of an LGT event and the proposed specialization of the M. mitochondrii strains, mIxholo1 and mIxholo2, for different I. holocyclus life stages highlight the complex interactions between M. mitochondrii and their tick hosts.

Metagenome diversity illuminates the origins of pathogen effectors

Recent metagenome-assembled genome (MAG) analyses have profoundly impacted Rickettsiology systematics. The discovery of basal lineages (novel families Mitibacteraceae and Athabascaceae) with predicted extracellular lifestyles exposed an evolutionary timepoint for the transition to host dependency, which seemingly occurred independent of mitochondrial evolution. Notably, these basal rickettsiae carry the Rickettsiales vir homolog (rvh) type IV secretion system and purportedly use rvh to kill congener microbes rather than parasitize host cells as described for later-evolving rickettsial pathogens. MAG analysis also substantially increased diversity for the genus Rickettsia and delineated a sister lineage (the novel genus Tisiphia) that stands to inform on the emergence of human pathogens from protist and invertebrate endosymbionts. Herein, we probed Rickettsiales MAG and genomic diversity for the distribution of Rickettsia rvh effectors to ascertain their origins. A sparse distribution of most Rickettsia rvh effectors outside of Rickettsiaceae lineages illuminates unique rvh evolution from basal extracellular species and other rickettsial families. Remarkably, nearly every effector was found in multiple divergent forms with variable architectures, indicating profound roles for gene duplication and recombination in shaping effector repertoires in Rickettsia pathogens. Lateral gene transfer plays a prominent role in shaping the rvh effector landscape, as evinced by the discovery of many effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchange between Rickettsia and Legionella species. Our study exemplifies how MAGs can yield insight into pathogen effector origins, particularly how effector architectures might become tailored to the discrete host cell functions of different eukaryotic hosts.IMPORTANCEWhile rickettsioses are deadly vector-borne human diseases, factors distinguishing Rickettsia pathogens from the innumerable bevy of environmental rickettsial endosymbionts remain lacking. Recent metagenome-assembled genome (MAG) studies revealed evolutionary timepoints for rickettsial transitions to host dependency. The rvh type IV secretion system was likely repurposed from congener killing in basal extracellular species to parasitizing host cells in later-evolving pathogens. Our analysis of MAG diversity for over two dozen rvh effectors unearthed their presence in some non-pathogens. However, most effectors were found in multiple divergent forms with variable architectures, indicating gene duplication and recombination-fashioned effector repertoires of Rickettsia pathogens. Lateral gene transfer substantially shaped pathogen effector arsenals, evinced by the discovery of effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchanges between Rickettsia and Legionella species. Our study exemplifies how MAGs yield insight into pathogen effector origins and evolutionary processes tailoring effectors to eukaryotic host cell biology.

Tick symbiosis

As obligate blood-feeders, ticks serve as vectors for a variety of pathogens that pose threats on both human and livestock health. The microbiota that ticks harbor play important roles in influencing tick nutrition, development, reproduction, and vector. These microbes also affect the capacity of ticks to transmit pathogens (vector competence). Therefore, comprehending the functions of tick microbiota will help in developing novel and effective tick control strategies. Here, we summarize the effects of main tick symbiotic bacteria on tick physiology and vector competency.

Single-cell genomics revealed Candidatus Grellia alia sp. nov. as an endosymbiont of Eutreptiella sp. (Euglenophyceae)

Though endosymbioses between protists and prokaryotes are widespread, certain host lineages have received disproportionate attention what may indicate either a predisposition to such interactions or limited studies on certain protist groups due to lack of cultures. The euglenids represent one such group in spite of microscopic observations showing intracellular bacteria in some strains. Here, we perform a comprehensive molecular analysis of a previously identified endosymbiont in the Eutreptiella sp. CCMP3347 using a single cell approach and bulk culture sequencing. The genome reconstruction of this endosymbiont allowed the description of a new endosymbiont Candidatus Grellia alia sp. nov. from the family Midichloriaceae. Comparative genomics revealed a remarkably complete conjugative type IV secretion system present in three copies on the plasmid sequences of the studied endosymbiont, a feature missing in the closely related Grellia incantans. This study addresses the challenge of limited host cultures with endosymbionts by showing that the genomes of endosymbionts reconstructed from single host cells have the completeness and contiguity that matches or exceeds those coming from bulk cultures. This paves the way for further studies of endosymbionts in euglenids and other protist groups. The research also provides the opportunity to study the diversity of endosymbionts in natural populations.

Tick-borne pathogens of zoonotic and veterinary importance in cattle ticks in Ghana

Ticks are important vectors involved in the transmission of pathogens of zoonotic and veterinary importance. In this study, ticks were collected from cattle in Navrongo, Kintampo, and Kumasi and screened for pathogen DNA using PCR and Sanger sequencing. A total of 454 ticks were collected, morphologically identified and confirmed using primers that target the 660-bp segment of the mitochondrial COI gene. The predominant tick species was Amblyomma variegatum (70.26%). DNA was extracted from 85 tick pools and screened for the presence of Rickettsia DNA based on the 639 bp of the outer membrane protein A (ompA) gene, Ehrlichia/Anaplasma DNA based on the 345 bp fragment of the 16SrRNA gene and Babesia/ Theileria DNA based on the 560 bp fragment of the ssrRNA gene. From the 85 tick pools, the DNA of pathogens detected were Rickettsia africae (36.47%), Rickettsia aeschlimannii (16.47%), Ehrlichia canis (2.35%), Babesia occultans (1.18%), Theileria velifera (1.18%) and a symbiont Candidatus Midichloria mitochondrii (8.24%). This study reports the first molecular detection of Candidatus Cryptoplasma californiense (1.18%) in Ghana. Coinfections were recorded in 8.24% of the tick pools. The findings of this study highlight the importance of tick species in Ghana and the need to adopt effective control measures to prevent pathogen spread.

Cross-alteration of murine skin and tick microbiome concomitant with pathogen transmission after Ixodes ricinus bite

BACKGROUND

Ticks are major vectors of diseases affecting humans such as Lyme disease or domestic animals such as anaplasmosis. Cross-alteration of the vertebrate host skin microbiome and the tick microbiome may be essential during the process of tick feeding and for the mechanism of pathogen transmission. However, it has been poorly investigated.

METHODS

We used mice bitten by field-collected ticks (nymphs and adult ticks) in different experimental conditions to investigate, by 16S rRNA gene metabarcoding, the impact of blood feeding on both the mouse skin microbiome and the tick microbiome. We also investigated by PCR and 16S rRNA gene metabarcoding, the diversity of microorganisms transmitted to the host during the process of tick bite at the skin interface and the dissemination of the pathogen in host tissues (blood, heart, and spleen).

RESULTS

Most of the commensal bacteria present in the skin of control mice were replaced during the blood-feeding process by bacteria originating from the ticks. The microbiome of the ticks was also impacted by the blood feeding. Several pathogens including tick-borne pathogens (Borrelia/Borreliella, Anaplasma, Neoehrlichia, Rickettsia) and opportunistic bacteria (Williamsia) were transmitted to the skin microbiome and some of them disseminated to the blood or spleen of the mice. In the different experiments of this study, skin microbiome alteration and Borrelia/Borreliella transmission were different depending on the tick stages (nymphs or adult female ticks).

CONCLUSIONS

Host skin microbiome at the bite site was deeply impacted by the tick bite, to an extent which suggests a role in the tick feeding, in the pathogen transmission, and a potentially important impact on the skin physiopathology. The diversified taxonomic profiles of the tick microbiome were also modified by the blood feeding. Video Abstract.

Metagenome diversity illuminates origins of pathogen effectors

Recent metagenome assembled genome (MAG) analyses have profoundly impacted Rickettsiology systematics. Discovery of basal lineages (Mitibacteraceae and Athabascaceae) with predicted extracellular lifestyles reveals an evolutionary timepoint for the transition to host dependency, which occurred independent of mitochondrial evolution. Notably, these basal rickettsiae carry the Rickettsiales vir homolog (rvh) type IV secretion system (T4SS) and purportedly use rvh to kill congener microbes rather than parasitize host cells as described for derived rickettsial pathogens. MAG analysis also substantially increased diversity for genus Rickettsia and delineated a basal lineage (Tisiphia) that stands to inform on the rise of human pathogens from protist and invertebrate endosymbionts. Herein, we probed Rickettsiales MAG and genomic diversity for the distribution of Rickettsia rvh effectors to ascertain their origins. A sparse distribution of most Rickettsia rvh effectors outside of Rickettsiaceae lineages indicates unique rvh evolution from basal extracellular species and other rickettsial families. Remarkably, nearly every effector was found in multiple divergent forms with variable architectures, illuminating profound roles for gene duplication and recombination in shaping effector repertoires in Rickettsia pathogens. Lateral gene transfer plays a prominent role shaping the rvh effector landscape, as evinced by the discover of many effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchange between Rickettsia and Legionella species. Our study exemplifies how MAGs can provide incredible insight on the origins of pathogen effectors and how their architectural modifications become tailored to eukaryotic host cell biology.

The Biological and Ecological Features of Northbound Migratory Birds, Ticks, and Tick-Borne Microorganisms in the African-Western Palearctic

Identifying the species that act as hosts, vectors, and vehicles of vector-borne pathogens is vital for revealing the transmission cycles, dispersal mechanisms, and establishment of vector-borne pathogens in nature. Ticks are common vectors for pathogens causing human and animal diseases, and they transmit a greater variety of pathogenic agents than any other arthropod vector group. Ticks depend on the movements by their vertebrate hosts for their dispersal, and tick species with long feeding periods are more likely to be transported over long distances. Wild birds are commonly parasitized by ticks, and their migration patterns enable the long-distance range expansion of ticks. The African-Palearctic migration system is one of the world's largest migrations systems. African-Western Palearctic birds create natural links between the African, European, and Asian continents when they migrate biannually between breeding grounds in the Palearctic and wintering grounds in Africa and thereby connect different biomes. Climate is an important geographical determinant of ticks, and with global warming, the distribution range and abundance of ticks in the Western Palearctic may increase. The introduction of exotic ticks and their microorganisms into the Western Palearctic via avian vehicles might therefore pose a greater risk for the public and animal health in the future.

What do we know about the microbiome of I. ricinus?

I. ricinus is an obligate hematophagous parasitic arthropod that is responsible for the transmission of a wide range of zoonotic pathogens including spirochetes of the genus Borrelia, Rickettsia spp., C. burnetii, Anaplasma phagocytophilum and Francisella tularensis, which are part the tick´s microbiome. Most of the studies focus on "pathogens" and only very few elucidate the role of "non-pathogenic" symbiotic microorganisms in I. ricinus. While most of the members of the microbiome are leading an intracellular lifestyle, they are able to complement tick´s nutrition and stress response having a great impact on tick´s survival and transmission of pathogens. The composition of the tick´s microbiome is not consistent and can be tied to the environment, tick species, developmental stage, or specific organ or tissue. Ovarian tissue harbors a stable microbiome consisting mainly but not exclusively of endosymbiotic bacteria, while the microbiome of the digestive system is rather unstable, and together with salivary glands, is mostly comprised of pathogens. The most prevalent endosymbionts found in ticks are Rickettsia spp., Ricketsiella spp., Coxiella-like and Francisella-like endosymbionts, Spiroplasma spp. and Candidatus Midichloria spp. Since microorganisms can modify ticks' behavior, such as mobility, feeding or saliva production, which results in increased survival rates, we aimed to elucidate the potential, tight relationship, and interaction between bacteria of the I. ricinus microbiome. Here we show that endosymbionts including Coxiella-like spp., can provide I. ricinus with different types of vitamin B (B2, B6, B7, B9) essential for eukaryotic organisms. Furthermore, we hypothesize that survival of Wolbachia spp., or the bacterial pathogen A. phagocytophilum can be supported by the tick itself since coinfection with symbiotic Spiroplasma ixodetis provides I. ricinus with complete metabolic pathway of folate biosynthesis necessary for DNA synthesis and cell division. Manipulation of tick´s endosymbiotic microbiome could present a perspective way of I. ricinus control and regulation of spread of emerging bacterial pathogens.

Detection of Endosymbiont Candidatus Midichloria mitochondrii and Tickborne Pathogens in Humans Exposed to Tick Bites, Italy

During 2021, we collected blood and serum samples from 135 persons exposed to tick bites in southern Italy. We serologically and molecularly screened for zoonotic tickborne pathogens and only molecularly screened for Candidatus Midichloria mitochondrii. Overall, 62 (45.9%) persons tested positive for tickborne pathogens. Coxiella burnetii was detected most frequently (27.4%), along with Rickettsia spp. (21.5%) and Borrelia spp. (10.4%). We detected Candidatus M. mitochondrii DNA in 46 (34.1%) participants who had statistically significant associations to tickborne pathogens (p<0.0001). Phylogenetic analysis of Candidatus M. mitochondrii sequences revealed 5 clades and 8 human sequence types that correlated with vertebrates, Ixodes spp. ticks, and countries in Europe. These data demonstrated a high circulation of tickborne pathogens and Candidatus M. mitochondrii DNA in persons participating in outdoor activities in southern Italy. Our study shows how coordinated surveillance among patients, clinicians, and veterinarians could inform a One Health approach for monitoring and controlling the circulation of tickborne pathogens.

The Genetic Diversity of Rickettsiella Symbionts in Ixodes ricinus Throughout Europe

Rickettsiella species are bacterial symbionts that are present in a great variety of arthropod species, including ixodid ticks. However, little is known about their genetic diversity and distribution in Ixodes ricinus, as well as their relationship with other tick-associated bacteria. In this study, we investigated the occurrence and the genetic diversity of Rickettsiella spp. in I. ricinus throughout Europe and evaluated any preferential and antagonistic associations with Candidatus Midichloria mitochondrii and the pathogens Borrelia burgdorferi sensu lato and Borrelia miyamotoi. Rickettsiella spp. were detected in most I. ricinus populations investigated, encompassing a wide array of climate types and environments. The infection prevalence significantly differed between geographic locations and was significantly higher in adults than in immature life stages. Phylogenetic investigations and protein characterization disclosed four Rickettsiella clades (I-IV). Close phylogenetic relations were observed between Rickettsiella strains of I. ricinus and other arthropod species. Isolation patterns were detected for Clades II and IV, which were restricted to specific geographic areas. Lastly, although coinfections occurred, we did not detect significant associations between Rickettsiella spp. and the other tick-associated bacteria investigated. Our results suggest that Rickettsiella spp. are a genetically and biologically diverse facultative symbiont of I. ricinus and that their distribution among tick populations could be influenced by environmental components.

Co-Occurrence of Francisella, Spotted Fever Group Rickettsia, and Midichloria in Avian-Associated Hyalomma rufipes

The migratory behavior of wild birds contributes to the geographical spread of ticks and their microorganisms. In this study, we aimed to investigate the dispersal and co-occurrence of Francisella and spotted fever group Rickettsia (SFGR) in ticks infesting birds migrating northward in the African-Western Palaearctic region (AWPR). Birds were trapped with mist nests across the Mediterranean basin during the 2014 and 2015 spring migration. In total, 575 ticks were collected from 244 birds. We screened the ticks for the species Francisella tularensis, the genus Francisella, and SFGR by microfluidic real-time PCR. Confirmatory analyses and metagenomic sequencing were performed on tick samples that putatively tested positive for F. tularensis during initial screenings. Hyalomma rufipes was the most common tick species and had a high prevalence of Francisella, including co-occurrence of Francisella and SFGR. Metagenomic analysis of total DNA extracted from two H. rufipes confirmed the presence of Francisella, Rickettsia, and Midichloria. Average nucleotide identity and phylogenetic inference indicated the highest identity of the metagenome-assembled genomes to a Francisella-like endosymbiont (FLE), Rickettsia aeschlimannii, and Midichloria mitochondrii. The results of this study suggest that (i) FLE- and SFGR-containing ticks are dispersed by northbound migratory birds in the AWPR, (ii) H. rufipes likely is not involved in transmission of F. tularensis in the AWPR, and (iii) a dual endosymbiosis of FLEs and Midichloria may support some of the nutritional requirements of H. rufipes.

The Symbiotic Continuum Within Ticks: Opportunities for Disease Control

Among blood-sucking arthropods, ticks are recognized as being of prime global importance because of their role as vectors of pathogens affecting human and animal health. Ticks carry a variety of pathogenic, commensal, and symbiotic microorganisms. For the latter, studies are available concerning the detection of endosymbionts, but their role in the physiology and ecology of ticks remains largely unexplored. This review paper focuses on tick endosymbionts of the genera Coxiella, Rickettsia, Francisella, Midichloria, and Wolbachia, and their impact on ticks and tick-pathogen interactions that drive disease risk. Tick endosymbionts can affect tick physiology by influencing nutritional adaptation, fitness, and immunity. Further, symbionts may influence disease ecology, as they interact with tick-borne pathogens and can facilitate or compete with pathogen development within the vector tissues. Rickettsial symbionts are frequently found in ticks of the genera of Ixodes, Amblyomma, and Dermacentor with relatively lower occurrence in Rhipicephalus, Haemaphysalis, and Hyalomma ticks, while Coxiella-like endosymbionts (CLEs) were reported infecting almost all tick species tested. Francisella-like endosymbionts (FLEs) have been identified in tick genera such as Dermacentor, Amblyomma, Ornithodoros, Ixodes, and Hyalomma, whereas Wolbachia sp. has been detected in Ixodes, Amblyomma, Hyalomma, and Rhipicephalus tick genera. Notably, CLEs and FLEs are obligate endosymbionts essential for tick survival and development through the life cycle. American dog ticks showed greater motility when infected with Rickettsia, indirectly influencing infection risk, providing evidence of a relationship between tick endosymbionts and tick-vectored pathogens. The widespread occurrence of endosymbionts across the tick phylogeny and evidence of their functional roles in ticks and interference with tick-borne pathogens suggests a significant contribution to tick evolution and/or vector competence. We currently understand relatively little on how these endosymbionts influence tick parasitism, vector capacity, pathogen transmission and colonization, and ultimately on how they influence tick-borne disease dynamics. Filling this knowledge gap represents a major challenge for future research.

The 'other' Rickettsiales: an overview of the family 'Candidatus Midichloriaceae'

The family 'Candidatus Midichloriaceae' constitutes the most diverse but least studied lineage within the important order of intracellular bacteria Rickettsiales. Midichloriaceae endosymbionts are found in many hosts, including terrestrial arthropods, aquatic invertebrates, and protists. Representatives of the family are not documented to be pathogenic, but some are associated with diseased fish or corals. Different genera display a range of unusual features, such as full sets of flagellar genes without visible flagella, or the ability to invade host mitochondria. Since studies on 'Ca. Midichloriaceae' tend to focus on the host, the family is rarely addressed as a unit and we therefore lack a coherent picture of its diversity. Here we provide four new midichloriaceae genomes and we survey molecular and ecological data from the entire family. Features like genome size, ecological context, and host transitions vary considerably even among closely related midichloriaceae, suggesting a high frequency of such shifts, incomplete sampling, or both. Important functional traits involved in energy metabolism, flagella and secretion systems were independently reduced multiple times with no obvious correspondence to host or habitat, corroborating the idea that many features of these 'professional symbionts' are largely independent of host identity. Finally, despite 'Ca. Midichloriaceae' being predominantly studied in ticks, our analyses show that the clade is mainly aquatic, with a few terrestrial offshoots. This highlights the importance of considering aquatic hosts, and protists in particular, when reconstructing the evolution of these endosymbionts and by extension all Rickettsiales. Importance Among endosymbiotic bacterial lineages, few are as intensely studied as Rickettsiales, which include the causative agents of spotted fever, typhus, and anaplasmosis. And yet, an important subgroup called 'Candidatus Midichloriaceae' receives little attention despite accounting for a third of the diversity of Rickettsiales and harbouring a wide range of bacteria with unique features, like the ability to infect mitochondria. Midichloriaceae are found in many hosts, from ticks to corals to unicellular protozoa, and studies on them tend to focus on the host groups. Here, for the first time since the establishment of this clade, we address the genomics, evolution, and ecology of 'Ca. Midichloriaceae' as a whole, highlighting trends and patterns, the remaining gaps in our knowledge, and its importance for the understanding of symbiotic processes in intracellular bacteria.

The bacterial biome of ticks and their wildlife hosts at the urban-wildland interface

Advances in sequencing technologies have revealed the complex and diverse microbial communities present in ticks (Ixodida). As obligate blood-feeding arthropods, ticks are responsible for a number of infectious diseases that can affect humans, livestock, domestic animals and wildlife. While cases of human tick-borne diseases continue to increase in the northern hemisphere, there has been relatively little recognition of zoonotic tick-borne pathogens in Australia. Over the past 5 years, studies using high-throughput sequencing technologies have shown that Australian ticks harbour unique and diverse bacterial communities. In the present study, free-ranging wildlife (n=203), representing ten mammal species, were sampled from urban and peri-urban areas in New South Wales (NSW), Queensland (QLD) and Western Australia (WA). Bacterial metabarcoding targeting the 16S rRNA locus was used to characterize the microbiomes of three sample types collected from wildlife: blood, ticks and tissue samples. Further sequence information was obtained for selected taxa of interest. Six tick species were identified from wildlife: Amblyomma triguttatum, Ixodes antechini, Ixodes australiensis, Ixodes holocyclus, Ixodes tasmani and Ixodes trichosuri. Bacterial 16S rRNA metabarcoding was performed on 536 samples and 65 controls, generating over 100 million sequences. Alpha diversity was significantly different between the three sample types, with tissue samples displaying the highest alpha diversity (P<0.001). Proteobacteria was the most abundant taxon identified across all sample types (37.3 %). Beta diversity analysis and ordination revealed little overlap between the three sample types (P<0.001). Taxa of interest included Anaplasmataceae, Bartonella, Borrelia, Coxiellaceae, Francisella, Midichloria, Mycoplasma and Rickettsia. Anaplasmataceae bacteria were detected in 17.7% (95/536) of samples and included Anaplasma, Ehrlichia and Neoehrlichia species. In samples from NSW, 'Ca. Neoehrlichia australis', 'Ca. Neoehrlichia arcana', Neoehrlichia sp. and Ehrlichia sp. were identified. A putative novel Ehrlichia sp. was identified from WA and Anaplasma platys was identified from QLD. Nine rodent tissue samples were positive for a novel Borrelia sp. that formed a phylogenetically distinct clade separate from the Lyme Borrelia and relapsing fever groups. This novel clade included recently identified rodent-associated Borrelia genotypes, which were described from Spain and North America. Bartonella was identified in 12.9% (69/536) of samples. Over half of these positive samples were obtained from black rats (Rattus rattus), and the dominant bacterial species identified were Bartonella coopersplainsensis and Bartonella queenslandensis. The results from the present study show the value of using unbiased high-throughput sequencing applied to samples collected from wildlife. In addition to understanding the sylvatic cycle of known vector-associated pathogens, surveillance work is important to ensure preparedness for potential zoonotic spillover events.

Tick exposure and risk of tick-borne pathogens infection in hunters and hunting dogs: a citizen science approach

Citizen science may be described as a research involving communities and individuals, other than scientists. Following this approach, along with the evidence of a high prevalence of Rickettsia spp. in Dermacentor marginatus from wild boars in hunting areas of southern Italy, this study aimed to assess the occurrence of tick‐borne pathogens (TBPs) in ticks collected from hunters and their hunting dogs. From October 2020 to May 2021, ticks were collected from wild boar hunters (n = 347) and their dogs (n = 422) in regions of southern Italy (i.e., Apulia, Basilicata, Calabria, Campania and Sicily). All ticks were morphologically identified, classified according to gender, feeding status, host, geographic origin, and molecularly screened for zoonotic bacteria. Adult ticks (n = 411) were collected from hunters (i.e., n = 29; 8.4%; mean of 1.6 ticks for person) and dogs (i.e., n = 200; 47.4%; mean of 1.8 ticks for animal) and identified at species level as D. marginatus (n = 240, 58.4%), Rhipicephalus sanguineus sensu lato (n = 135, 32.8%), Rhipicephalus turanicus (n = 27, 6.6%) and Ixodes ricinus (n = 9, 2.2%). Overall, 45 ticks (i.e., 10.9%, 95% CI: 8.3‐14.3) tested positive for at least one tick‐borne agent, being Rickettsia slovaca the most frequent species (n = 37, 9.0%), followed by Rickettsia raoultii, Rickettsia aeschlimannii, Rickettsia monacensis, Coxiella burnetii, Borrelia lusitaniae and Candidatus Midichloria mitochondrii (n = 2, 0.5% each). Data herein presented demonstrate a relevant risk of exposure to TBPs for hunters and hunting dogs during the hunting activities. Therefore, the role of hunters to monitor the circulation of ticks in rural areas may be considered an effective example of the citizen science approach, supporting the cooperation toward private and public health stakeholders.

Modeling the Life Cycle of the Intramitochondrial Bacterium "Candidatus Midichloria mitochondrii" Using Electron Microscopy Data

“Candidatus Midichloria mitochondrii” is a Gram-negative bacterium that lives in strict intracellular symbiosis with the hard tick Ixodes ricinus, forming one of the most intriguing endosymbiosis described to date. The bacterium is capable of durably colonizing the host mitochondria, a peculiar tropism that makes “Ca. Midichloria mitochondrii” a very interesting tool to study the physiology of these cellular organelles. The interaction between the symbiont and the organelle has, however, been difficult to characterize. A parallelism with the predatory bacterium Bdellovibrio bacteriovorus has been drawn, suggesting the hypothesis that “Ca. Midichloria mitochondrii” could prey on mitochondria and consume them to multiply. We studied the life cycle of the bacterium within the host oocytes using a multidisciplinary approach, including electron microscopy, molecular biology, statistics, and systems biology. Our results were not coherent with a predatory-like behavior by “Ca. Midichloria mitochondrii” leading us to propose a novel hypothesis for its life cycle. Based on our results, we here present a novel model called the “mitochondrion-to-mitochondrion hypothesis.” Under this model, the bacterium would be able to move from mitochondrion to mitochondrion, possibly within a mitochondrial network. We show that this model presents a good fit with quantitative electron microscopy data.

Insect vectors' saliva and gut microbiota as a blessing in disguise: probability versus possibility

Drawing of host blood is a natural phenomenon during the bite of blood-probing insect vectors. Along with the blood meal, the vectors introduce salivary components and a trail of microbiota. In the case of infected vectors, the related pathogen accompanies the aforementioned biological components. In addition to Anopheles gambiae or Anopheles stephensi, the bites of other nonmalarial vectors cannot be ignored in malaria-endemic regions. Similarly, the bite incidence of Phlebotomus papatasi cannot be ignored in visceral leishmaniasis-endemic regions. Even the chances of getting bitten by uninfected vectors are higher than the infected vectors. We have discussed the probability or possibility of uninfected, infected, and/or nonvector's saliva and gut microbiota as a therapeutic option leading to the initial deterrent to pathogen establishment.

Molecular characterization of bacterial communities of two neotropical tick species (Amblyomma aureolatum and Ornithodoros brasiliensis) using rDNA 16S sequencing

Ticks are one of the main vectors of pathogens for humans and animals worldwide. However, they harbor non-pathogenic microorganisms that are important for their survival, facilitating both their nutrition and immunity. We investigated the bacterial communities associated with two neotropical tick species of human and veterinary potential health importance from Brazil: Amblyomma aureolatum and Ornithodoros brasiliensis. In A. aureolatum (adult ticks collected from wild canids from Southern Brazil), the predominant bacterial phyla were Proteobacteria (98.68%), Tenericutes (0.70%), Bacteroidetes (0.14%), Actinobacteria (0.13%), and Acidobacteria (0.05%). The predominant genera were Francisella (97.01%), Spiroplasma (0.70%), Wolbachia (0.51%), Candidatus Midichloria (0.25%), and Alkanindiges (0.13%). The predominant phyla in O. brasiliensis (adults, fed and unfed nymphs collected at the environment from Southern Brazil) were Proteobacteria (90.27%), Actinobacteria (7.38%), Firmicutes (0.77%), Bacteroidetes (0.44%), and Planctomycetes (0.22%). The predominant bacterial genera were Coxiella (87.71%), Nocardioides (1.73%), Saccharopolyspora (0.54%), Marmoricola (0.42%), and Staphylococcus (0.40%). Considering the genera with potential importance for human and animal health which can be transmitted by ticks, Coxiella sp. was found in all stages of O. brasiliensis, Francisella sp. in all stages of A. aureolatum and in unfed nymphs of O. brasiliensis, and Rickettsia sp. in females of A. aureolatum from Banhado dos Pachecos (BP) in Viamão municipality, Brazil, and in females and unfed nymphs of O. brasiliensis. These results deepen our understanding of the tick-microbiota relationship in Ixodidae and Argasidae, driving new studies with the focus on the manipulation of tick microbiota to prevent outbreaks of tick-borne diseases in South America.

When bacteria meet mitochondria: The strange case of the tick symbiont Midichloria mitochondrii†

Mitochondria are key eukaryotic organelles that perform several essential functions. Not surprisingly, many intracellular bacteria directly or indirectly target mitochondria, interfering with innate immunity, energy production or apoptosis, to make the host cell a more hospitable niche for bacterial replication. The alphaproteobacterium Midichloria mitochondrii has taken mitochondrial targeting to another level by physically colonising mitochondria, as shown by transmission electron micrographs of bacteria residing in the mitochondrial intermembrane space. This unique localization provokes a number of questions around the mechanisms allowing, and reasons driving intramitochondrial tropism. We suggest possible scenarios that could lead to this peculiar localization and hypothesize potential costs and benefits of mitochondrial colonisation for the bacterium and its host.

Symbiont dynamics during the blood meal of Ixodes ricinus nymphs differ according to their sex

Ticks harbour rich and diverse microbiota and, among the microorganisms associated with them, endosymbionts are the subject of a growing interest due to their crucial role in the biology of their arthropod host. Midichloria mitochondrii is the main endosymbiont of the European tick Ixodes ricinus and is found in abundance in all I. ricinus females, while at a much lower density in males, where it is even absent in 56 % of the individuals. This endosymbiont is also known to increase in numbers after the blood meal of larvae, nymphs or females. Because of this difference in the prevalence of M. mitochondrii between the two sexes, surveying the density of these bacteria in nymphs that will become either females or males could help to understand the behaviour of Midichloria in its arthropod host. To this aim, we have set up an experimental design by building 3 groups of unfed nymphs based on their scutum and hypostome lengths. After engorgement, weighing and moulting of a subset of the nymphs, a significant difference in sex-ratio among the 3 groups was observed. In parallel, Midichloria load in individual nymphs was quantified by qPCR both before and after engorgement. No difference in either body mass or Midichloria load was observed at the unfed stage, but following engorgement, both features were significantly different between each size group. Our results demonstrate that symbiont dynamics during nymphal engorgement is different between the two sexes, resulting in a significantly higher Midichloria load in nymphs that will become females. The consequences of those findings on our understanding of the interplay between the endosymbiont and its arthropod host are discussed.

Induced Transient Immune Tolerance in Ticks and Vertebrate Host: A Keystone of Tick-Borne Diseases?

Ticks and tick transmitted infectious agents are increasing global public health threats due to increasing abundance, expanding geographic ranges of vectors and pathogens, and emerging tick-borne infectious agents. Greater understanding of tick, host, and pathogen interactions will contribute to development of novel tick control and disease prevention strategies. Tick-borne pathogens adapt in multiple ways to very different tick and vertebrate host environments and defenses. Ticks effectively pharmacomodulate by its saliva host innate and adaptive immune defenses. In this review, we examine the idea that successful synergy between tick and tick-borne pathogen results in host immune tolerance that facilitates successful tick infection and feeding, creates a favorable site for pathogen introduction, modulates cutaneous and systemic immune defenses to establish infection, and contributes to successful long-term infection. Tick, host, and pathogen elements examined here include interaction of tick innate immunity and microbiome with tick-borne pathogens; tick modulation of host cutaneous defenses prior to pathogen transmission; how tick and pathogen target vertebrate host defenses that lead to different modes of interaction and host infection status (reservoir, incompetent, resistant, clinically ill); tick saliva bioactive molecules as important factors in determining those pathogens for which the tick is a competent vector; and, the need for translational studies to advance this field of study. Gaps in our understanding of these relationships are identified, that if successfully addressed, can advance the development of strategies to successfully disrupt both tick feeding and pathogen transmission.

Update on the intricate tango between tick microbiomes and tick-borne pathogens

The recent development of high-throughput NGS technologies, (ie, next-generation sequencing) has highlighted the complexity of tick microbial communities-which include pathogens, symbionts, and commensals-and also their dynamic variability. Symbionts and commensals can confer crucial and diverse benefits to their hosts, playing nutritional roles or affecting fitness, development, nutrition, reproduction, defence against environmental stress and immunity. Nonpathogenic tick bacteria may also play a role in modifying tick-borne pathogen colonization and transmission, as relationships between microorganisms existing together in one environment can be competitive, exclusive, facilitating or absent, with many potential implications for both human and animal health. Consequently, ticks represent a compelling yet challenging system in which to investigate the composition and both the functional and ecological implications of tick bacterial communities, and thus merits greater attention. Ultimately, deciphering the relationships between microorganisms carried by ticks as well as symbiont-tick interactions will garner invaluable information, which may aid in some future arthropod-pest and vector-borne pathogen transmission control strategies. This review outlines recent research on tick microbiome composition and dynamics, highlights elements favouring the reciprocal influence of the tick microbiome and tick-borne agents and finally discusses how ticks and tick-borne diseases might potentially be controlled through tick microbiome manipulation in the future.

General Microbiota of the Soft Tick Ornithodoros turicata Parasitizing the Bolson Tortoise (Gopherus flavomarginatus) in the Mapimi Biosphere Reserve, Mexico

The general bacterial microbiota of the soft tick Ornithodoros turicata found on Bolson tortoises (Gopherus flavomarginatus) were analyzed using next generation sequencing. The main aims of the study were to establish the relative abundance of bacterial taxa in the tick, and to document the presence of potentially pathogenic species for this tortoise, other animals, and humans. The study was carried-out in the Mapimi Biosphere Reserve in the northern-arid part of Mexico. Bolson tortoises (n = 45) were inspected for the presence of soft ticks, from which 11 tortoises (24.4%) had ticks in low loads (1–3 ticks per individual). Tick pools (five adult ticks each) were analyzed through 16S rRNA V3–V4 region amplification in a MiSeq Illumina, using EzBioCloud as a taxonomical reference. The operational taxonomic units (OTUs) revealed 28 phyla, 84 classes, 165 orders, 342 families, 1013 genera, and 1326 species. The high number of taxa registered for O. turicata may be the result of the variety of hosts that this tick parasitizes as they live inside G. flavomarginatus burrows. While the most abundant phyla were Proteobacteria, Actinobacteria, and Firmicutes, the most abundant species were two endosymbionts of ticks (Midichloria-like and Coxiella-like). Two bacteria documented as pathogenic to Gopherus spp. were registered (Mycoplasma spp. and Pasteurella testudinis). The bovine and ovine tick-borne pathogens A. marginale and A. ovis, respectively, were recorded, as well as the zoonotic bacteria A. phagocytophilum,Coxiella burnetii, and Neoehrlichia sp. Tortoises parasitized with O. turicata did not show evident signs of disease, which could indicate a possible ecological role as a reservoir that has yet to be demonstrated. In fact, the defense mechanisms of this tortoise against the microorganisms transmitted by ticks during their feeding process are still unknown. Future studies on soft ticks should expand our knowledge about what components of the microbiota are notable across multiple host–microbe dynamics. Likewise, studies are required to better understand the host competence of this tortoise, considered the largest terrestrial reptile in North America distributed throughout the Chihuahuan Desert since the late Pleistocene.

Bacterial community profiling highlights complex diversity and novel organisms in wildlife ticks

Ticks Acari:Ixodida transmit a greater variety of pathogens than any other blood-feeding group of arthropods. While numerous microbes have been identified inhabiting Australian Ixodidae, some of which are related to globally important tick-borne pathogens, little is known about the bacterial communities within ticks collected from Australian wildlife. In this study, 1,019 ticks were identified on 221 hosts spanning 27 wildlife species. Next-generation sequencing was used to amplify the V1-2 hypervariable region of the bacterial 16S rRNA gene from 238 ticks; Amblyomma triguttatum (n = 6), Bothriocroton auruginans (n = 11), Bothriocroton concolor (n = 20), Haemaphysalis bancrofti (n = 10), Haemaphysalis bremneri (n = 4), Haemaphysalis humerosa (n = 13), Haemaphysalis longicornis (n = 4), Ixodes antechini (n = 29), Ixodes australiensis (n = 26), Ixodes fecialis (n = 13), Ixodes holocyclus (n = 37), Ixodes myrmecobii (n = 1), Ixodes ornithorhynchi (n = 10), Ixodes tasmani (n = 51) and Ixodes trichosuri (n = 3). After bioinformatic analyses, over 14 million assigned bacterial sequences revealed the presence of recently described bacteria 'Candidatus Borrelia tachyglossi', 'Candidatus Neoehrlichia australis', 'Candidatus Neoehrlichia arcana' and 'Candidatus Ehrlichia ornithorhynchi'. Furthermore, three novel Anaplasmataceae species were identified in the present study including; a Neoehrlichia sp. in I. australiensis and I. fecialis collected from quenda (Isoodon fusciventer) (Western Australia), an Anaplasma sp. from one B. concolor from echidna (Tachyglossus aculeatus) (New South Wales), and an Ehrlichia sp. from a single I. fecialis parasitising a quenda (WA). This study highlights the diversity of bacterial genera harboured within wildlife ticks, which may prove to be of medical and/or veterinary importance in the future.

The scale affects our view on the identification and distribution of microbial communities in ticks

Ticks transmit the highest variety of pathogens impacting human and animal health worldwide. It is now well established that ticks also harbour a microbial complex of coexisting symbionts, commensals and pathogens. With the development of high throughput sequencing technologies, studies dealing with such diverse bacterial composition in tick considerably increased in the past years and revealed an unexpected microbial diversity. These data on diversity and composition of the tick microbes are increasingly available, giving crucial details on microbial communities in ticks and improving our knowledge on the tick microbial community. However, consensus is currently lacking as to which scales (tick organs, individual specimens or species, communities of ticks, populations adapted to particular environmental conditions, spatial and temporal scales) best facilitate characterizing microbial community composition of ticks and understanding the diverse relationships among tick-borne bacteria. Temporal or spatial scales have a clear influence on how we conduct ecological studies, interpret results, and understand interactions between organisms that build the microbiome. We consider that patterns apparent at one scale can collapse into noise when viewed from other scales, indicating that processes shaping tick microbiome have a continuum of variability that has not yet been captured. Based on available reports, this review demonstrates how much the concept of scale is crucial to be considered in tick microbial community studies to improve our knowledge on tick microbe ecology and pathogen/microbiota interactions.

Rickettsia buchneri, symbiont of the deer tick Ixodes scapularis, can colonise the salivary glands of its host

Vertically-transmitted bacterial symbionts are widespread in ticks and have manifold impacts on the epidemiology of tick-borne diseases. For instance, they may provide essential nutrients to ticks, affect vector competence, induce immune responses in vertebrate hosts, or even evolve to become vertebrate pathogens. The deer or blacklegged tick Ixodes scapularis harbours the symbiont Rickettsia buchneri in its ovarian tissues. Here we show by molecular, proteomic and imaging methods that R. buchneri is also capable of colonising the salivary glands of wild I. scapularis. This finding has important implications for the diagnosis of rickettsial infections and for pathogen-symbiont interactions in this notorious vector of Lyme borreliosis.

Seropositivity to Midichloria mitochondrii (order Rickettsiales) as a marker to determine the exposure of humans to tick bite

Ixodes ricinus is the most common tick species parasitizing humans in Europe, and the main vector of Borrelia burgdorferi sensu lato, the causative agent of Lyme disease in the continent. This tick species also harbors the endosymbiont Midichloria mitochondrii, and there is strong evidence that this bacterium is inoculated into the vertebrate host during the blood meal. A high proportion of tick bites remains unnoticed due to rarity of immediate symptoms, implying the risk of occult tick-borne infections in turn a potential risk factor for the onset of chronic-degenerative diseases. Since suitable tools to determine the previous exposure to I. ricinus bites are needed, this work investigated whether seropositivity toward a protein of M. mitochondrii (rFliD) could represent a marker for diagnosis of I. ricinus bite. We screened 274 sera collected from patients from several European countries, at different risk of tick bite, using an ELISA protocol. Our results show a clear trend indicating that positivity to rFliD is higher where the tick bite can be regarded as certain/almost certain, and lower where there is an uncertainty on the bite, with the highest positivity in Lyme patients (47.30%) and the lowest (2.00%) in negative controls. According to the obtained results, M. mitochondrii can be regarded as a useful source of antigens, with the potential to be used to assess the exposure to ticks harboring this bacterium. In prospect, additional antigens from M. mitochondrii and tick salivary glands should be investigated and incorporated in a multi-antigen test for tick bite diagnosis.

Tissue tropism and metabolic pathways of Midichloria mitochondrii suggest tissue-specific functions in the symbiosis with Ixodes ricinus

A wide range of arthropod species harbour bacterial endosymbionts in various tissues, many of them playing important roles in the fitness and biology of their hosts. In several cases, many different symbionts have been reported to coexist simultaneously within the same host and synergistic or antagonistic interactions can occur between them. While the associations with endosymbiotic bacteria have been widely studied in many insect species, in ticks such interactions are less investigated. The females and immatures of Ixodes ricinus (Ixodidae), the most common hard tick in Europe, harbour the intracellular endosymbiont "Candidatus Midichloria mitochondrii" with a prevalence up to 100%, suggesting a mutualistic relationship. Considering that the tissue distribution of a symbiont might be indicative of its functional role in the physiology of the host, we investigated M. mitochondrii specific localization pattern and the corresponding abundance in selected organs of I. ricinus females. We paired these experiments with in silico analysis of the metabolic pathways of M. mitochondrii, inferred from the available genome sequence, and additionally compared the presence of these pathways in seven other symbionts commonly harboured by ticks to try to obtain a comparative understanding of their biological effects on the tick hosts. M. mitochondrii was found to be abundant in ovaries and tracheae of unfed I. ricinus, and in ovaries, Malpighian tubules and salivary glands of semi-engorged females. These results, together with the in silico metabolic reconstruction allow to hypothesize that the bacterium could play multiple tissue-specific roles in the host, both enhancing the host fitness (supplying essential nutrients, enhancing the reproductive fitness, helping in the anti-oxidative defence, in the energy production and in the maintenance of homeostasis and water balance) and/or for ensuring its presence in the host population (nutrients acquisition, vertical and horizontal transmission). The ability of M. mitochondrii to colonize different tissues allows to speculate that distinctive sub-populations may display different specializations in accordance with tissue tropism. Our hypotheses should be corroborated with future nutritional and physiological experiments for a better understanding of the mechanisms underlying this symbiotic interaction.

Multi-locus sequence typing of Ixodes ricinus and its symbiont Candidatus Midichloria mitochondrii across Europe reveals evidence of local co-cladogenesis in Scotland

Ticks have relatively complex microbiomes, but only a small proportion of the bacterial symbionts recorded from ticks are vertically transmitted. Moreover, co-cladogenesis between ticks and their symbionts, indicating an intimate relationship over evolutionary history driven by a mutualistic association, is the exception rather than the rule. One of the most widespread tick symbionts is Candidatus Midichloria, which has been detected in all of the major tick genera of medical and veterinary importance. In some species of Ixodes, such as the sheep tick Ixodes ricinus (infected with Candidatus Midichloria mitochondrii), the symbiont is fixed in wild adult female ticks, suggesting an obligate mutualism. However, almost no information is available on genetic variation in Candidatus M. mitochondrii or possible co-cladogenesis with its host across its geographic range. Here, we report the first survey of Candidatus M. mitochondrii in I. ricinus in Great Britain and a multi-locus sequence typing (MLST) analysis of tick and symbiont between British ticks and those collected in continental Europe. We show that while the prevalence of the symbiont in nymphs collected in England is similar to that reported from the continent, a higher prevalence in nymphs and adult males is apparent in Wales. In general, Candidatus M. mitochondrii exhibits very low levels of sequence diversity, although a consistent signal of host-symbiont coevolution was apparent in Scotland. Moreover, the tick MLST scheme revealed that Scottish specimens form a clade that is partially separated from other British ticks, with almost no contribution of continental sequence types in this north-westerly border of the tick's natural range. The low diversity of Candidatus M. mitochondrii, in contrast with previously reported high rates of polymorphism in I. ricinus mitogenomes, suggests that the symbiont may have swept across Europe recently via a horizontal, rather than vertical, transmission route.

The microbiota of hematophagous ectoparasites collected from migratory birds

Arthropod vectors are responsible for the transmission of human pathogens worldwide. Several arthropod species are bird ectoparasites, however, no study to date has characterized their microbiota as a whole. We sampled hematophagous ectoparasites that feed on migratory birds and performed 16S rRNA gene metabarcoding to characterize their microbial community. A total of 194 ectoparasites were collected from 115 avian hosts and classified into three groups: a) Hippoboscidae diptera; b) ticks; c) other arthropods. Metabarcoding showed that endosymbionts were the most abundant genera of the microbial community, including Wolbachia for Hippoboscidae diptera, Candidatus Midichloria for ticks, Wolbachia and Arsenophonus for the other arthropod group. Genera including pathogenic species were: Rickettsia, Borrelia, Coxiella, Francisella, Bartonella, Anaplasma. Co-infection with Borrelia-Rickettsia and Anaplasma-Rickettsia was also observed. A global overview of the microbiota of ectoparasites sampled from migratory birds was obtained with the use of 16S rRNA gene metabarcoding. A novel finding is the first identification of Rickettsia in the common swift louse fly, Crataerina pallida. Given their possible interaction with pathogenic viruses and bacteria, the presence of endosymbionts in arthropods merits attention. Finally, molecular characterization of genera, including both pathogenic and symbiont species, plays a pivotal role in the design of targeted molecular diagnostics.

Midichloria mitochondrii, endosymbiont of Ixodes ricinus: evidence for the transmission to the vertebrate host during the tick blood meal

Ticks are important vectors of a variety of pathogens affecting humans and other animals, but they also harbor numerous microorganisms whose role is still limitedly investigated. Ixodes ricinus harbors the endosymbiont Midichloria mitochondrii, which is localized in ovaries and in salivary glands. The bacterium is vertically transmitted and is present in 100% of wild adult females, while prevalence values drop after some generations under laboratory conditions. Molecular and serological evidences showed that M. mitochondrii molecules are transmitted to the vertebrate hosts by I. ricinus during the blood meal. Our work was focused on monitoring M. mitochondrii antigens and DNA in a vertebrate model after infestation with I. ricinus for a time-span of four months. Two groups of rabbits were infested with I. ricinus females, respectively from the wild (naturally infected with the symbiont) and laboratory strain (lab; considered devoid of M. mitochondrii after quantitative PCR investigations) and screened using molecular and serological assays at nine time points. M. mitochondrii presence was detected in rabbits infested with wild I. ricinus ticks, but surprisingly also in those infested with lab ticks, albeit at later time points. This result prompted a more sensitive molecular screening of lab ticks, which were found to harbor very low symbiont loads. Our results indicate that transmission of the bacterium occurs even at low bacterial loads, and that antibody response against M. mitochondrii antigens begins within one week post-infestation with wild I. ricinus. Circulating DNA was detected in the blood of rabbits belonging to both groups up to the end of the experiment, suggesting a replication of the symbiont inside the vertebrate host.

Multi-locus phylogenetics of the Midichloria endosymbionts reveals variable specificity of association with ticks

Candidatus Midichloria mitochondrii is a maternally inherited bacterium of ticks with a unique intra-mitochondrial lifestyle. Here, we investigate on the evolutionary history of these associations and the degree of Midichloria-tick specificity. While previous surveys used the 16S rRNA gene as an exclusive molecular marker, we rather developed a multi-locus typing method based on four more variable housekeeping genes (groEL, rpoB, dnaK and ftsZ) and on one flagellum gene (fliC) present in Midichloria genomes. Using this method, multi-locus phylogenetic analyses revealed the structuring of a wide Midichloria genetic diversity into three distinct lineages associated with ticks. Overall, two distinct evolutionary strategies are obvious depending on lineage: two Midichloria lineages are generalists with infections acquired through horizontal transfers between distantly related tick species but one other Midichloria lineage rather show a high specificity degree to the Ixodes tick genus. This pattern suggests a capacity of certain Midichloria strains to maintain infections in only limited range of related tick species. These different infection strategies of Midichloria highlight an unexpected variability in their dependency to their tick hosts. We further conjecture that this pattern is also likely to indicate variability in their effects on ticks.

Molecular and Serological Evidence of the Presence of Midichloria mitochondrii in Roe Deer ( Capreolus capreolus) in France

Midichloria mitochondrii is a tick-borne intracellular bacterium of the order Rickettsiales, found with high prevalence in the sheep tick ( Ixodes ricinus). Midichloria mitochondrii is capable of vertical transmission in the tick, but recently evidence of potential horizontal transmission to the tick hosts through the blood meal has been reported. We investigated the presence of the bacterium in the blood of roe deer ( Capreolus capreolus) collected from an area known to be highly infested with I. ricinus ticks. We collected blood and sera samples for 3 yr in Gardouch (Haute Garonne, France) and subjected them to molecular screening through PCR and to serological investigation using enzyme-linked immunosorbent assay and western blot. Bacterial DNA was detected in the blood of four of seven animals, but only at one or two points in time, whereas all sera were positive for M. mitochondrii antigens at all times. Our results indicated that the presence of the bacterium in the blood is transient, but the antibody response appeared to be long-lasting, possibly due to constant exposure to tick bites, and thus to repeated injection of bacteria. The role of M. mitochondrii in the mammalian host, and its interaction with other tick-borne bacteria, remains unknown.

Control of Lyme borreliosis and other Ixodes ricinus-borne diseases

Lyme borreliosis (LB) and other Ixodes ricinus-borne diseases (TBDs) are diseases that emerge from interactions of humans and domestic animals with infected ticks in nature. Nature, environmental and health policies at (inter)national and local levels affect the risk, disease burden and costs of TBDs. Knowledge on ticks, their pathogens and the diseases they cause have been increasing, and resulted in the discovery of a diversity of control options, which often are not highly effective on their own. Control strategies involving concerted actions from human and animal health sectors as well as from nature managers have not been formulated, let alone implemented. Control of TBDs asks for a “health in all policies” approach, both at the (inter)national level, but also at local levels. For example, wildlife protection and creating urban green spaces are important for animal and human well-being, but may increase the risk of TBDs. In contrast, culling or fencing out deer decreases the risk for TBDs under specific conditions, but may have adverse effects on biodiversity or may be societally unacceptable. Therefore, in the end, nature and health workers together must carry out tailor-made control options for the control of TBDs for humans and animals, with minimal effects on the environment. In that regard, multidisciplinary approaches in environmental, but also medical settings are needed. To facilitate this, communication and collaboration between experts from different fields, which may include patient representatives, should be promoted.

Patterns of Midichloria infection in avian-borne African ticks and their trans-Saharan migratory hosts

Background

Ticks are obligate haematophagous ectoparasites of vertebrates and frequently parasitize avian species that can carry them across continents during their long-distance migrations. Ticks may have detrimental effects on the health state of their avian hosts, which can be either directly caused by blood-draining or mediated by microbial pathogens transmitted during the blood meal. Indeed, ticks host complex microbial communities, including bacterial pathogens and symbionts. Midichloria bacteria (Rickettsiales) are widespread tick endosymbionts that can be transmitted to vertebrate hosts during the tick bite, inducing an antibody response. Their actual role as infectious/pathogenic agents is, however, unclear.

Methods

We screened for Midichloria DNA African ticks and blood samples collected from trans-Saharan migratory songbirds at their arrival in Europe during spring migration.

Results

Tick infestation rate was 5.7%, with most ticks belonging to the Hyalomma marginatum species complex. Over 90% of Hyalomma ticks harboured DNA of Midichloria bacteria belonging to the monophylum associated with ticks. Midichloria DNA was detected in 43% of blood samples of avian hosts. Tick-infested adult birds were significantly more likely to test positive to the presence of Midichloria DNA than non-infested adults and second-year individuals, suggesting a long-term persistence of these bacteria within avian hosts. Tick parasitism was associated with a significantly delayed timing of spring migration of avian hosts but had no significant effects on body condition, whereas blood Midichloria DNA presence negatively affected fat deposits of tick-infested avian hosts.

Conclusions

Our results show that ticks effectively transfer Midichloria bacteria to avian hosts, supporting the hypothesis that they are infectious to vertebrates. Bird infection likely enhances the horizontal spread of these bacteria across haematophagous ectoparasite populations. Moreover, we showed that Midichloria and tick parasitism have detrimental non-independent effects on avian host health during migration, highlighting the complexity of interactions involving ticks, their vertebrate hosts, and tick-borne bacteria.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2669-z) contains supplementary material, which is available to authorized users.

Rethinking Coxiella infections in Australia

Coxiellaburnetii is the causative agent of coxiellosis in animals and Q fever in humans. Despite being a vaccine preventable disease, Q fever remains a frequently reported zoonotic infection in Australia. Recently, a Coxiella species was identified in brown dog ticks (Rhipicephalus sanguineus) in urban and rural regions of Australia. Further molecular characterisation revealed that it is genetically identical to ‘Candidatus Coxiella massiliensis’ (KM079627) described in R. sanguineus ticks removed from humans with eschars in France and serologic cross-reactivity among ‘Ca. Coxiella massiliensis’ and C.burnetii may occur. This report highlights the need for molecular testing of seropositive companion animals and humans to determine which species of Coxiella they are infected with, in order to further assess Coxiella species associated with Coxiella infections in Australia.

Endosymbiosis and its significance in dermatology

Proposed at the beginning of the twentieth century to explain the origin of eukaryotic organelles from prokaryotes, endosymbiosis is now medically defined by various interaction patterns between microorganisms and their residing hosts, best exemplified by the bacterial endosymbiont Wolbachia identified in arthropods and filarial nematodes, which can influence normal development, reproduction, survival and transmission of the hosts. Based on the transmission modes, vertical or horizontal, and the function of the endosymbionts, the host-symbiont dependence can be divided into primary or secondary. In dermatology, the role of endosymbionts in skin ectoparasitosis has aroused great interests in the past years. Riesia pediculicola is a primary bacterial endosymbiont in body lice Pediculus humanus, and supplement their hosts with vitamin B, especially pantothenic acid. In cimicosis, the Gram-negative Wolbachia can synthesize biotin and riboflavin, which are crucial for the growth and reproduction of the bedbug Cimex lectularius. In human demodicosis and rosacea, further study is required to prove the pathogenic role of the Gram-negative bacteria Bacillus oleronius or the Gram-positive bacteria Bacillus cereus demonstrated in the Demodex mites. The high infection rate of adult female ticks Ixodes ricinus with the Gram-negative bacteria Midichloria mitochondrii present in the mitochondria in diverse ovarian cells, with the high seroprevalence rate in tick-exposed subjects, raises the possibility that this non-pathogenic endosymbiont may play a role in immune response and successful transmission of the tick-borne pathogen. The anaerobic protozoan Trichomonas vaginalis and bacteria Mycoplasma hominis are two obligate parasites in the urogenital epithelium, with partially overlapping symptoms. Intracellular localization of Mycoplasma hominis can avoid host immune response and penetration of antibiotics, while Trichomonas vaginalis infected with Mycoplasma hominis seems to have a higher cytopathic activity and amoeboid transformation rate. Further study on the biology and pathogenesis of different endosymbionts in dermatological parasitosis will help for the development of new treatment modalities.

A Coxiella mutualist symbiont is essential to the development of Rhipicephalus microplus

The cattle tick Rhipicephalus microplus is a hematophagous ectoparasite that causes important economic losses in livestock. Different species of ticks harbor a symbiont bacterium of the genus Coxiella. It was showed that a Coxiella endosymbiont from R. microplus (CERM) is a vertically transmitted mutualist symbiont, comprising 98% of the 16S rRNA sequences in both eggs and larvae. Sequencing of the bacterial genome revealed genes for biosynthetic pathways for several vitamins and key metabolic cofactors that may provide a nutritional complement to the tick host. The CERM was abundant in ovary and Malpighian tubule of fully engorged female. Tetracycline treatment of either the tick or the vertebrate host reduced levels of bacteria in progeny in 74% for eggs and 90% for larvae without major impact neither on the reproductive fitness of the adult female or on embryo development. However, CERM proved to be essential for the tick to reach the adult life stage, as under antibiotic treatment no tick was able to progress beyond the metanymph stage. Data presented here suggest that interference in the symbiotic CERM-R. microplus relationship may be useful to the development of alternative control methods, highlighting the interdependence between ticks and their endosymbionts.

The Physiology of Phagocytosis in the Context of Mitochondrial Origin

How mitochondria came to reside within the cytosol of their host has been debated for 50 years. Though current data indicate that the last eukaryote common ancestor possessed mitochondria and was a complex cell, whether mitochondria or complexity came first in eukaryotic evolution is still discussed. In autogenous models (complexity first), the origin of phagocytosis poses the limiting step at eukaryote origin, with mitochondria coming late as an undigested growth substrate. In symbiosis-based models (mitochondria first), the host was an archaeon, and the origin of mitochondria was the limiting step at eukaryote origin, with mitochondria providing bacterial genes, ATP synthesis on internalized bioenergetic membranes, and mitochondrion-derived vesicles as the seed of the eukaryote endomembrane system. Metagenomic studies are uncovering new host-related archaeal lineages that are reported as complex or phagocytosing, although images of such cells are lacking. Here we review the physiology and components of phagocytosis in eukaryotes, critically inspecting the concept of a phagotrophic host. From ATP supply and demand, a mitochondrion-lacking phagotrophic archaeal fermenter would have to ingest about 34 times its body weight in prokaryotic prey to obtain enough ATP to support one cell division. It would lack chemiosmotic ATP synthesis at the plasma membrane, because phagocytosis and chemiosmosis in the same membrane are incompatible. It would have lived from amino acid fermentations, because prokaryotes are mainly protein. Its ATP yield would have been impaired relative to typical archaeal amino acid fermentations, which involve chemiosmosis. In contrast, phagocytosis would have had great physiological benefit for a mitochondrion-bearing cell.

Micropathogen Community Analysis in Hyalomma rufipes via High-Throughput Sequencing of Small RNAs

Ticks are important vectors in the transmission of a broad range of micropathogens to vertebrates, including humans. Because of the role of ticks in disease transmission, identifying and characterizing the micropathogen profiles of tick populations have become increasingly important. The objective of this study was to survey the micropathogens of Hyalomma rufipes ticks. Illumina HiSeq2000 technology was utilized to perform deep sequencing of small RNAs (sRNAs) extracted from field-collected H. rufipes ticks in Gansu Province, China. The resultant sRNA library data revealed that the surveyed tick populations produced reads that were homologous to St. Croix River Virus (SCRV) sequences. We also observed many reads that were homologous to microbial and/or pathogenic isolates, including bacteria, protozoa, and fungi. As part of this analysis, a phylogenetic tree was constructed to display the relationships among the homologous sequences that were identified. The study offered a unique opportunity to gain insight into the micropathogens of H. rufipes ticks. The effective control of arthropod vectors in the future will require knowledge of the micropathogen composition of vectors harboring infectious agents. Understanding the ecological factors that regulate vector propagation in association with the prevalence and persistence of micropathogen lineages is also imperative. These interactions may affect the evolution of micropathogen lineages, especially if the micropathogens rely on the vector or host for dispersal. The sRNA deep-sequencing approach used in this analysis provides an intuitive method to survey micropathogen prevalence in ticks and other vector species.

Bacteria of the Family 'Candidatus Midichloriaceae' in Sympatric Zones of Ixodes Ticks: Genetic Evidence for Vertical Transmission

Ixodes ticks transmit infectious agents and also harbor their own parasites and symbionts. The presumptive endosymbiont of Ixodes ricinus, 'Candidatus Midichloria mitochondrii', has a unique ability to invade mitochondria within tick ovarian cells and is transovarially transmitted with 100% efficiency. A closely related bacterium, provisionally named Montezuma (now 'Candidatus Lariskella arthropodarum'), was isolated from the Ixodes persulcatus ticks and human blood in 2004 as well as from Ixodes pavlovskyi in 2015. These microorganisms belong to the family 'Candidatus Midichloriaceae fam. nov.' and were detected not only in tick salivary glands, but also in animal blood. Nevertheless, the relative importance of vertical and horizontal routes for their transmission or maintenance in natural tick populations remains unclear. We analyzed the prevalence of L. arthropodarum and M. mitochondrii in two sympatric zones, where I. persulcatus/I. ricinus and I. persulcatus/I. pavlovskyi cohabit and produce interspecific hybrids. A specificity of the associations of L. arthropodarum with I. persulcatus (100%) and M. mitochondrii with I. ricinus (96.2%) was observed in the sympatric zone in Estonia, possibly showing poor contribution of the horizontal route to the overall prevalence of endosymbionts. L. arthropodarum was observed probably multiplying in I. pavlovskyi and also subjected to transovarial transmission, but much less efficiently compared to I. persulcatus. We revealed two new genetic variants of the rrl-rrf intergenic spacer of L. arthropodarum isolated from I. pavlovskyi ticks that possibly could indicate an ongoing process of adaptation of the microorganism to a new host species.

Diversity and Universality of Endosymbiotic Rickettsia in the Fish Parasite Ichthyophthirius multifiliis

Although the presence of endosymbiotic rickettsial bacteria, specifically Candidatus Megaira, has been reported in diverse habitats and a wide range of eukaryotic hosts, it remains unclear how broadly Ca. Megaira are distributed in a single host species. In this study we seek to address whether Ca. Megaira are present in most, if not all isolates, of the parasitic ciliate Ichthyophthirius multifiliis. Conserved regions of bacterial 16S rRNA genes were either PCR amplified, or assembled from deep sequencing data, from 18 isolates/populations of I. multifiliis sampled worldwide (Brazil, Taiwan, and USA). We found that rickettsial rRNA sequences belonging to three out of four Ca. Megaira subclades could be consistently detected in all I. multifiliis samples. I. multifiliis collected from local fish farms tend to be inhabited by the same subclade of Ca. Megaira, whereas those derived from pet fish are often inhabited by more than one subclade of Ca. Megaira. Distributions of Ca. Megaira in I. multifiliis thus better reflect the travel history, but not the phylogeny, of I. multifiliis. In summary, our results suggest that I. multifiliis may be dependent on this endosymbiotic relationship, and the association between Ca. Megaira and I. multifiliis is more diverse than previously thought.

Bacterial pathogens and endosymbionts in ticks

Ticks collected from goats in northern Greece were tested for the presence of tick-borne bacteria. Among adult ticks, 37 (57.8%) were Rhipicephalus bursa, 11 (17.2%) Dermacentor marginatus, 10 (15.6%) Ixodes ricinus, 3 (4.7%) Rhipicephalus sanguineus sensu lato and 2 (3.1%) Haemaphysalis parva; one (1.6%) Rhipicephalus spp. tick was nymph. Rickettsia monacensis, Rickettsia massilae, Anaplasma phagocytophilum and Anaplasma platys were detected in I. ricinus and Rh. bursa ticks. A variety of Coxiella-like endosymbionts were detected in all tick genera tested, forming distinct clades from Coxiella burnetii in the phylogenetic tree based on the 16S rRNA gene. An additional endosymbiont, Candidatus Midichloria mitochondrii, was detected in most of the I. ricinus ticks. Surveillance for human pathogens in ticks provides knowledge helpful for the public health, while further studies are needed to determine the role of endosymbionts in tick physiology, vector competence and probably in public health.

Molecular screening for Midichloria in hard and soft ticks reveals variable prevalence levels and bacterial loads in different tick species

Candidatus Midichloria mitochondrii, symbiont of the sheep tick Ixodes ricinus, was the first described member of the family Candidatus Midichloriaceae, order Rickettsiales. Recent reports are expanding our view of this family, now including numerous bacteria of great biological and medical interest, indicating a widespread distribution with an increasing range of hosts, with ticks being strongly represented. Here we present a molecular screening of 17 tick species, detecting and quantifying bacteria of the family Midichloriaceae in seven of them, including the first report of a representative of this family in a soft tick species (Argasidae), Ornithodoros maritimus. Based on sequence identity and phylogenetic analysis we propose that all these bacterial symbionts of ticks could be members of the genus Midichloria. The performed screening highlights different prevalence levels and variable bacterial loads in different tick species including one, Ixodes aulacodi, where the bacterium is present in all examined individuals, like in I. ricinus. This result prompts us to hypothesize different roles of Midichloria bacteria in different tick species.

Co-infection of Ticks: The Rule Rather Than the Exception

Introduction

Ticks are the most common arthropod vectors of both human and animal diseases in Europe, and the Ixodes ricinus tick species is able to transmit a large number of bacteria, viruses and parasites. Ticks may also be co-infected with several pathogens, with a subsequent high likelihood of co-transmission to humans or animals. However few data exist regarding co-infection prevalences, and these studies only focus on certain well-known pathogens. In addition to pathogens, ticks also carry symbionts that may play important roles in tick biology, and could interfere with pathogen maintenance and transmission. In this study we evaluated the prevalence of 38 pathogens and four symbionts and their co-infection levels as well as possible interactions between pathogens, or between pathogens and symbionts.

Methodology/principal findings

A total of 267 Ixodes ricinus female specimens were collected in the French Ardennes and analyzed by high-throughput real-time PCR for the presence of 37 pathogens (bacteria and parasites), by rRT-PCR to detect the presence of Tick-Borne encephalitis virus (TBEV) and by nested PCR to detect four symbionts. Possible multipartite interactions between pathogens, or between pathogens and symbionts were statistically evaluated. Among the infected ticks, 45% were co-infected, and carried up to five different pathogens. When adding symbiont prevalences, all ticks were infected by at least one microorganism, and up to eight microorganisms were identified in the same tick. When considering possible interactions between pathogens, the results suggested a strong association between Borrelia garinii and B. afzelii, whereas there were no significant interactions between symbionts and pathogens.

Conclusion/significance

Our study reveals high pathogen co-infection rates in ticks, raising questions about possible co-transmission of these agents to humans or animals, and their consequences to human and animal health. We also demonstrated high prevalence rates of symbionts co-existing with pathogens, opening new avenues of enquiry regarding their effects on pathogen transmission and vector competence.

Ticks transmit more pathogens than any other arthropod, and one single species can transmit a large variety of bacteria and parasites. Because co-infection might be much more common than previously thought, we evaluated the prevalence of 38 known or neglected tick-borne pathogens in Ixodes ricinus ticks. Our results demonstrated that co-infection occurred in almost half of the infected ticks, and that ticks could be infected with up to five pathogens. Moreover, as it is well established that symbionts can affect pathogen transmission in arthropods, we also evaluated the prevalence of four symbiont species and demonstrated that all ticks were infected by at least one microorganism. This work highlights the co-infection phenomenon in ticks, which may have important implications for human and animal health, emphasizing the need for new diagnostic tests better adapted to tick-borne diseases. Finally, the high co-occurrence of symbionts and pathogens in ticks, reveals the necessity to also account for these interactions in the development of new alternative strategies to control ticks and tick-borne disease.

A House for Two--Double Bacterial Infection in Euplotes woodruffi Sq1 (Ciliophora, Euplotia) Sampled in Southeastern Brazil

Several ciliated protists form symbiotic associations with a diversity of microorganisms, leading to drastic impact on their ecology and evolution. In this work, two Euplotes spp. sampled in Rio de Janeiro, Brazil, were identified based on morphological and molecular features as Euplotes woodruffi strain Sq1 and E. encysticus strain Sq2 and investigated for the presence of endosymbionts. While E. woodruffi Sq1 stably hosts two bacterial populations, namely Polynucleobacter necessarius (Betaproteobacteria) and a new member of the family "Candidatus Midichloriaceae" (Alphaproteobacteria, Rickettsiales), here described as "Candidatus Bandiella woodruffii," branching with a broad host range bacterial group found in association with cnidarians, sponges, euglenoids, and some arthropods; in E. encysticus Sq2 no symbiotic bacterium could be detected. The dispersion ability of this novel bacterium was tested by co-incubating E. woodruffi Sq1 with three different ciliate species. Among the tested strains "Ca. B. woodruffii" could only be detected in association with E. encysticus Sq2 with a prevalence of 20 % after 1 week and 40 % after 2 weeks, maintaining this level for up to 6 months. Nevertheless, this apparent in vitro association was abolished when E. woodruffi Sq1 donor was removed from the microcosm, suggesting that this bacterium has the capacity for at least a short-term survival outside its natural host and the aptitude to ephemerally interact with other organisms. Together, these findings strongly suggest the need for more detailed investigations to evaluate the host range for "Ca. B. woodruffii" and any possible pathogenic effect of this bacterium on other organisms including humans.

"Candidatus Fokinia solitaria", a Novel "Stand-Alone" Symbiotic Lineage of Midichloriaceae (Rickettsiales)

Recently, the family Midichloriaceae has been described within the bacterial order Rickettsiales. It includes a variety of bacterial endosymbionts detected in different metazoan host species belonging to Placozoa, Cnidaria, Arthropoda and Vertebrata. Representatives of Midichloriaceae are also considered possible etiological agents of certain animal diseases. Midichloriaceae have been found also in protists like ciliates and amoebae. The present work describes a new bacterial endosymbiont, "Candidatus Fokinia solitaria", retrieved from three different strains of a novel Paramecium species isolated from a wastewater treatment plant in Rio de Janeiro (Brazil). Symbionts were characterized through the full-cycle rRNA approach: SSU rRNA gene sequencing and fluorescence in situ hybridization (FISH) with three species-specific oligonucleotide probes. In electron micrographs, the tiny rod-shaped endosymbionts (1.2 x 0.25-0.35 μm in size) were not surrounded by a symbiontophorous vacuole and were located in the peripheral host cytoplasm, stratified in the host cortex in between the trichocysts or just below them. Frequently, they occurred inside autolysosomes. Phylogenetic analyses of Midichloriaceae apparently show different evolutionary pathways within the family. Some genera, such as "Ca. Midichloria" and "Ca. Lariskella", have been retrieved frequently and independently in different hosts and environmental surveys. On the contrary, others, such as Lyticum, "Ca. Anadelfobacter", "Ca. Defluviella" and the presently described "Ca. Fokinia solitaria", have been found only occasionally and associated to specific host species. These last are the only representatives in their own branches thus far. Present data do not allow to infer whether these genera, which we named "stand-alone lineages", are an indication of poorly sampled organisms, thus underrepresented in GenBank, or represent fast evolving, highly adapted evolutionary lineages.