Ticks Resist Skin Commensals with Immune Factor of Bacterial Origin

The impact of volatiles on tick-host interaction and vector competence

Ticks are obligatory hematophagous arachnids, serving as vectors for a wide array of pathogens that can be transmitted to humans or animals. The ability of tick-borne pathogens to maintain within natural reservoirs is intricately influenced by the attractiveness of ticks to their animal hosts, including humans. However, the complex dynamics of tick behavior and host-seeking strategies remain understudied. This review aims to summarize the impact of volatiles or odors on tick behavior and vector competence. Our literature review has identified a selection of compounds, such as 1-octen-3-ol, hexanal, heptanal, nonanal, 6-methyl-5-hepten-2-one, acetone, and octanal, as having the potential to impact both ticks' and mosquitos' behaviors. In addition, carbon dioxide (CO2) is a universal attractant for hematophagous arthropods. Moreover, we have gathered some clues indicating that volatiles emitted by infected animal hosts might play a role in the transmission of tick-borne pathogens. Nonetheless, our understanding of this phenomenon remains largely inadequate, particularly with regarding to whether the tick microbiome or the skin microbiota of the feeding mammals, including humans, can actively modulate tick-host-seeking behavior. Further investigations in this emerging field hold immense promise for the development of innovative strategies aimed at controlling vectors and curtailing the spread of tick-borne diseases.

Insight Into the Dynamics of the Ixodes ricinus Nymphal Midgut Proteome

Ticks are ectoparasites that feed on blood and have an impressive ability to consume and process enormous amounts of host blood, allowing extremely long periods of starvation between blood meals. The central role in the parasitic lifestyle of ticks is played by the midgut. This organ efficiently stores and digests ingested blood and serves as the primary interface for the transmission of tick-borne pathogens. In this study, we used a label-free quantitative approach to perform a novel dynamic proteomic analysis of the midgut of Ixodesricinus nymphs, covering their development from unfed to pre-molt stages. We identified 1534 I. ricinus-specific proteins with a relatively low proportion of host proteins. This proteome dataset, which was carefully examined by manual scrutiny, allowed precise annotation of proteins important for blood meal processing and their dynamic changes during nymphal ontogeny. We focused on midgut molecules related to lipid hydrolysis, storage, and transport, opening a yet unexplored avenue for studying lipid metabolism in ticks. Further dynamic profiling of the tick's multi-enzyme digestive network, protease inhibitors, enzymes involved in redox homeostasis and detoxification, antimicrobial peptides, and proteins responsible for midgut colonization by Borrelia spirochetes promises to uncover new targets for targeting tick nymphs, the most critical life stage for transmission the pathogens that cause tick-borne diseases.

Tick extracellular vesicles in host skin immunity and pathogen transmission

Ticks can transmit a variety of human pathogens, including intracellular and extracellular bacteria, viruses, and protozoan parasites. Historically, their saliva has been of immense interest due to its anticoagulant, anti-inflammatory, and anesthetic properties. Only recently, it was discovered that tick saliva contains extracellular vesicles (EVs). Briefly, it has been observed that proteins associated with EVs are important for multiple tick-borne intracellular microbial lifestyles. The impact of tick EVs on viral and intracellular bacterial pathogen transmission from the tick to the mammalian host has been shown experimentally. Additionally, tick EVs interact with the mammalian skin immune system at the bite site. The interplay between tick EVs, the transmission of pathogens, and the host skin immune system affords opportunities for future research.

Multiomics Reveals Symbionts, Pathogens, and Tissue-Specific Microbiome of Blacklegged Ticks (Ixodes scapularis) from a Lyme Disease Hot Spot in Southeastern Ontario, Canada

Ticks in the family Ixodidae are important vectors of zoonoses, including Lyme disease (LD), which is caused by spirochete bacteria from the Borreliella (Borrelia) burgdorferi sensu lato complex. The blacklegged tick (Ixodes scapularis) continues to expand across Canada, creating hot spots of elevated LD risk at the leading edge of its expanding range. Current efforts to understand the risk of pathogen transmission associated with I. scapularis in Canada focus primarily on targeted screens, while natural variation in the tick microbiome remains poorly understood. Using multiomics consisting of 16S metabarcoding and ribosome-depleted, whole-shotgun RNA transcriptome sequencing, we examined the microbial communities associated with adult I. scapularis (n = 32), sampled from four tissue types (whole tick, salivary glands, midgut, and viscera) and three geographical locations within a LD hot spot near Kingston, Ontario, Canada. The communities consisted of both endosymbiotic and known or potentially pathogenic microbes, including RNA viruses, bacteria, and a Babesia sp. intracellular parasite. We show that β-diversity is significantly higher between the bacterial communities of individual tick salivary glands and midguts than that of whole ticks. Linear discriminant analysis effect size (LEfSe) determined that the three potentially pathogenic bacteria detected by V4 16S rRNA sequencing also differed among dissected tissues only, including a Borrelia strain from the B. burgdorferi sensu lato complex, Borrelia miyamotoi, and Anaplasma phagocytophilum. Importantly, we find coinfection of I. scapularis by multiple microbes, in contrast to diagnostic protocols for LD, which typically focus on infection from a single pathogen of interest (B. burgdorferi sensu stricto). IMPORTANCE As a vector of human health concern, blacklegged ticks (Ixodes scapularis) transmit pathogens that cause tick-borne diseases (TBDs), including Lyme disease (LD). Several hot spots of elevated LD risk have emerged across Canada as I. scapularis expands its range. Focusing on a hot spot in southeastern Ontario, we used high-throughput sequencing to characterize the microbiome of whole ticks and dissected salivary glands and midguts. Compared with whole ticks, salivary glands and midguts were more diverse and associated with distinct bacterial communities that are less dominated by Rickettsia endosymbiont bacteria and are enriched for pathogenic bacteria, including a B. burgdorferi sensu lato-associated Borrelia sp., Borrelia miyamotoi, and Anaplasma phagocytophilum. We also found evidence of coinfection of I. scapularis by multiple pathogens. Overall, our study highlights the challenges and opportunities associated with the surveillance of the microbiome of I. scapularis for pathogen detection using metabarcoding and metatranscriptome approaches.

Antimicrobial resistance gene lack in tick-borne pathogenic bacteria

Tick-borne infections, including those of bacterial origin, are significant public health issues. Antimicrobial resistance (AMR), which is one of the most pressing health challenges of our time, is driven by specific genetic determinants, primarily by the antimicrobial resistance genes (ARGs) of bacteria. In our work, we investigated the occurrence of ARGs in the genomes of tick-borne bacterial species that can cause human infections. For this purpose, we processed short/long reads of 1550 bacterial isolates of the genera Anaplasma (n = 20), Bartonella (n = 131), Borrelia (n = 311), Coxiella (n = 73), Ehrlichia (n = 13), Francisella (n = 959) and Rickettsia (n = 43) generated by second/third generation sequencing that have been freely accessible at the NCBI SRA repository. From Francisella tularensis, 98.9% of the samples contained the FTU-1 beta-lactamase gene. However, it is part of the F. tularensis representative genome as well. Furthermore, 16.3% of them contained additional ARGs. Only 2.2% of isolates from other genera (Bartonella: 2, Coxiella: 8, Ehrlichia: 1, Rickettsia: 2) contained any ARG. We found that the odds of ARG occurrence in Coxiella samples were significantly higher in isolates related to farm animals than from other sources. Our results describe a surprising lack of ARGs in these bacteria and suggest that Coxiella species in farm animal settings could play a role in the spread of AMR.

Identification and Classification of Papain-like Cysteine Proteinases

Papain-like cysteine peptidases form a big and highly diverse superfamily of proteins involved in many important biological functions, such as protein turnover, deubiquitination, tissue remodeling, blood clotting, virulence, defense, and cell wall remodeling. High sequence and structure diversity observed within these proteins hinders their comprehensive classification as well as the identification of new representatives. Moreover, in general protein databases, many families already classified as papain-like lack details regarding their mechanism of action or biological function. Here, we use transitive remote homology searches and 3D modeling to newly classify 21 families to the papain-like cysteine peptidase superfamily. We attempt to predict their biological function, and provide structural chacterization of 89 protein clusters defined based on sequence similarity altogether spanning 106 papain-like families. Moreover, we systematically discuss observed diversity in sequences, structures, and catalytic sites. Eventually, we expand the list of human papain-related proteins by seven representatives, including dopamine receptor-interacting protein (DRIP1) as potential deubiquitinase, and centriole duplication regulating CEP76 as retaining catalytically active peptidase-like domain. The presented results not only provide structure-based rationales to already existing peptidase databases but also may inspire further experimental research focused on peptidase-related biological processes.

New insights into the impact of microbiome on horizontal and vertical transmission of a tick-borne pathogen

BACKGROUND

The impact of host skin microbiome on horizontal transmission of tick-borne pathogens , and of pathogen associated transstadial and transovarial changes in tick microbiome are largely unknown, but are important to control increasingly emerging tick-borne diseases worldwide.

METHODS

Focusing on a rickettsiosis pathogen, Rickettsia raoultii, we used R. raoultii-positive and R. raoultii-negative Dermacentor spp. tick colonies to study the involvement of skin microbiota in cutaneous infection with rickettsiae in laboratory mice, and the function of the tick microbiome on maintenance of rickettsiae through all tick developmental stages (eggs, larvae, nymphs, adults) over two generations.

RESULTS

We observed changes in the skin bacteria community, such as Chlamydia, not only associated with rickettsial colonization but also with tick feeding on skin. The diversity of skin microbiome differed between paired tick-bitten and un-bitten sites. For vertical transmission, significant differences in the tick microbiota between pathogenic rickettsia-positive and -negative tick chorts was observed across all developmental stages at least over two generations, which appeared to be a common pattern not only for R. raoultii but also for another pathogenic species, Candidatus Rickettsia tarasevichiae. More importantly, bacterial differences were complemented by functional shifts primed for genetic information processing during blood feeding. Specifically, the differences in tick microbiome gene repertoire between pathogenic Rickettsia-positive and -negative progenies were enriched in pathways associated with metabolism and hormone signals during vertical transmission.

CONCLUSIONS

We demonstrate that host skin microbiome might be a new factor determining the transmission of rickettsial pathogens through ticks. While pathogenic rickettsiae infect vertebrate hosts during blood-feeding by the tick, they may also manipulate the maturation of the tick through changing the functional potential of its microbiota over the tick's life stages. The findings here might spur the development of new-generation control methods for ticks and tick-borne pathogens. Video Abstract.

Immune evasion strategies of major tick-transmitted bacterial pathogens

Tick-transmitted bacterial pathogens thrive in enzootic infection cycles, colonizing disparate vertebrate and arthropod tissues, often establishing persistent infections. Therefore, the evolution of robust immune evasion strategies is central to their successful persistence or transmission between hosts. To survive in nature, these pathogens must counteract a broad range of microbicidal host responses that can be localized, tissue-specific, or systemic, including a mix of these responses at the host-vector interface. Herein, we review microbial immune evasion strategies focusing on Lyme disease spirochetes and rickettsial or tularemia agents as models for extracellular and intracellular tick-borne pathogens, respectively. A better understanding of these adaptive strategies could enrich our knowledge of the infection biology of relevant tick-borne diseases, contributing to the development of future preventions.

A Deeper Insight into the Tick Salivary Protein Families under the Light of Alphafold2 and Dali: Introducing the TickSialoFam 2.0 Database

Hard ticks feed for several days or weeks on their hosts and their saliva contains thousands of polypeptides belonging to dozens of families, as identified by salivary transcriptomes. Comparison of the coding sequences to protein databases helps to identify putative secreted proteins and their potential functions, directing and focusing future studies, usually done with recombinant proteins that are tested in different bioassays. However, many families of putative secreted peptides have a unique character, not providing significant matches to known sequences. The availability of the Alphafold2 program, which provides in silico predictions of the 3D polypeptide structure, coupled with the Dali program which uses the atomic coordinates of a structural model to search the Protein Data Bank (PDB) allows another layer of investigation to annotate and ascribe a functional role to proteins having so far being characterized as "unique". In this study, we analyzed the classification of tick salivary proteins under the light of the Alphafold2/Dali programs, detecting novel protein families and gaining new insights relating the structure and function of tick salivary proteins.

Tick microbial associations at the crossroad of horizontal and vertical transmission pathways

BACKGROUND

Microbial communities can affect disease risk by interfering with the transmission or maintenance of pathogens in blood-feeding arthropods. Here, we investigated whether bacterial communities vary between Ixodes ricinus nymphs which were or were not infected with horizontally transmitted human pathogens.

METHODS

Ticks from eight forest sites were tested for the presence of Borrelia burgdorferi sensu lato, Babesia spp., Anaplasma phagocytophilum, and Neoehrlichia mikurensis by quantitative polymerase chain reaction (qPCR), and their microbiomes were determined by 16S rRNA amplicon sequencing. Tick bacterial communities clustered poorly by pathogen infection status but better by geography. As a second approach, we analysed variation in tick microorganism community structure (in terms of species co-infection) across space using hierarchical modelling of species communities. For that, we analysed almost 14,000 nymphs, which were tested for the presence of horizontally transmitted pathogens B. burgdorferi s.l., A. phagocytophilum, and N. mikurensis, and the vertically transmitted tick symbionts Rickettsia helvetica, Rickettsiella spp., Spiroplasma ixodetis, and Candidatus Midichloria mitochondrii.

RESULTS

With the exception of Rickettsiella spp., all microorganisms had either significant negative (R. helvetica and A. phagocytophilum) or positive (S. ixodetis, N. mikurensis, and B. burgdorferi s.l.) associations with M. mitochondrii. Two tick symbionts, R. helvetica and S. ixodetis, were negatively associated with each other. As expected, both B. burgdorferi s.l. and N. mikurensis had a significant positive association with each other and a negative association with A. phagocytophilum. Although these few specific associations do not appear to have a large effect on the entire microbiome composition, they can still be relevant for tick-borne pathogen dynamics.

CONCLUSIONS

Based on our results, we propose that M. mitochondrii alters the propensity of ticks to acquire or maintain horizontally acquired pathogens. The underlying mechanisms for some of these remarkable interactions are discussed herein and merit further investigation. Positive and negative associations between and within horizontally and vertically transmitted symbionts.

Antibacterial potency of Type VI amidase effector toxins is dependent on substrate topology and cellular context

Members of the bacterial T6SS amidase effector (Tae) superfamily of toxins are delivered between competing bacteria to degrade cell wall peptidoglycan. Although Taes share a common substrate, they exhibit distinct antimicrobial potency across different competitor species. To investigate the molecular basis governing these differences, we quantitatively defined the functional determinants of Tae1 from Pseudomonas aeruginosa PAO1 using a combination of nuclear magnetic resonance and a high-throughput in vivo genetic approach called deep mutational scanning (DMS). As expected, combined analyses confirmed the role of critical residues near the Tae1 catalytic center. Unexpectedly, DMS revealed substantial contributions to enzymatic activity from a much larger, ring-like functional hot spot extending around the entire circumference of the enzyme. Comparative DMS across distinct growth conditions highlighted how functional contribution of different surfaces is highly context-dependent, varying alongside composition of targeted cell walls. These observations suggest that Tae1 engages with the intact cell wall network through a more distributed three-dimensional interaction interface than previously appreciated, providing an explanation for observed differences in antimicrobial potency across divergent Gram-negative competitors. Further binding studies of several Tae1 variants with their cognate immunity protein demonstrate that requirements to maintain protection from Tae activity may be a significant constraint on the mutational landscape of tae1 toxicity in the wild. In total, our work reveals that Tae diversification has likely been shaped by multiple independent pressures to maintain interactions with binding partners that vary across bacterial species and conditions.

Current debates and advances in tick microbiome research

The main importance of ticks resides in their ability to harbor pathogens that can be transmitted to terrestrial vertebrates including humans. Recently, studies have focused on the taxonomic and functional composition of the tick microbiome, its microbial diversity and variation under different factors including tick species, sex, and environment among others. Of special interest are the interactions between the tick, the microbiome and pathogens since tick microbiome can influence pathogen colonization within the tick vector, and potentially, transmission to the vertebrate host. In this review, we tackled a synthesis on the growing field of tick microbiomes. We focus on the current state of tick microbiome research, addressing controversial and hotly debated topics and advances in the precise manipulation of tick microbiome. Furthermore, we discuss the innovative anti-tick microbiota vaccines as a possible tool for microbiome modulation and thus, control of tick-borne diseases. Deciphering tick-microbiome pathogen interactions can spur new strategies to control tick-borne diseases via modulation of tick microbiome.

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Whether the diversity observed in tick microbiomes concerns the biology or the methodology remains an open question.

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Tick immunity must play a major role in selecting ‘who stays and who leaves’ the microbiome.

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Anti-tick microbiota vaccines can target specific bacteria and subsequently modulate tick microbiome.

Host blood meal identity modifies vector gene expression and competency

A vector's susceptibility and ability to transmit a pathogen—termed vector competency—determines disease outcomes, yet the ecological factors influencing tick vector competency remain largely unknown. Ixodes pacificus, the tick vector of Borrelia burgdorferi (Bb) in the western U.S., feeds on rodents, birds, and lizards. Rodents and birds are reservoirs for Bb and infect juvenile ticks, while lizards are refractory to Bb and cannot infect feeding ticks. Additionally, the lizard bloodmeal contains borreliacidal properties, clearing previously infected feeding ticks of their Bb infection. Despite I. pacificus feeding on a range of hosts, it is undetermined how the host identity of the larval bloodmeal affects future nymphal vector competency. We experimentally evaluate the influence of larval host bloodmeal on Bb acquisition by nymphal I. pacificus. Larval I. pacificus were fed on either lizards or mice and after molting, nymphs were fed on Bb‐infected mice. We found that lizard‐fed larvae were significantly more likely to become infected with Bb during their next bloodmeal than mouse‐fed larvae. We also conducted the first RNA‐seq analysis on whole‐bodied I. pacificus and found significant upregulation of tick antioxidants and antimicrobial peptides in the lizard‐fed group. Our results indicate that the lizard bloodmeal significantly alters vector competency and gene regulation in ticks, highlighting the importance of host bloodmeal identity in vector‐borne disease transmission and upends prior notions about the role of lizards in Lyme disease community ecology.

Horizontally acquired cellulases assist the expansion of dietary range in Pristionchus nematodes

Horizontal gene transfer (HGT) enables the acquisition of novel traits via non-Mendelian inheritance of genetic material. HGT plays a prominent role in the evolution of prokaryotes, whereas in animals, HGT is rare and its functional significance is often uncertain. Here, we investigate horizontally acquired cellulase genes in the free-living nematode model organism Pristionchus pacificus. We show that these cellulase genes 1) are likely of eukaryotic origin, 2) are expressed, 3) have protein products that are secreted and functional, and 4) result in endo-cellulase activity. Using CRISPR/Cas9, we generated an octuple cellulase mutant, which lacks all eight cellulase genes and cellulase activity altogether. Nonetheless, this cellulase-null mutant is viable and therefore allows a detailed analysis of a gene family that was horizontally acquired. We show that the octuple cellulase mutant has associated fitness costs with reduced fecundity and slower developmental speed. Furthermore, by using various Escherichia coli K-12 strains as a model for cellulosic biofilms, we demonstrate that cellulases facilitate the procurement of nutrients from bacterial biofilms. Together, our analysis of cellulases in Pristionchus provides comprehensive evidence from biochemistry, genetics, and phylogeny, which supports the integration of horizontally acquired genes into the complex life history strategy of this soil nematode.

The cyclic dinucleotide 2'3'-cGAMP induces a broad antibacterial and antiviral response in the sea anemone Nematostella vectensis

In mammals, cyclic dinucleotides (CDNs) bind and activate STING to initiate an antiviral type I interferon response. CDNs and STING originated in bacteria and are present in most animals. By contrast, interferons are believed to have emerged in vertebrates; thus, the function of CDN signaling in invertebrates is unclear. Here, we use a CDN, 2'3' cyclic guanosine monophosphate-adenosine monophosphate (2'3'-cGAMP), to activate immune responses in a model cnidarian invertebrate, the starlet sea anemone Nematostella vectensis Using RNA sequencing, we found that 2'3'-cGAMP induces robust transcription of both antiviral and antibacterial genes in N. vectensis Many of the antiviral genes induced by 2'3'-cGAMP are homologs of vertebrate interferon-stimulated genes, implying that the interferon response predates the evolution of interferons. Knockdown experiments identified a role for NF-κB in specifically inducing antibacterial genes downstream of 2'3'-cGAMP. Some of these putative antibacterial genes were also found to be induced during Pseudomonas aeruginosa infection. We characterized the protein product of one of the putative antibacterial genes, the N. vectensis homolog of Dae4, and found that it has conserved antibacterial activity. This work suggests that a broad antibacterial and antiviral transcriptional response is an evolutionarily ancestral output of 2'3'-cGAMP signaling in animals.

Phylogenetic Analysis Indicates That Evasin-Like Proteins of Ixodid Ticks Fall Into Three Distinct Classes

Chemokines are structurally related proteins that activate leucocyte migration in response to injury or infection. Tick saliva contains chemokine-binding proteins or evasins which likely neutralize host chemokine function and inflammation. Biochemical characterisation of 50 evasins from Ixodes, Amblyomma and Rhipicephalus shows that they fall into two functional classes, A and B, with exclusive binding to either CC- or CXC- chemokines, respectively. Class A evasins, EVA1 and EVA4 have a four-disulfide-bonded core, whereas the class B evasin EVA3 has a three-disulfide-bonded “knottin” structure. All 29 class B evasins have six cysteine residues conserved with EVA3, arrangement of which defines a Cys6-motif. Nineteen of 21 class A evasins have eight cysteine residues conserved with EVA1/EVA4, the arrangement of which defines a Cys8-motif. Two class A evasins from Ixodes (IRI01, IHO01) have less than eight cysteines. Many evasin-like proteins have been identified in tick salivary transcriptomes, but their phylogenetic relationship with respect to biochemically characterized evasins is not clear. Here, using BLAST searches of tick transcriptomes with biochemically characterized evasins, we identify 292 class A and 157 class B evasins and evasin-like proteins from Prostriate (Ixodes), and Metastriate (Amblyomma, Dermacentor, Hyalomma, Rhipicephalus) ticks. Phylogenetic analysis shows that class A evasins/evasin-like proteins segregate into two classes, A1 and A2. Class A1 members are exclusive to Metastriate ticks and typically have a Cys8-motif and include EVA1 and EVA4. Class A2 members are exclusive to Prostriate ticks, lack the Cys8-motif, and include IHO01 and IRI01. Class B evasins/evasin-like proteins are present in both Prostriate and Metastriate lineages, typically have a Cys6-motif, and include EVA3. Most evasins/evasin-like proteins in Metastriate ticks belong to class A1, whereas in Prostriate species they are predominantly class B. In keeping with this, the majority of biochemically characterized Metastriate evasins bind CC-chemokines, whereas the majority of Prostriate evasins bind CXC-chemokines. While the origin of the structurally dissimilar classes A1 and A2 is yet unresolved, these results suggest that class B evasin-like proteins arose before the divergence of Prostriate and Metastriate lineages and likely functioned to neutralize CXC-chemokines and support blood feeding.

Horizontal Transfer of Microbial Toxin Genes to Gall Midge Genomes

A growing body of evidence has underscored the role of horizontal gene transfer (HGT) in animal evolution. Previously, we discovered the horizontal transfer of the gene encoding the eukaryotic genotoxin cytolethal distending toxin B (cdtB) from the pea aphid Acyrthosiphon pisum secondary endosymbiont (APSE) phages to drosophilid and aphid nuclear genomes. Here, we report cdtB in the nuclear genome of the gall-forming "swede midge" Contarinia nasturtii (Diptera: Cecidomyiidae) via HGT. We searched all available gall midge genome sequences for evidence of APSE-to-insect HGT events and found five toxin genes (aip56, cdtB, lysozyme, rhs, and sltxB) transferred horizontally to cecidomyiid nuclear genomes. Surprisingly, phylogenetic analyses of HGT candidates indicated APSE phages were often not the ancestral donor lineage of the toxin gene to cecidomyiids. We used a phylogenetic signal statistic to test a transfer-by-proximity hypothesis for animal HGT, which suggested that microbe-to-insect HGT was more likely between taxa that share environments than those from different environments. Many of the toxins we found in midge genomes target eukaryotic cells, and catalytic residues important for toxin function are conserved in insect copies. This class of horizontally transferred, eukaryotic cell-targeting genes is potentially important in insect adaptation.

Grappling with the tick microbiome

Ixodes scapularis and Ixodes pacificus are the predominant vectors of multiple human pathogens, including Borrelia burgdorferi, one of the causative agents of Lyme disease in North America. Differences in the habitats and host preferences of these closely related tick species present an opportunity to examine key aspects of the tick microbiome. While advances in sequencing technologies have accelerated a descriptive understanding of the tick microbiome, molecular and mechanistic insights into the tick microbiome are only beginning to emerge. Progress is stymied by technical difficulties in manipulating the microbiome and by biological variables related to the life cycle of Ixodid ticks. This review highlights these challenges and examines avenues to understand the significance of the tick microbiome in tick biology.

The genus Anaplasma: drawing back the curtain on tick-pathogen interactions

Tick-borne illnesses pose a serious concern to human and veterinary health and their prevalence is on the rise. The interactions between ticks and the pathogens they carry are largely undefined. However, the genus Anaplasma, a group of tick-borne bacteria, has been instrumental in uncovering novel paradigms in tick biology. The emergence of sophisticated technologies and the convergence of entomology with microbiology, immunology, metabolism and systems biology has brought tick-Anaplasma interactions to the forefront of vector biology with broader implications for the infectious disease community. Here, we discuss the use of Anaplasma as an instrument for the elucidation of novel principles in arthropod-microbe interactions. We offer an outlook of the primary areas of study, outstanding questions and future research directions.

Tick Immune System: What Is Known, the Interconnections, the Gaps, and the Challenges

Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.

Beating Them with Their Own Stick-Tick Uses Amidase of Bacterial Origin as Part of Its Immune Arsenal

As vectors of microbial diseases in vertebrates, ticks are excellent at regulating bacterial proliferation in and around them. In a recent issue of Cell, Hayes et al. (2020) reveal acarid toxins of bacterial origin that help eliminate microbes that are pathogenic to black-legged ticks but commensal to their vertebrate hosts.