The technical scope of the review was deliberately narrow, and that narrowness is its strength. The authors restricted inclusion to studies that used Illumina-based next-generation sequencing of the 16S rRNA V3-V4 hypervariable region, the most consistently applied method in tick microbiome research. By amplifying and sequencing this specific stretch of the bacterial ribosomal gene, researchers can profile entire bacterial communities without culturing a single organism. Restricting the synthesis to a single sequencing platform and gene region reduced methodological heterogeneity, allowing the authors to compare bacterial richness and diversity across continents with far more confidence than previous narrative reviews could achieve.

What emerged from the synthesis is a striking pattern: hard ticks across the globe harbor a limited core microbiota dominated by just four bacterial phyla, Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria, and this dominance holds regardless of tick species, sex, host, geographic region, or the tissue analyzed. Within that phylum-level framework, two endosymbionts stood out as consistently dominant across studies: Coxiella and Rickettsia. These genera, which include notorious pathogenic species, also exist as non-pathogenic symbionts that appear to play fundamental roles in tick biology, potentially supplying B vitamins and other nutrients that the blood-only diet of ticks fails to provide.

The review went beyond simple taxonomic lists by applying network and co-occurrence analyses to the reported data. Using presence-absence heatmaps, UpSet plots, and bipartite networks constructed in RStudio, the authors identified a putative core set of bacterial genera associated with Ixodidae ticks: Acinetobacter, Bacillus, Corynebacterium, Coxiella, and Rickettsia. Known symbionts such as Coxiella and Rickettsia were positioned centrally in the network, reflecting their common detection across tick genera, while environmentally acquired soil bacteria such as Sphingobium and Ralstonia sat at the network peripheries. The authors classified bacterial genera as core, moderate, or rare based on their reported occurrence across the reviewed tick genera, using thresholds adapted from the core microbiome criteria of Shade and Handelsman.

Ecological and physiological factors proved to be powerful shapers of the tick microbiome. Males typically showed greater microbial richness than females, although some studies found no significant sex-related differences, suggesting that sex effects are species dependent and likely influenced by the presence of specific endosymbionts. Across developmental stages, dominant bacteria such as Francisella, Candidatus Midichloria mitochondrii, and Rickettsia persisted from larva to adult, but community richness and evenness declined as ticks matured, likely due to blood-meal feeding and physiological filtering. Host-collected, engorged ticks displayed increased bacterial diversity with significant overlap between tick and host blood bacteria, while field-collected ticks harbored more diverse communities than laboratory-reared colonies, which appeared depauperate and dominated by endosymbionts.

Tick organs themselves act as microbial filters. The midgut exhibited the highest bacterial diversity across studies and displayed a dynamic microbiota profile, whereas reproductive tissues and Malpighian tubules displayed the lowest diversity and richness, maintaining restricted, specialized assemblages that likely ensure vertical transmission of symbionts to offspring. This organ-level filtering was consistent across geographic regions, with internal organs of Dermacentor and Amblyomma ticks consistently harboring higher diversity in the midgut than in reproductive tissues. Only a small subset of studies, roughly 10 percent, targeted specific tissues rather than whole ticks, highlighting how much organ-level resolution remains missing from the field.

Geographic location, surprisingly, had limited influence on tick microbiome composition in many comparisons, with similar bacterial communities reported in Haemaphysalis, Ixodes, and Rhipicephalus ticks sampled from Japan, Peninsular Malaysia, Southwest China, France, and Germany. Yet other studies reported significant geographic variation in other parts of Asia, Europe, and North America, suggesting that while some tick-associated bacterial communities may be conserved across broad regions, local environmental factors such as soil type, vegetation, and available hosts can drive microbiome divergence among geographically distinct populations. The effects of pathogen infection status on microbiome composition were similarly inconsistent across studies, and only three investigations of temporal fluctuations were found, rendering conclusions about how tick microbiomes change over time essentially inconclusive.

The implications for disease control are considerable. Non-pathogenic microbiome members can exert variable effects on pathogens within the tick, and in Ixodes ricinus, the endosymbiont Wolbachia has been shown to limit the presence and transmission of Anaplasma, Borrelia, Neoehrlichia, and Rickettsia in field populations. Anti-microbiota vaccines, which exploit the capacity of vertebrate host immunoglobulins to remain functional within tick hemolymph, have been developed to modulate the tick microbiome in a taxon-specific manner. Functional redundancy in the tick microbiome, where many bacterial genera contribute to a single metabolic pathway, suggests that ticks have evolved mechanisms to select for microbial communities conferring ecological advantages. Meanwhile, Wolbachia-based paratransgenesis, successfully deployed in mosquito vectors to induce pathogen refractoriness, remains biologically challenging to apply in ticks.

The quality assessment component of the review exposed sobering gaps in methodological rigor. Applying the Joanna Briggs Institute checklists, the STORMS reporting checklist, and AMSTAR2, the authors found that none of the studies investigating ticks collected from humans or domestic livestock reported host exposure to acaricides, a variable expected to influence microbiome composition. Negative controls such as DNA extraction blanks and PCR no-template controls were included in only 37 of 68 studies, and positive mock community controls in just 15. Surface decontamination protocols varied widely, and while DNA extracts were universally stored at -20 or -80 degrees Celsius, field storage conditions ranged from live transport under controlled temperature and humidity to 70 to 100 percent ethanol. Only 45 studies deposited raw sequencing data in public repositories such as NCBI and EMBL-EBI, and full computer-readable results were available for less than half of the reviewed work.

The authors also flagged a disproportionate geographic distribution of research, with Asia contributing 44 percent of studies, led by China with 17 of 68, and the United States contributing 15, while Africa accounted for only five studies and South America and Australasia just three each. This paucity hampers the broad generalizability of the findings, particularly for regions where tick-borne disease burdens are high but microbiome research infrastructure is limited. Looking forward, the review points to standardized protocols, improved reporting of host, sex, and developmental stage metadata, adoption of the MIxS and MIMARKS data standards, and functional studies as priorities. The authors also highlight CRISPR/Cas gene-editing technologies, already demonstrated in Ixodes scapularis through embryo injection and ReMOT Control delivery, as promising tools for unraveling how tick microbiomes influence tick biology and pathogen transmission, potentially opening entirely new avenues for interrupting the cycle of tick-borne disease.

Subject of Research: Global variations and implications of microbiome communities in Ixodidae ticks: a systematic review and quality assessment

Article Title: Global variations and implications of microbiome communities in Ixodidae ticks: a systematic review and quality assessment

Article References: Global variations and implications of microbiome communities in Ixodidae ticks: a systematic review and quality assessment. (n.d.). https://doi.org/10.3389/fbrio.2026.1868676

Image Credits: AI Generated

DOI: 10.3389/fbrio.2026.1868676

Keywords: Global, variations, implications, microbiome, communities, Ixodidae, ticks, systematic, review, quality, assessment, scientific research