Symbiont-mediated antisense RNA delivery controls Nosema ceranae infections in Apis mellifera.

Nosema ceranae is a main parasite for honeybees (Apis mellifera) which causes colony collapse in spring. Effective management of N. ceranae infections in bees is imperative for beekeepers. RNA interference (RNAi) has been proven a promising method to control bee pathogens, including IAPV, Varroa destructor, and Nosema. Most studies in this field focused on oral inoculation of double-stranded RNA (dsRNA). We developed an easier method with long-term RNAi effects by engineering the bee symbiont, Bacillus subtilis, to deliver single-stranded antisense RNA (asRNA) in the bee guts, targeting N. ceranae genes. We interfered with the expression of a spore wall protein (SWP12) and a polar tube protein (PTP3) of N. ceranae, resulting in a 60.5% increase in bee lifespan and a 72.7% decrease in Nosema spore load. Our research introduced a novel approach to bee parasite control: B. subtilis-mediated asRNA delivery. Our strategy simplifies the procedure of RNAi, presenting a more efficient mechanism with both prophylactic and therapeutic effects on N. ceranae-infected bees.

Effects of developmental stages, sex difference, and diet types of the host marmalade hoverfly (Episyrphus balteatus) on symbiotic bacteria.

Introduction: Symbiotic bacteria play key roles in a variety of important life processes of insects such as development, reproduction and environmental adaptation, and the elucidation of symbiont population structure and dynamics is crucial for revealing the underlying regulatory mechanisms. The marmalade hoverfly (Episyrphus balteatus) is not only a remarkable aphid predator, but also a worldwide pollinator second to honeybees. However, its symbiont composition and dynamics remain unclear. Methods: Herein, we investigate the symbiotic bacterial dynamics in marmalade hoverfly throughout whole life cycle, across two sexes, and in its prey Megoura crassicauda by 16S rRNA sequencing. Results: In general, the dominant phyla were Proteobacteria and Firmicutes, and the dominant genera were Serratia and Wolbachia. Serratia mainly existed in the larval stage of hoverfly with the highest relative abundance of 86.24% in the 1st instar larvae. Wolbachia was found in adults and eggs with the highest relative abundance of 62.80% in eggs. Significant difference in species diversity was observed between the adults feeding on pollen and larvae feeding on M. crassicauda, in which the dominant symbiotic bacteria were Asaia and Serratia, respectively. However, between two sexes, the symbionts exhibited high similarity in species composition. In addition, our results suggested that E. balteatus obtainded Serratia mainly through horizontal transmission by feeding on prey aphids, whereas it acquired Wolbachia mainly through intergeneration vertical transmission. Taken together, our study revealed the effects of development stages, diet types and genders of E. balteatus on symbionts, and explored transmission modes of dominant bacteria Serratia and Wolbachia. Discussion: Our findings lay a foundation for further studying the roles of symbiotic bacteria in E. balteatus life cycle, which will benefit for revealing the co-adaptation mechanisms of insects and symbiotic bacteria.

Genome sequences of Pseudomonas sp. strain FFPRI_1 and Photorhabdus temperata strain FFPRI_2, isolated from the entomopathogenic nematode, Heterorhabditis megidis.

The complete genome sequences of two bacteria isolated from an entomopathogenic nematode are reported. The genome sizes of Pseudomonas sp. strain FFPRI_1 and Photorhabdus temperata strain FFPRI_2 are 7,521,420 bp long with 63% G+C content and 5,483,510 bp long with 43.8% G+C content, respectively.

Rapid Loss of Nutritional Symbionts in an Endemic Hawaiian Herbivore Radiation Is Associated with Plant Galling Habit.

Insect herbivores frequently cospeciate with symbionts that enable them to survive on nutritionally unbalanced diets. While ancient symbiont gain and loss events have been pivotal for insect diversification and feeding niche specialization, evidence of recent events is scarce. We examine the recent loss of nutritional symbionts (in as little as 1 MY) in sap-feeding Pariaconus, an endemic Hawaiian insect genus that has undergone adaptive radiation, evolving various galling and free-living ecologies on a single host-plant species, Metrosideros polymorpha within the last ∼5 MY. Using 16S rRNA sequencing, we investigated the bacterial microbiomes of 19 Pariaconus species and identified distinct symbiont profiles associated with specific host-plant ecologies. Phylogenetic analyses and metagenomic reconstructions revealed significant differences in microbial diversity and functions among psyllids with different host-plant ecologies. Within a few millions of years, Pariaconus species convergently evolved the closed-gall habit twice. This shift to enclosed galls coincided with the loss of the Morganella-like symbiont that provides the essential amino acid arginine to free-living and open-gall sister species. After the Pariaconus lineage left Kauai and colonized younger islands, both open- and closed-gall species lost the Dickeya-like symbiont. This symbiont is crucial for synthesizing essential amino acids (phenylalanine, tyrosine, and lysine) as well as B vitamins in free-living species. The recurrent loss of these symbionts in galling species reinforces evidence that galls are nutrient sinks and, combined with the rapidity of the evolutionary timeline, highlights the dynamic role of insect-symbiont relationships during the diversification of feeding ecologies. We propose new Candidatus names for the novel Morganella-like and Dickeya-like symbionts.

Gill-associated ammonia oxidizers are widespread in teleost fish.

Recent advances in sequencing methods have greatly expanded the knowledge of teleost-associated microorganisms. While fish-gut microbiomes are comparatively well studied, less attention has gone toward other, external organ-microbiome associations. Gills are particularly interesting to investigate due to their functions in gas exchange, osmoregulation, and nitrogen excretion. We recently discovered a branchial symbiosis between nitrogen-cycling bacteria and teleosts (zebrafish and carp), in which ammonia-oxidizing Nitrosomonas and denitrifying bacteria together convert toxic ammonia excreted by the fish into harmless dinitrogen (N2) gas. This symbiosis can function as a "natural biofilter" in fish gills and can potentially occur in all ammonotelic fish species, but it remains unknown how widespread this symbiosis is. In this study, we analyzed all publicly available gill microbiome data sets and checked for the presence of Nitrosomonas. We discovered that more than half of the described fish gill microbiomes contain 16S rRNA gene sequences of ammonia-oxidizing bacteria (AOB). The presence of gill-specific AOB was shown in both wild and aquacultured fish, as well as in marine and freshwater fish species. Based on these findings, we propose that ammonia oxidizers are widespread in teleost fish gills. These gill-associated AOB can significantly affect fish nitrogen excretion, and the widespread nature of this association suggests that the gill-associated AOB can have similar impacts on more fish species. Future research should address the contribution of these microorganisms to fish nitrogen metabolism and the fundamental characteristics of this novel symbiosis.IMPORTANCERecent advances in sequencing have increased our knowledge of teleost-associated microbiota, but the gill microbiome has received comparatively little attention. We recently discovered a consortium of nitrogen-cycling bacteria in the gills of common carp and zebrafish, which are able to convert (toxic) ammonia into harmless dinitrogen gas. These microorganisms thus function as a natural nitrogen biofilter. We analyzed all available gill microbiome data sets to determine how widespread gill-associated ammonia-oxidizing bacteria (AOB) are. More than half of the data sets contained AOB, representing both aquacultured and wild fish from freshwater and marine habitats. In total, 182 amplicon sequencing variants were obtained, of which 115 were found specifically in the gills and not the environmental microbiomes. As gill-associated AOB are apparently widespread in teleost fish, it is important to study their impact on host nitrogen excretion and the potential to reduce ammonia accumulation in (recirculating) aquaculture of relevant fish species.

Networking the desert plant microbiome, bacterial and fungal symbionts structure and assortativity in co-occurrence networks.

In nature, microbes do not thrive in seclusion but are involved in complex interactions within- and between-microbial kingdoms. Among these, symbiotic associations with mycorrhizal fungi and nitrogen-fixing bacteria are namely known to improve plant health, while providing resources to benefit other microbial members. Yet, it is not clear how these microbial symbionts interact with each other or how they impact the microbiota network architecture. We used an extensive co-occurrence network analysis, including rhizosphere and roots samples from six plant species in a natural desert in AlUla region (Kingdom of Saudi Arabia) and described how these symbionts were structured within the plant microbiota network. We found that the plant species was a significant driver of its microbiota composition and also of the specificity of its interactions in networks at the microbial taxa level. Despite this specificity, a motif was conserved across all networks, i.e., mycorrhizal fungi highly covaried with other mycorrhizal fungi, especially in plant roots-this pattern is known as assortativity. This structural property might reflect their ecological niche preference or their ability to opportunistically colonize roots of plant species considered non symbiotic e.g., H. salicornicum, an Amaranthaceae. Furthermore, these results are consistent with previous findings regarding the architecture of the gut microbiome network, where a high level of assortativity at the level of bacterial and fungal orders was also identified, suggesting the existence of general rules of microbiome assembly. Otherwise, the bacterial symbionts Rhizobiales and Frankiales covaried with other bacterial and fungal members, and were highly structural to the intra- and inter-kingdom networks. Our extensive co-occurrence network analysis of plant microbiota and study of symbiont assortativity, provided further evidence on the importance of bacterial and fungal symbionts in structuring the global plant microbiota network.

Supplementation of diethyl aminoethyl hexanoate for enhancing antibiotics removal by different microalgae-based system.

With the growth of the aquaculture industry, antibiotic residues in treated wastewater have become a serious ecological threat. The effects of supplementation with diethyl aminoethyl hexanoate (DA-6) on the removal of tetracycline (TC), ciprofloxacin (CPFX), and sulfamonomethoxine (SMM) from aquaculture wastewater by different microalgae-based systems were examined and systematically analyzed. The results demonstrated that C. vulgaris -S395-2-C. rosea symbiont performed best under 0.2 mg L-1 antibiotic treatment for antibiotic removal. At 10-7 M, DA-6 significantly enhanced C. vulgaris-S395-2-C. rosea symbiont removal of CPFX and SMM at 0.20 mg L-1. The removal of TC, CPFX and SMM by this strain under optimal conditions was 99.2 ± 0.4 %, 86.3 ± 6.3 %, and 91.3 ± 5.7 %, respectively. These results suggest that DA-6 may act on microalgae-bacteria-fungi three-phase symbionts for the removal of multiple antibiotics from aquaculture wastewater.

The burrower bug Macroscytus japonensis (Hemiptera: Cydnidae) acquires obligate symbiotic bacteria from the environment.

Many plant-feeding stinkbugs belonging to the infraorder Pentatomomorpha possess a specialized symbiotic organ at the posterior end of the midgut, in which mutualistic bacterial symbionts are harbored extracellularly. In species of the superfamily Pentatomoidea, these symbionts typically are verticallytransmitted from host mothers to offspring, whereas in species of the superfamilies Coreoidea and Lygaeoidea they are acquired from the environment. In the pentatomoid family Cydnidae, vertical symbiont transmission has been reported in several species. Here, we report the first case of environmental symbiont acquisition in Cydnidae, observed in the burrower bug Macroscytus japonensis. A comprehensive survey of 72 insect samples from 23 sites across the Japanese archipelago revealed that (1) symbionts exhibit remarkably high diversity, forming six distinct phylogenetic groups within the Enterobacteriaceae of the γ-Proteobacteria, (2) most symbionts are cultivable and closely related to free-living Pantoea-allied bacteria, and (3) symbiont phylogenetic groups do not reflect the host phylogeny. Microbial inspection of eggs revealed the absence of bacteria on the egg surface. These results strongly suggest that symbionts are acquired from the environment, not vertical transmission. Rearing experiments confirmed environmental symbiont acquisition. When environmental symbiont sources were experimentally withheld, nymphs became aposymbiotic and died before molting to the second instar, indicating that nymphs environmentally acquire symbionts during the first-instar stage and that symbionts are essential for nymphal growth and survival. This study highlights Cydnidae as the only pentatomoid family that includes species that environmentally acquire symbionts and those that vertically transmit symbionts, providing an ideal platform for comparative studies of the ecological and environmental factors that influence the evolution of symbiont transmission modes.

Symbiotic Algae of Acoel Species in the Seto Inland Sea and Symbiont Selectivity in the Hosts.

Praesagittifera naikaiensis is an acoel flatworm that inhabits the sandy beaches in the intertidal zone of the Seto Inland Sea. This species carries Tetraselmis sp., a green unicellular chlorophyte, as a symbiont in its body, and depends on algal photosynthetic products to survive. However, the eggs of P. naikaiensis contain no symbiotic algae, and juvenile P. naikaiensis acquire symbionts from the surrounding environment through horizontal transfer after hatching, thereby establishing new symbiotic relationships in each generation. Other acoel species, Symsagittifera spp., also inhabit the Seto Inland Sea shores and acquire symbiotic green algae via horizontal transfers. To characterize their symbionts, these acoels were collected from a wide area of the Seto Inland Sea and partial nucleotide sequences of the chloroplast ribulose diphosphate carboxylase large subunit (rbcL) of the symbiotic algae were determined and used for molecular phylogenetic analysis. Symbionts of both P. naikaiensis and Symsagittifera spp. belonged to the genus Tetraselmis but were phylogenetically distant, and both species established symbiotic relationships with different symbionts even when they were sympatric. To test whether each species selects specific algae in the environment for symbiosis, we established algal strains from P. naikaiensis and Symsagittifera sp. symbionts and conducted uptake experiments on aposymbiotic juveniles of P. naikaiensis. The results suggest that symbiotic algae from Symsagittifera could be taken up by P. naikaiensis juveniles, but were unable to establish a normal symbiotic relationship with the juveniles.

Progress and future directions for seaweed holobiont research.

In the marine environment, seaweeds (i.e. marine macroalgae) provide a wide range of ecological services and economic benefits. Like land plants, seaweeds do not provide these services in isolation, rather they rely on their associated microbial communities, which together with the host form the seaweed holobiont. However, there is a poor understanding of the mechanisms shaping these complex seaweed-microbe interactions, and of the evolutionary processes underlying these interactions. Here, we identify the current research challenges and opportunities in the field of seaweed holobiont biology. We argue that identifying the key microbial partners, knowing how they are recruited, and understanding their specific function and their relevance across all seaweed life history stages are among the knowledge gaps that are particularly important to address, especially in the context of the environmental challenges threatening seaweeds. We further discuss future approaches to study seaweed holobionts, and how we can apply the holobiont concept to natural or engineered seaweed ecosystems.

Current paradigms and future challenges in harnessing gut bacterial symbionts of insects for biodegradation of plastic wastes.

The ubiquitous incorporation of plastics into daily life, coupled with inefficient recycling practices, has resulted in the accumulation of millions of metric tons of plastic waste, that poses a serious threat to the Earth's sustainability. Plastic pollution, a global problem, disrupts the ecological balance and endangers various life forms. Efforts to combat plastic pollution are underway, with a promising avenue being biological degradation facilitated by certain insects and their symbiotic gut microorganisms, particularly bacteria. This review consolidates existing knowledge on plastic degradation by insects and their influence on gut microbiota. Additionally, it delves into the potential mechanisms employed by insects in symbiosis with gut bacteria, exploring the bioconversion of waste plastics into value-added biodegradable polymers through mineralization. These insights hold significant promise for the bio-upcycling of plastic waste, opening new horizons for future biomanufacturing of high-value chemicals from plastic-derived compounds. Finally, we weigh the pros and cons of future research endeavors related to the bioprospection of plastic-degrading bacteria from underexplored insect species. We also underscore the importance of bioengineering depolymerases with novel characteristics, aiming for their application in the remediation and valorization of waste plastics.

Repeated horizontal acquisition of lagriamide-producing symbionts in Lagriinae beetles.

Microbial symbionts associate with multicellular organisms on a continuum from facultative associations to mutual codependency. In some of the oldest intracellular symbioses there is exclusive vertical symbiont transmission, and co-diversification of symbiotic partners over millions of years. Such symbionts often undergo genome reduction due to low effective population sizes, frequent population bottlenecks, and reduced purifying selection. Here, we describe multiple independent acquisition events of closely related defensive symbionts followed by genome erosion in a group of Lagriinae beetles. Previous work in Lagria villosa revealed the dominant genome-eroded symbiont of the genus Burkholderia produces the antifungal compound lagriamide and protects the beetle's eggs and larvae from antagonistic fungi. Here, we use metagenomics to assemble 11 additional genomes of lagriamide-producing symbionts from seven different host species within Lagriinae from five countries, to unravel the evolutionary history of this symbiotic relationship. In each host species, we detected one dominant genome-eroded Burkholderia symbiont encoding the lagriamide biosynthetic gene cluster (BGC). Surprisingly, however, we did not find evidence for host-symbiont co-diversification, or for a monophyly of the lagriamide-producing symbionts. Instead, our analyses support at least four independent acquisition events of lagriamide-encoding symbionts and subsequent genome erosion in each of these lineages. By contrast, a clade of plant-associated relatives retained large genomes but secondarily lost the lagriamide BGC. In conclusion, our results reveal a dynamic evolutionary history with multiple independent symbiont acquisitions characterized by high degree of specificity. They highlight the importance of the specialized metabolite lagriamide for the establishment and maintenance of this defensive symbiosis.

Mauritone A, a new polyketide from a fungal-bacterial symbiont Aspergillus spelaeus GXIMD 04541/Sphingomonas echinoides GXIMD 04532.

A new polyketide, mauritone A (1) with six known polyketides curvulone B (2), curvularin (3), 12-oxocurvularin (4), (10E,15S)-10,11-dehydrocurvularin (5), (11R,15S)-11-hydroxycurvularin (6), and (11S,15S)-11-hydroxycurvularin (7) were isolated from the fungal-bacterial symbiont Aspergillus spelaeus GXIMD 04541/Sphingomonas echinoides GXIMD 04532 derived from Mauritia arabica. Their structures were elucidated by extensive spectral analysis. All compounds (1-7) were evaluated for their anti-inflammatory effects. The inhibitory effects of 4, 5, and 7 on nitric oxide (NO) production were found to be significant, with IC50 values of 5.5 ± 0.26, 2.0 ± 0.31, and 8.3 ± 0.62 µM, respectively, surpassing that of the positive control quercetin (10.6 ± 0.64 µM). Compounds 3 and 6 exhibited moderate inhibition of NO, with IC50 values of 18.6 ± 0.53 and 12.7 ± 0.45 µM, respectively.

Trade-offs in a reef-building coral after six years of thermal acclimation.

There is growing evidence that reef-building corals can acclimate to novel and challenging thermal conditions. However, potential trade-offs that accompany acclimation remain largely unexplored. We investigated physiological trade-offs in colonies of a globally abundant coral species (Pocillopora acuta) that were acclimated ex situ to an elevated temperature of 31 °C (i.e., 1 °C above their bleaching threshold) for six years. By comparing them to conspecifics maintained at a cooler temperature, we found that the energy storage of corals was prioritized over skeletal growth at the elevated temperature. This was associated with the formation of higher density skeletons, lower calcification rates and consequently lower skeletal extension rates, which entails ramifications for future reef-building processes, structural complexity and reef community composition. Furthermore, symbionts were physiologically compromised at 31 °C and had overall lower energy reserves, likely due to increased exploitation by their host, resulting in an overall lower stress resilience of the holobiont. Our study shows how biological trade-offs of thermal acclimation unfold, helping to refine our picture of future coral reef trajectories. Importantly, our observations in this six-year study do not align with observations of short-term studies, where elevated temperatures were often associated with the depletion of energy reserves, highlighting the importance of studying acclimation of organisms at relevant biological scales.

The Microbiota of Moon Snail Egg Collars is Shaped by Host-Specific Factors.

Moon Snails lay eggs using a mixture of mucus and sediment to form an egg mass commonly referred to as an egg collar. These collars do not appear to experience micro-biofouling or predation and this observation led us to hypothesize that the egg collars possess a chemically-rich microbiota that protect the egg collars from pathogens. Herein, we sought to gain an understanding of the bacterial composition of the egg collars by amplifying and sequencing the 16S rRNA gene from egg collar and sediment samples collected at four distinct geographical regions in SW Florida. Relative abundance and non-metric multidimensional scaling plots revealed distinct differences in the bacterial composition between the egg collar and sediment samples. In addition, the egg collars had a lower α-diversity than the sediment, with specific genera being significantly enriched in the egg collars. Analysis of microorganisms consistent across two seasons suggests that Flavobacteriaceae make up a large portion of the core microbiota (36 - 58% of 16S sequences). We also investigated the natural product potential of the egg collar microbiota by sequencing a core biosynthetic gene, the adenylation domains (AD), within the gene clusters of non-ribosomal peptide synthetase (NRPS). AD sequences matched multiple modules within known bioactive NRPs biosynthetic gene clusters, suggesting production is possible within the egg collar system and lays the foundation for future studies into the chemical and ecological role of this microbiota.

Seaweeds as holobionts: Current state, challenges, and potential applications.

Seaweeds play a strong ecological and economical role along the world's coastlines, where they support industries (e.g., aquaculture, bioproducts) and essential ecosystem services (e.g., biodiversity, fisheries, carbon capture). Evidence from wild and cultured seaweeds suggests that microorganisms play crucial roles in their health and functioning, prompting the need for considering seaweeds and their microbiome as a coherent entity or "holobiont." Here we show that the number of studies investigating seaweed hosts and their microbiome have increased in the last two decades. This likely reflects the increase in the appreciation of the importance of microbiomes for eukaryotic hosts, improved molecular approaches used to characterize their interactions, and increasing interest in commercial use of seaweeds. However, although increasing, most studies of seaweed holobionts have focused on (i) a few seaweed species of ecological or commercial significance, (ii) interactions involving only bacteria, and (iii) descriptive rather than experimental approaches. The relatively few experimental studies have mostly focused on manipulating abiotic factors to examine responses of seaweeds and their microbiome. Of the few studies that directly manipulated microorganisms to investigate their effects on seaweeds, most were done in laboratory or aquaria. We emphasize the need to move beyond the descriptions of patterns to experimental approaches for understanding causation and mechanisms. We argue that such experimental approaches are necessary for a better understanding of seaweed holobionts, for management actions for wild and cultivated seaweeds, and to better integrate studies of seaweed holobionts with the broader fields of seaweed ecology and biology, which are strongly experimental.

Evolutionary history of tyrosine-supplementing endosymbionts in pollen-feeding beetles.

Many insects feeding on nutritionally challenging diets like plant sap, leaves, or wood engage in ancient associations with bacterial symbionts that supplement limiting nutrients or produce digestive or detoxifying enzymes. However, the distribution, function, and evolutionary dynamics of microbial symbionts in insects exploiting other plant tissues or relying on a predacious diet remain poorly understood. Here, we investigated the evolutionary history and function of the intracellular gamma-proteobacterial symbiont "Candidatus Dasytiphilus stammeri" in soft-winged flower beetles (Coleoptera, Melyridae, Dasytinae) that transition from saprophagy or carnivory to palynivory (pollen-feeding) between larval and adult stage. Reconstructing the distribution of the symbiont within the Dasytinae phylogeny unraveled not only a long-term coevolution, originating from a single acquisition event with subsequent host-symbiont codiversification, but also several independent symbiont losses. The analysis of 20 different symbiont genomes revealed that their genomes are severely eroded. However, the universally retained shikimate pathway indicates that the core metabolic contribution to their hosts is the provisioning of tyrosine for cuticle sclerotization and melanization. Despite the high degree of similarity in gene content and order across symbiont strains, the capacity to synthesize additional essential amino acids and vitamins and to recycle urea is retained in some but not all symbionts, suggesting ecological differences among host lineages. This report of tyrosine-provisioning symbionts in insects with saprophagous or carnivorous larvae and pollen-feeding adults expands our understanding of tyrosine supplementation as an important symbiont-provided benefit across a broad range of insects with diverse feeding ecologies.

Bacterial Symbiont Mediates Aphid Antiviral Systems to Inhibit the Propagation of a New Pathogenic Densovirus in Chinese Wheat Aphid, Sitobion miscanthi.

Sitobion miscanthi L-type symbiont (SMLS) is a bacterial symbiont commonly found in the wheat aphid S. miscanthi. A new aphid densovirus, S. miscanthi densovirus (SmDV), was recently identified in S. miscanthi. In this study, the similar cellular tropism of SmDV and SMLS in aphid embryos was uncovered using in situ hybridization. SmDV infection significantly decreased the longevity and number of S. miscanthi offspring. However, the SmDV titers were significantly suppressed after SMLS transmission, thus reducing the negative effects of SmDV infection on S. miscanthi fitness. Moreover, an integrative analysis of RNA-seq datasets showed that SMLS inhibited the expression of genes related to the phosphatidylinositol 3-kinase (Pl3K)/Akt pathways and further induced the expression of antiviral factors associated with the apoptosis and FoxO signaling pathways. These results indicate that SMLS mediates host antiviral defenses to inhibit the propagation of SmDV, which was further verified by an RNA interference assay.

Distinct biogeographical patterns in snail gastrointestinal tract bacterial communities compared with sediment and water.

The factors that influence the distribution of bacterial community composition are not well understood. The role of geographical patterns, which suggest limited dispersal, is still a topic of debate. Bacteria associated with hosts face unique dispersal challenges as they often rely on their hosts, which provide specific environments for their symbionts. In this study, we examined the effect of biogeographic distances on the bacterial diversity and composition of bacterial communities in the gastrointestinal tract of Ampullaceana balthica. We compared the effects on the host-associated bacterial community to those on bacterial communities in water and sediment. This comparison was made using 16S ribosomal RNA gene sequencing. We found that the bacterial communities we sampled in Estonia, Denmark, and Northern Germany varied between water, sediment, and the gastrointestinal tract. They also varied between countries within each substrate. This indicates that the type of substrate is a dominant factor in determining bacterial community composition. We separately analyzed the turnover rates of water, sediment, and gastrointestinal bacterial communities over increasing geographic distances. We observed that the turnover rate was lower for gastrointestinal bacterial communities compared to water bacterial communities. This implies that the composition of gastrointestinal bacteria remains relatively stable over distances, while water bacterial communities exhibit greater variability. However, the gastrointestinal tract had the lowest percentage of country-specific amplicon sequence variants, suggesting bacterial colonization from local bacterial communities. Since the overlap between the water and gastrointestinal tract was highest, it appears that the gastrointestinal bacterial community is colonized by the water bacterial community. Our study confirmed that biogeographical patterns in host-associated communities differ from those in water and sediment bacterial communities. These host-associated communities consist of numerous facultative symbionts derived from the water bacterial community.