More deterministic assembly constrains the diversity of gut microbiota in freshwater snails

Invasive apple snails with their core microbes are underestimated hotspots for disseminating antibiotic resistance genes and virulence factors in aquatic habitats

Antibiotic resistance in pathogens is a threat to human health. The invasive apple snail (Pomacea canaliculata), widely distributed and linked to human activities, is a potential vector for human pathogens. However, its role in spreading antibiotic resistance genes (ARGs) is poorly understood. This study assessed the microbiological risk of this snail by sampling from five interconnected habitats: feces of invasive apple snails and native snails, ditch water, sediment, and soil. Using metagenomic and 16S rRNA sequencing, we analyzed the microbial communities and quantified the ARGs and virulence factors (VFs). Results showed that invasive apple snails carried significantly higher levels of ARGs and VFs compared with the native snails and environmental samples. ARGs and VFs were primarily found co-occurring in Aeromonas and Citrobacter freundii, with Aeromonas identified as the core microbe selected by invasive apple snails. Furthermore, the abundance and community dissimilarity of Aeromonas positively correlated with those of ARGs and VFs, both directly and indirectly through mobile genetic elements. This suggests Aeromonas may play a key role in disseminating ARGs and VFs across habitats. Overall, this study highlights the invasive apple snail as a significant vector for ARGs and virulent pathogens, providing critical insights for risk assessment and targeted management within the One Health framework.

Stochastic processes govern gut bacterial community assembly in a Schistosoma mansoni-transmitting snail, Biomphalaria straminea

BACKGROUND

Studies have revealed extensive taxonomic classifications and patterns of gut microbial diversity in snails, with limited focus on community assembly processes. To better understand the balance between stochastic and deterministic processes in the snail gut microbial assembly and their associations with snail fitness, we used the freshwater snail Biomphalaria straminea as a model and analyzed the gut bacterial communities from 118 samples via high-throughput sequencing of the 16S rRNA gene.

RESULTS

This study reveals that Proteobacteria and Bacteroidota dominate the gut microbiota of B. straminea. Snails from different laboratory habitats exhibit similar gut bacterial diversity but significantly different community structures. The assembly of gut bacterial communities in both laboratory and wild samples is predominantly influenced by stochastic processes rather than deterministic processes, as evidenced by the neutral community model (NCM). Furthermore, during the snail invasion and adaptation to a new environment, stochastic processes are more crucial than deterministic ones in shaping the snail gut microbiota. This indicates that the interplay between stochastic and deterministic processes in the snail gut microbial assembly is associated with host fitness during snail adaptation to a new environment. Based on the null model analysis, we also found that stochastic processes (based on dispersal limitation, homogenizing dispersal, and undominated processes) play a larger role than deterministic (based on homogeneous selection and variable selection) in driving the snail gut bacterial community assembly. Furthermore, the significant difference in the proportions of dispersal limitation and undominated processes is linked to both adaptive and non-adaptive snails.

CONCLUSIONS

This study demonstrates that stochastic processes govern the assembly of the gut microbiota in B. straminea. Furthermore, snail adaptation is associated with the interplay between stochastic and deterministic processes in gut microbial composition. This study provides a better understanding of the dynamic patterns of the gut microbial community in freshwater gastropods and may contribute to the development of strategies for controlling intermediate hosts and schistosomiasis.

Bacterial Diversity Associated with Terrestrial and Aquatic Snails

The introduction of the holobiont concept has triggered scientific interest in depicting the structural and functional diversity of animal microbial symbionts, which has resulted in an unprecedented wealth of such cross-domain biological associations. The steadfast technological progress in nucleic acid-based approaches would cause one to expect that scientific works on the microbial symbionts of animals would be balanced at least for the farmed animals of human interest. For some animals, such as ruminants and a few farmed fish species of financial significance, the scientific wealth of the microbial worlds they host is immense and ever growing. The opposite happens for other animals, such as snails, in both the wild and farmed species. Snails are evolutionary old animals, with complex ecophysiological roles, living in rich microbial habitats such as soil and sediments or water. In order to create a stepping stone for future snail microbiome studies, in this literature review, we combined all the available knowledge to date, as documented in scientific papers, on any microbes associated with healthy and diseased terrestrial and aquatic snail species from natural and farmed populations. We conducted a Boolean search in Scopus, Web of Science, and ScienceDirect until June 2024, identifying 137 papers, of which 60 were used for original data on snail bacterial communities in the gastrointestinal tract, hepatopancreas, and feces. We provide a synthesis on how representative this knowledge is towards depicting the possible snail core microbiota, as well as the steps that need to be taken in the immediate future to increase the in-depth and targeted knowledge of the bacterial component in snail holobionts.