The Hidden Microbial World in the Gut of the Terrestrial Snail Cornu aspersum maxima and the Unexpected Negative Effects of Synbiotics.

The intestinal microbiome plays a pivotal role in the physiology and health of terrestrial gastropods yet remains largely unexplored. This study aimed to characterize the gut microbial communities of the farmed snail Cornu aspersum maxima and to assess the effects of dietary supplementation with the probiotic Lactobacillus plantarum, the prebiotic inulin, and their combination (synbiotic) on microbial diversity, snail growth, and survival. In total, 300 sexually immature snails (~9.8 g average body mass, ~5 months old) were assigned to four dietary groups (Control, Probiotic, Prebiotic, Synbiotic), each comprising three replicates of 25 snails. Individuals of similar size were placed in each container within the treatment groups. The Probiotic group received 1.25 mg of L. plantarum (1010 CFU/g) per 5 g of feed, the Prebiotic group received 1 g of inulin per 5 g of feed, and the Synbiotic group received both supplements at the same dosages. Over the 60-day trial, the gut microbiota was analyzed via 16S rRNA amplicon sequencing using Oxford Nanopore technology. The results revealed significant treatment-dependent shifts (p < 0.05) in microbial composition at both the phylum and genus levels. The dominant bacterial phyla identified were Proteobacteria and Actinobacteria, while a notable increase in unclassified microbial taxa was observed, especially in the inulin-supplemented groups. Despite its known probiotic properties, L. plantarum was not detected post-administration, suggesting a transient effect. The synbiotic group exhibited significantly higher microbial diversity (Shannon index, p < 0.05) but also the highest mortality rate. All groups showed limited weight gain, with reductions observed after day 30. Potentially pathogenic genera (e.g., Klebsiella, Mycoplasma, Staphylococcus) were detected but showed reduced abundance in the probiotic- and prebiotic-treated groups, suggesting a protective effect. Overall, probiotic supplementation with L. plantarum enhanced the abundance of beneficial Actinobacteria and reduced potentially pathogenic taxa, while the prebiotic inulin promoted the growth of unclassified but potentially beneficial genera. However, synbiotic administration, despite increasing microbial diversity, was associated with adverse outcomes including 100% mortality at day 60. These findings highlight both the potential and the risks of dietary manipulation of the snail microbiome, underscoring the need for cautious application of synbiotics in snail farming. They also underscore the dynamic nature of the snail gut microbiome and its responsiveness to dietary interventions, providing valuable insights for sustainable snail farming and future probiotic applications in invertebrate species.