Microbial colonization during early phases of amphibian life

Background: In the last decade, there has been a growing interest in the universality of vertical transmission of microbial community. While in invertebrates such transmission has been proven in several species, a debate is still ongoing in vertebrate animals. Hence, we sought to study this matter in a well-established developmental animal model: Xenopus laevis. Materials and methods: X. laevis larvae were grown in sterile and non-sterile FETAX (Amphibian embryo developmental test) to the desired stage. Maternal stools and larvae were collected and stored until use. We analyzed stools by Next Generation Sequencing techniques (NGS) and larvae, collected under sterile conditions, by fluorescent in situ hybridization (FISH) using labeled probes targeting 16S rRNA. Results: As reported for humans, Bacteroidetes and Firmicutes are the predominant phyla in the X. laevis gut (relative abundance of 46.5% and 24.2%, respectively), followed by Fusobacteria (12.8%) and Proteobacteria (11.6%). Coherently, at family level, Bacteroidaceae accounts for 33% of all the microbial population, overcoming any Gram-positive taxon belonging to Firmicutes. FISH studies, using Eubacteria probes, confirmed microbes’ presence in the lumen of the developing amphibian gut with a clear difference in abundance depending on the growing conditions. Conclusions: This study reports, for the first time to our knowledge, the microbial composition of maternal and larval Xenopus laevis.