Impacts of a novel defensive symbiosis on the nematode host microbiome

Abstract Background: Bacteria adapted to live within animals can protect their hosts against harmful infections. Beyond antagonism with pathogens, a ‘defensive’ bacterial symbiont could engage in additional interactions with other colonizing micro-organisms. A single bacterium might thus have cascading ecological impacts on the whole microbiome that are rarely investigated. Here, we assess the role of a defensive symbiont as a driver of host-associated microbiota composition by using a bacterial species ( Enterococcus faecalis ) that was previously experimentally adapted ( Enterococcus faecalis ) to a nematode host model ( Caenorhabditis elegans ). Results: An analysis of 16S rRNA data from C. elegans exposed to E. faecalis and subsequently reared in soil, reveal that symbiont adaptation to host environment or its protective potential had minimal impact on microbiota diversity. Whilst the abundance of Pseudomonas was higher in the microbiota of hosts with protective E.faecalis (and another protective species tested), three other genera – Serratia, Klebsiella and Salinispora – were less abundant in hosts colonized by all E. faecalis strains. In addition, the protective effect of E. faecalis against opportunistic Staphylococcus aureus pathogens was maintained despite multi-species interactions within the microbiota. Conclusions: Our results reveal the degree to which a new, evolving symbiont can colonise and maintain its conferred phenotype (i.e., pathogen-resistance) with minimal disruption to the host microbiota diversity.