Abundance and environmental host range of the SXT/R391 ICEs in aquatic environmental communities

Mobile genetic elements (MGEs) such as plasmids or integrative conjugative elements (ICEs) are widely involved in the horizontal transfer of antibiotic resistant genes (ARGs), but their environmental host-range and reservoirs remain poorly known, as mainly assessed through the analysis of culturable and clinical bacterial isolates. In this study, we used a gradual approach for determining the environmental abundance and host-range of ICEs belonging to the SXT/R391 family, otherwise well known to bring ARGs in Vibrio spp. epidemic clones and other pathogens. First, by screening a set of aquatic bacteria libraries covering 1794 strains, we found that almost 1% of the isolates hosted an SXT/R391 element, all belonging to a narrow group of non-O1/non-O139 Vibrio cholerae. However, when SXT/R391 ICEs were then quantified in various aquatic communities, they appeared to be ubiquitous and relatively abundant, from 10^-6 to 10^-3 ICE copies per 16 S rDNA. Finally, the molecular exploration of the SXT/R391 host-range in two river ecosystems impacted by anthropogenic activities, using the single-cell genomic approach epicPCR, revealed several new SXT/R391 hosts mostly in the Proteobacteria phylum. Some, such as the pathogen Arcobacter cryaerophilus (Campylobacteraceae), have only been encountered in discharged treated wastewaters and downstream river waters, thus revealing a likely anthropogenic origin. Others, such as the non-pathogenic bacterium Neptunomonas acidivorans (Oceanospirillaceae), were solely identified in rivers waters upstream and downstream the treated wastewaters discharge points and may intrinsically belong to the SXT/R391 environmental reservoir. This work points out that not only the ICEs of the SXT/R391 family are more abundant in the environment than anticipated, but also that a variety of unsuspected hosts may well represent a missing link in the environmental dissemination of MGEs from and to bacteria of anthropogenic origin.