Small and equipped: the rich repertoire of antibiotic resistance genes in Candidate Phyla Radiation genomes

Microbes belonging to Candidate Phyla Radiation (CPR) have joined the tree of life as a new unique branch, thanks to the intensive application of metagenomics and advances of sequencing technologies. Despite their ultra-small size, reduced genome and metabolic pathways which mainly depend on symbiotic/exo-parasitic relationship with their bacterial host, CPR microbes are abundant and ubiquitous in almost all environments and are consequently survivors in highly competitive circumstances within microbial communities. They have been eventually identified by 16S rRNA analysis and represent more than 26% of microbial diversity. CPR microbes were able to survive in this context, although their defence mechanisms and phenotypic characteristic remain, however, poorly explored. Here, we conducted a thorough in-silico analysis on 4,062 CPR genomes to test whether these ultrasmall microorganisms might encode for antibiotic resistance (AR)-like enzymes. We used an adapted AR screening criteria with an exhaustive consensus database and complementary steps conferring their resistance functions. We conclude by reporting the surprising discovery of rich reservoir of divergent AR-like genes (n= 30,545 HITs, mean=7.5 HITs/genome [0-41] encoding for 89 AR enzymes, distributed across the 13 CPR phyla, and associated with 14 different chemical classes of antimicrobials. However, most HITs found (93.6%) were linked to glycopeptide, beta-lactams, macrolide-lincosamide-streptogramin, tetracycline and aminoglycoside resistance. Moreover, a distinct AR profile was discerned between the microgenomates group and Candidatus Parcubacteria, and between each of them and other CPR phyla. CPR cells seem to be active players during microbial competitive interactions and are well-equipped for the microbial combat in different habitats, supporting their natural survival/persistence and continued existence.