Comparative genomics of global optrA-carrying Enterococcus faecalis uncovers common genetic features and a chromosomal hotspot for optrA acquisition

Linezolid-resistant Enterococcus faecalis (LREfs) carrying optrA are increasingly reported globally from multiple sources, but we still lack a comprehensive analysis of human and animal optrA-LREfs strains. We investigated the phylogenetic structure, genetic content [antimicrobial resistance (AMR), virulence, prophages, plasmidome] and optrA-containing platforms of 28 publicly available optrA-positive E. faecalis genomes from different hosts in 7 countries. In the genome-level analysis, in house databases with 57 virulence and 391 plasmid replication genes were tested for the first time. Our analysis showed a diversity of clones and adaptive gene sequences related to a wide range of genera, mainly but not exclusive from Firmicutes. The content in AMR and virulence genes was highly identical in contrast to the diversity of phages and plasmids observed. Epidemiologically unrelated clones (ST476-like and ST21-like) obtained from human clinical and animal hosts in different continents over 5 years (2012-2017) were phylogenetically related (3-122 SNPs difference). They also exhibited identical AMR and virulence profiles, highlighting a global spread of optrA-positive strains with relevant adaptive traits in livestock and that they might originate from an animal reservoir. optrA was located on the chromosome within a Tn6674-like element (n=9) or on medium-size plasmids (30-60 kb; n=14) belonging to main plasmid families (RepA_N/Inc18/Rep_3). In most cases, the immediate gene vicinity of optrA was identical in chromosomal (Tn6674) and plasmid (impB-fexA-optrA) backbones. Tn6674 was always inserted in the same ∆radC integration site and embedded in a 32 kb chromosomal platform common to diverse strains from different origins (patients, healthy humans, and animals) in Europe, Africa, and Asia during 2012-2018. This platform is conserved among hundreds of E. faecalis genomes and we here propose a conserved chromosomal hotspot for optrA integration. The finding of optrA in strains sharing identical adaptive features and genetic backgrounds across different hosts and countries suggest the occurrence of common and independent genetic events occurring in distant regions, and might explain the easy de novo generation of optrA-positive strains. It also anticipates a dramatic increase of optrA carriage and spread with a serious impact in the efficacy of linezolid for the treatment of Gram-positive infections.