FIGURE 4.

Core genome SNP-based phylogeny and diverse flanking genetic environments of 28 optrA-positive Enterococcus faecium isolates. A maximum-likelihood phylogenetic tree based on core genome single-nucleotide polymorphisms (SNPs) was constructed for 28 representative optrA-positive Enterococcus faecium isolates. These isolates were selected from a larger collection based on the presence of complete optrA-flanking sequences and distinct genetic structures, after excluding genetically closely related isolates with identical optrA-associated contexts. The phylogenetic tree is shown on the left, with corresponding metadata including isolation source, country, host, year of isolation, and strain identification. The genetic environments surrounding the optrA gene are displayed on the right, with arrows representing predicted open reading frames colored according to gene function. Despite the pronounced phylogenetic diversity of the host strains, the optrA gene is embedded within multiple distinct flanking genetic structures, many of which are associated with mobile genetic elements such as insertion sequences and transposase-related genes, highlighting the role of horizontal gene transfer in the dissemination of optrA.