Apparent nosocomial adaptation of Enterococcus faecalis predates the modern hospital era

Enterococcus faecalis is a commensal and nosocomial pathogen, which is also ubiquitous in animals and insects, representing a classical generalist microorganism. Here, we study E. faecalis isolates ranging from the pre-antibiotic era in 1936 up to 2018, covering a large set of host species including wild birds, mammals, healthy humans, and hospitalised patients. We sequence the bacterial genomes using short- and long-read techniques, and identify multiple extant hospital-associated lineages, with last common ancestors dating back as far as the 19th century. We find a population cohesively connected through homologous recombination, a metabolic flexibility despite a small genome size, and a stable large core genome. Our findings indicate that the apparent hospital adaptations found in hospital-associated E. faecalis lineages likely predate the “modern hospital” era, suggesting selection in another niche, and underlining the generalist nature of this nosocomial pathogen. Enterococcus faecalis is a commensal microorganism of animals, insects and humans, but also a nosocomial pathogen. Here, the authors analyse genomic sequences from E. faecalis isolates from animals and humans, and find that the last common ancestors of multiple hospital-associated lineages date to the pre-antibiotic era.