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Objectives To investigate the genomic diversity and β-lactam susceptibilities of Enterococcus faecalis collected from patients with infective endocarditis (IE). Methods We collected 60 contemporary E. faecalis isolates from definite or probable IE cases identified between 2018 and 2021 at the University of Pittsburgh Medical Center. We used whole-genome sequencing to study bacterial genomic diversity and employed antibiotic checkerboard assays and a one-compartment pharmacokinetic–pharmacodynamic (PK/PD) model to investigate bacterial susceptibility to ampicillin and ceftriaxone both alone and in combination. Results Genetically diverse E. faecalis were collected, however, isolates belonging to two STs, ST6 and ST179, were collected from 21/60 (35%) IE patients. All ST6 isolates encoded a previously described mutation upstream of penicillin-binding protein 4 (pbp4) that is associated with pbp4 overexpression. ST6 isolates had higher ceftriaxone MICs and higher fractional inhibitory concentration index values for ampicillin and ceftriaxone (AC) compared to other isolates, suggesting diminished in vitro AC synergy against this lineage. Introduction of the pbp4 upstream mutation found among ST6 isolates caused increased ceftriaxone resistance in a laboratory E. faecalis isolate. PK/PD testing showed that a representative ST6 isolate exhibited attenuated efficacy of AC combination therapy at humanized antibiotic exposures. Conclusions We find evidence for diminished in vitro AC activity among a subset of E. faecalis IE isolates with increased pbp4 expression. These findings suggest that alternate antibiotic combinations against diverse contemporary E. faecalis IE isolates should be evaluated. [ABSTRACT FROM AUTHOR]

Vancomycin-resistant enterococcal (VRE) bacteremia is associated with higher mortality rates and longer hospitalizations than vancomycin-sensitive enterococcal (VSE) bacteremia. A 67-year-old man with a right psoas abscess and pacemaker-associated tricuspid valve endocarditis in September 2020 grew VSE Enterococcus faecium from blood cultures that cleared after administration of intravenous vancomycin and gentamicin. Subsequently, he underwent tricuspid valve repair, pacemaker removal, and partial lead extraction. Valve and postoperative blood cultures grew VRE E. faecium, which cleared after administration of intravenous daptomycin. One VSE and two VRE isolates were collected and sequenced. All isolates belonged to E. faecium multilocus sequence type ST17 and were closely related, having <20 mutations in pairwise genome comparisons. Vancomycin resistance was due to the acquisition of a plasmid-encoded VanA operon. None of the isolates encoded the virulence factors asa1, gelE, cylA, or hyl; all encoded a homologue of efaAfm. VSE E. faecium, but not VRE E. faecium isolates, encoded a glucose transporter gene mutation. Two VRE E. faecium isolates formed more robust biofilms than the VSE E. faecium isolate (p < 0.001). The VRE E. faecium isolates, which generated larger biofilms than the VSE E. faecium isolate, could have remained protected in the heart valve and only caused bacteremia when disrupted during cardiac surgery. This study demonstrates that bacteria detected in the bloodstream of patients with endocarditis may not fully represent the organisms adherent to the cardiac valves or indwelling devices. [ABSTRACT FROM AUTHOR]