Differential ampicillin/ceftriaxone susceptibility among diverse Enterococcus faecalis from infective endocarditis

Enterococcus faecalis is a leading cause of infective endocarditis (IE), especially among older patients with comorbidities. Here we investigated the genomic diversity and antimicrobial susceptibility of 33 contemporary E. faecalis isolates from definite or probable IE cases at the University of Pittsburgh Medical Center (UPMC) between 2018 and 2020. Isolates belonging to two multi-locus sequence types (STs), ST6 and ST179, were isolated from nearly 40% of IE patients. Both of these dominant STs carried known beta-lactam resistance-associated mutations affecting the low-affinity penicillin-binding protein 4 (PBP4). We assessed the ability of ampicillin and ceftriaxone (AC) both alone and in combination to inhibit genetically diverse E. faecalis IE isolates in checkerboard synergy assays and an in vitro one-compartment pharmacokinetic-pharmacodynamic (PK-PD) model of AC treatment. ST6 isolates as well as an isolate with a mutation in the PP2C-type protein phosphatase IreP had higher ceftriaxone MICs compared to other isolates, and showed diminished in vitro synergy of AC. Additionally, both ST6 and ST179 isolates exhibited regrowth after 48 hours of humanized exposures to AC. Overall, we found evidence for diminished in vitro AC activity among E. faecalis IE isolates with PBP4 and IreP mutations. This study highlights the need to evaluate alternate antibiotic combinations in clinical practice against diverse contemporary E. faecalis IE isolates.