In vitro activity of ceftobiprole plus ampicillin against Enterococcus faecalis causing infective endocarditis: phenotypic and genotypic characterization

Abstract

Objectives: To study the in vitro activity of ceftobiprole alone and in combination with ampicillin against Enterococcus faecalis causing infective endocarditis (EFIE).

Methods: A total of 44 consecutive EFIE collected at Hospital Universitari de Bellvitge (2011-21) were studied. The activity of ampicillin/ceftobiprole was compared with that of ampicillin/ceftriaxone through four agar diffusion methods [double disc synergy test (DDST), crossing gradient diffusion strip (GDS), MIC:MIC proportion and overlapping GDS], chequerboard and time-kill assays. All available strains were studied through WGS.

Results: All EFIE isolates were susceptible to ampicillin and 36 had ceftobiprole MICs of ≤1 mg/L. Increased ceftobiprole MIC (n = 8, range 6-32 mg/L) was associated with clonal complex 2 and a deletion in the promoter region of pbp4 (-82delA). Ampicillin/ceftobiprole and ampicillin/ceftriaxone demonstrated synergism in 36 strains through DDST and crossing GDS methods. The MIC:MIC proportion and overlapping GDS methods yielded a lower proportion of synergism (35 and 4 for ampicillin/ceftriaxone; and 30 and 3 for ampicillin/ceftobiprole, respectively). For strains with high ceftobiprole MICs, no synergy was observed in any combination. FIC index (FICI) of strains with mutations in pbp4 were similar to those of WT strains. FICI of strains with -82delA were significantly higher.

Conclusion: Ampicillin/ceftobiprole showed comparable in vitro activity to ampicillin/ceftriaxone against EFIE. Mutations in the promoter region of pbp4 could compromise the activity of the double β-lactam therapy. The study of ceftobiprole MIC together with DDST is an easy and useful methodology to detect isolates harbouring these mutations, with no synergism to β-lactam combinations.