C HIRZEL¹, L HIRZBERGER¹, H FURRER¹, A ENDIMIANI<sup>2

1</sup>Department of Infectious Diseases, Bern University Hospital, University of Bern, Bern, Switzerland, ²Institute for Infectious Diseases, University of Bern, Bern, Switzerland

Introduction: A. urinae may cause severe infections (bacteremia and endocarditis) associated with high mortality. However, data on bactericidal and synergistic activity for clinically implemented antibiotics is scarce.

Methods: We performed time-kill (TK) analyses on 2 clinical isolates (AU1 and AU2) and ATCC700306 for penicillin (PG), ceftriaxone (CRO), gentamicin (GEN), daptomycin (DAP) and their combinations. TK experiments were performed in TH broth (supplemented with 50 µg/ml calcium for experiments with DAP). All experiments were performed at least 3 times at antibiotic concentrations of 0.5x, 1x, 2x, and 4x the MICs. Using the EUCAST criteria for viridans streptococci AU1 and AU2 were CRO-resistant (MICs, 2 µg/ml). ATCC was GEN high-level resistant (MIC, 512 µg/ml), whereas all strains were PG- and DAP-susceptible (MICs <0.125 and ≤1 µg/ml, respectively). CFU/ml count was determined at 0, 2, 4, 6, 8, 24, 30, and 48 hrs. Bactericidal activity was defined as a ≥3-log₁₀ decrease in CFU/ml compared to the initial inoculum at 8 and 24 hrs. Synergism was defined as a ≥2-log₁₀ decrease in CFU/ml at 8 and 24 hrs comparing the antimicrobial combination to the most active single antimicrobial drug.

Results: PG or CRO alone were not bactericidal for all strains, whereas DAP exhibited bactericidal activity at all MICs for AU2 and ATCC. The combination of PG or CRO with GEN was bactericidal for AU1 and AU2 at concentrations ≥1x MICs. Bactericidal synergism was detected for PG or CRO combined with GEN in the 2 clinical isolates. PG plus CRO showed non-bactericidal synergism for ATCC. DAP with GEN was synergistic at 1x MICs for AU1, whereas the killing activity of DAP was too pronounced to detect potential synergism in AU2.

Discussion: We demonstrated that the combination of PG or CRO with GEN is synergistic and bactericidal. Moreover, our data suggests that DAP may represent a potential bactericidal treatment alternative against A. urinae. This finding could be important for the treatment of patients with a β-lactam allergy or renal insufficiency.

This work was supported by grant 84800508 2014-15 from Bern University Hospital and University of Bern.