1136. Sub Inhibitory Concentrations of Ceftriaxone (CRO) Revert Daptomycin Resistance (DAP-R) in Enterococcus faecalis
Session: Oral Abstract Session: New Considerations for Improved Treatments of Community Infections
Friday, October 4, 2013: 3:00 PM
Room: The Moscone Center: 220-226
Background: Ampicillin (AMP) plus daptomycin (DAP) have been shown to be synergistic in vitro against an AMP and DAP-R E. faecium. Indeed, the addition of AMP to DAP was successfully used to treat a patient with persistent bacteremia due to an AMP-R E. faecium. However, the mechanisms of the synergism between DAP and β-lactams are unknown. To gain insights into these mechanisms, we sought to evaluate the effect of the association of CRO plus DAP on MICs and growth kinetics against a DAP-S and DAP-R E. faecalis clinical strain pair. We hypothesized that development of DAP-R in E. faecalis results in cell envelope changes that increase the activity of CRO.

Methods: E. faecalis isolates in this study include, i) S613, a DAP-S (MIC 1 mg/L) bloodstream clinical isolate recovered from a patient, and ii) R712, a DAP-R derivative of S613 (MIC 8 mg/L) recovered from the same patient after two weeks of DAP therapy. DAP Etest was performed on Mueller-Hinton (MH) agar in the presence and absence of CRO (8 mg/L, 1/32 X MIC). Growth curves were performed in triplicate with an initial bacterial inoculum of 107 CFU/mL in MH broth using CRO concentrations of 64 mg/L (1/4X MIC) and 16 mg/L (1/16X MIC). Optical density (A620) was measured at 0, 2, 4, 6, 8 and 24 h.

Results: MICs of S613 and R712 to CRO were 256 mg/L. DAP MIC of R712 was markedly decreased (6 fold) in the presence of CRO (from 8 to 1.5 mg/L [breakpoint 4 mg/L]). No change in DAP MIC was observed in S613 in the presence of CRO. Addition of CRO at concentrations of 64 mg/L and 16 mg/L did not affect the in vitro growth of S613. Conversely, growth of R712 was significantly impaired in the presence of CRO 64 mg/L (1/4X MIC, [p<0.05]). The growth defect was observed at CRO concentrations as low as 16 mg/L, with a final A620 at 24h that was more than 3 fold lower than R712 grown in the absence of CRO.

Conclusion: Sub inhibitory concentrations of CRO were sufficient to revert DAP resistance in E. faecalis. Our findings indicate that the pathway leading to DAP resistance in E. faecalis results in impaired in vitro growth in the presence of CRO without evident changes in MIC. This suggest that CRO mediated growth impairment of E. faecalis is responsible for the synergistic activity with DAP observed in vitro.

Jose Munita, MD1,2, Jinnethe Reyes, BSc2,3, Truc Tran2, Diana Panesso, MSc2,3 and Cesar Arias, MD, PhD2,3, (1)Clinica Alemana - Universidad Del Desarrollo School of Medicine, Santiago, Chile, (2)University of Texas Medical School At Houston, Houston, TX, (3)Bacterial Molecular Genetics Unit, Universidad El Bosque, Bogota, Colombia


J. Munita, None

J. Reyes, None

T. Tran, None

D. Panesso, None

C. Arias, Pfizer Inc.: Consultant and Speaker's Bureau, Consulting fee, Research support and Speaker honorarium
Cubist: Consultant and Speaker's Bureau, Consulting fee, Research support and Speaker honorarium
Forest Pharmaceuticals: Investigator, Research support
Novartis: Speaker's Bureau, Speaker honorarium

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