2181. Avibactam Prevents the in vitro Development of Imipenem Resistance in non-Carbapenemase-Producing Klebsiella pneumoniae

BACKGROUND: Carbapenem resistance (CR) in Klebsiella pneumoniae is most frequently mediated by the production of carbapenemases. However, non-carbapenemase-producing K. pneumoniae (non-CP-Kpn) have been well documented. Overexpression of narrow β-lactamases can play an important role in the development of CR in the absence of carbapenemases. Here, we investigated the role of avibactam, a potent β-lactamase inhibitor, in preventing the in vitro development of CR in a non-CP-Kpn. METHODS: We used SCL-1, a non-CP-Kpn strain recovered from the bloodstream of a patient that developed CR in vivo during treatment with imipenem (IMI). SCL-1 was exposed to serial passages of increasing concentrations of IMI alone [0.25-16 μg/mL] or in combination with avibactam (IMI-AVI) during 8 days. In the case of IMI-AVI, the concentration of AVI was fixed at 4 μg/mL. Evolution assays were performed in triplicate and considered 3 independent evolutionary lines (ELs). MICs at each time-point were measured to IMI and IMI-AVI by broth microdilution as per CLSI. Growth curves of SCL-1 and evolved strains were performed to assess fitness. The expression levels of narrow spectrum β-lactamases bla(OXA-1),bla(OXA-10), and of the ESBL bla(CTX-M-15) were measured by RT-qPCR. RESULTS: In vitro exposure to IMI led to CR, with the MICs increasing from 0.5 μg/mL to 8 μg/mL in all 3 ELs. In contrast, all ELs exposed to IMI-AVI remained fully susceptible to IMI after 8 days, with a final MIC of 1 μg/mL. In addition, the MIC to IMI of the evolved, IMI-resistant strains, in the presence of AVI (4 μg/mL) dropped from 8 to 1 μg/mL. Growth curves revealed that IMI-resistant strains evolved in IMI alone exhibited a growth defect as compared to both SCL-1 and the susceptible strains evolved in IMI-AVI. Finally, compared to the SCL-1, the expression level of bla(OXA-1) was significantly increased (p< 0.05) in all evolved isolates that developed IMI resistance (MIC=8 μg/mL). No significant changes were observed in the expression of bla(OXA-10) and bla(CTX-M-15) (Figure 1). [Figure: see text] CONCLUSION: Our findings suggest that the addition of AVI can prevent the in vitro development of IMI resistance in non-CP-Kpn. Our results have important implications for the clinical use of AVI as a strategy to combat CR in non-CP-Kpn infections. DISCLOSURES: All Authors: No reported disclosures