1230. Evaluation of the Interplay Between β-lactamases and Porin Production on β-Lactam MICs in Klebsiella pneumoniae

BACKGROUND: K. pneumoniae can emerge resistant to β-lactam antibiotics through the production of β-lactamase enzymes and/or loss of the outer membrane porins, OmpK35, OmpK36, and/or PhoE. While both mechanisms are hypothesized to work synergistically, β-lactamases have been the focus of previous studies. As a result, the contribution of outer membrane porin loss to the β-lactam minimum inhibitory concentration (MIC) is unknown. The objective of this study was to evaluate the contribution of specific β-lactamases and porin production to β-lactam susceptibility. We hypothesize that production of a β-lactamase in a clinical isolate deficient in 3 major porins will result in higher β-lactam MICs but not always a resistant phenotype. METHODS: The structural gene and promoter of CTX-M-14, CTX-M-15, and CMY-2 were cloned into a low copy number vector and transformed into Kp 23, a wild-type clinical isolate, and KPM 20, a clinical isolate deficient in OmpK35/36 and PhoE. MICs to ceftolozane/tazobactam, cefotaxime, ceftazidime, cefepime, and meropenem were determined by E-test. Kp 23 and KPM 20 were characterized by Western blot and whole genome sequencing. RESULTS: Production of CMY-2 alone led to a resistant phenotype for ceftolozane/tazobactam, cefotaxime, and ceftazidime regardless of porin production (Figure 1). CMY-2 production in KPM 20 resulted in non-susceptibility to meropenem. Both clones were susceptible to cefepime. Production of CTX-M-14 and CTX-M-15 in Kp 23 resulted in only cefotaxime resistance. Production of CTX-M-14 and CTX-M-15 in KPM 20 resulted in isolates non-susceptible to all antibiotics tested. [Image: see text] Figure 1. MICs of K. pneumoniae clones against panel of β-lactam antibiotics. CONCLUSION: When evaluating clinical isolates, it is impossible to determine the contribution of individual resistance mechanisms in the susceptibility pattern. This study demonstrated that resistance is not solely dependent on the β-lactamase produced and that the impact of porin deficiency varies with the antibiotic being evaluated. These data suggest that antibiotic selection may be more nuanced and that a broader range of therapeutics may be available given the appropriate diagnostic tools. Understanding the contributions of all resistance mechanisms is necessary to inform selection of the most appropriate antibiotic therapy. DISCLOSURES: Nancy D. Hanson, PhD, Merck (Grant/Research Support)