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Background Ceftazidime–avibactam (CZA) is a novel β-lactam / β-lactamase inhibitor with in vitro activity against multidrug-resistant Gram-negatives, including those harboring CMY-2 enzymes. Treatment-emergent resistance to CZA has been described in KPC-producing Klebsiella pneumoniae but has not been described in non-carbapenemase-producing, carbapenem-resistant Enterobacteriaceae (CRE). Methods A patient with an intra-abdominal infection due to a carbapenem-resistant E. coli (ertapenem MIC 16 µg/mL; meropenem MIC 2 µg/mL; CZA MIC 2 µg/mL; carbapenemase negative) was treated with CZA. On day 48 of therapy, a CZA resistant, carbapenem-sensitive E. coli was identified from abdominal drainage (CZA MIC ≥256 µg/mL; meropenem MIC 0.19 µg/mL). Illumina MiSeq whole-genome sequencing (WGS) was performed on both isolates to identify potential resistance mechanisms. The ResFinder database was used to identify known β-lactamase enzymes, and in silico modeling of β-lactamase structure was assessed. Results WGS revealed that both isolates were ST410 E. coli, with the sole difference in β-lactam resistance determinants between the two being a novel CMY β-lactamase harbored on an Inc1-type conjugative plasmid in the second isolate. The novel CMY has 4 amino acid substitutions relative to CMY-2: A134E, Q140K, V231S, and N366Y. The V231S substitution is found in CMY-42 and has previously been associated with increased ceftazidime hydrolysis. The remaining three substitutions have not previously been identified. Previous studies have identified that substitutions at position 366 influence the rate of ceftazidime hydrolysis rate. Preliminary protein structure analysis suggests that positions 140 and 366 are in the active site. No other differences in β-lactam resistance determinants were identified between the first and second isolates. Conclusion To our knowledge, we have identified the first case of CMY-associated CZA resistance. Given the widespread and transferrable nature of CMY enzymes, this finding raises concern for additional cases of resistance with increasing usage of CZA. Further analysis is needed to identify the mechanism by which this enzyme confers CZA resistance. Disclosures Samuel L. Aitken, PharmD, Melinta Therapeutics: Grant/Research Support, Research Grant; Merck, Sharpe, and Dohme: Advisory Board; Shionogi: Advisory Board.

605. Identification of a Novel CMY-Variant Enzyme in a Clinical Escherichia coli Strain with Treatment-Emergent Ceftazidime–Avibactam Resistance