612. Molecular Mechanisms Leading to Ceftolozane–Tazobactam Resistance in Clinical Isolates of Pseudomonas aeruginosa from Five Latin American Countries

BACKGROUND: Ceftolozane–tazobactam (C/T) is a combination of an antipseudomonal cephalosporin with a known β-lactamase inhibitor, with a potent in vitro activity against P. aeruginosa (Pae), without activity against carbapenemases. Among the mechanisms of resistance to C/T that have emerged, substitutions in the Pseudomonal-derived cephalosporinase (PDC), in AmpR, and in some ESBLs, are the most commonly described. The aim of this study was to identify the molecular mechanisms responsible for the in vitro non-susceptibility (NS) to C/T in a group of clinical Pae strains from Latin America. METHODS: Clinical Pae isolates (n = 508) were collected between January 2016 and October 2017 from 20 hospitals located in Argentina, Brazil, Chile, Colombia, and Mexico. Minimum inhibitory concentrations (MICs) to C/T were determined by standard broth microdilution and interpreted according to CLSI M100 S28 breakpoints. Production of carbapenemases in Pae isolates displaying NS to C/T was assessed by carbaNP® followed by PCR to detect blaKPC, blaNDM-1, blaVIM and blaIMP. Illumina whole-genome sequencing (WGS) was performed for isolates in which NS to C/T was not mediated by carbapenemases. The presence of mutations in PDC, AmpR, oprD and dacB as compared with PAO1, was evaluated. RESULTS: According to the CLSI breakpoints, 162/508 (32%) Pae isolates demonstrated NS to C/T. Due to the absence of growth, only 151/162 were further processed. Table 1 summarizes the results obtained by carbaNP®, PCR and WGS performed on these isolates. In 53% of the isolates, NS to C/T was explained by the production of at least one carbapenemase, KPC or VIM. WGS revealed that in addition to substitutions in PDC and AmpR, some isolates carried mutations in oprD and dacB (encoding PBP4) genes. The molecular mechanism of resistance in 4/56 isolates is yet to be determined. CONCLUSION: Carbapenemase production is the most common mechanism of resistance to C/T detected in this study. VIM and KPC were detected in equal proportions, while none of the isolates was found to carry IMP or NDM. Further studies are warranted to establish the role of the novel substitutions found in PDC and AmpR, as well as the degree to which the mutations found in oprD and dacB contribute to the NS phenotype in some isolates. [Image: see text] DISCLOSURES: All authors: No reported disclosures.