Clinical and Genomic Epidemiology of -Carrying Carbapenem-Resistant Isolates in Metropolitan Atlanta, 2012 to 2017.

Colistin is a last-resort antibiotic for multidrug-resistant Gram-negative infections. Recently, the ninth allele of the mobile colistin resistance () gene family, designated was reported. However, its clinical and public health significance remains unclear. We queried genomes of carbapenem-resistant (CRE) for from a convenience sample of clinical isolates collected between 2012 and 2017 through the Georgia Emerging Infections Program, a population- and laboratory-based surveillance program. Isolates underwent phenotypic characterization and whole-genome sequencing. Phenotypic characteristics, genomic features, and clinical outcomes of -positive and -negative CRE cases were then compared. Among 235 sequenced CRE genomes, 13 (6%) were found to harbor , all of which were Enterobacter cloacae complex. The median MIC and rates of heteroresistance and inducible resistance to colistin were similar between -positive and -negative isolates. However, rates of resistance were higher among -positive isolates across most antibiotic classes. All cases had significant health care exposures. The 90-day mortality was similarly high in both -positive (31%) and -negative (7%) CRE cases. Nucleotide identity and phylogenetic analysis did not reveal geotemporal clustering. -positive isolates had a significantly higher number of median [range] antimicrobial resistance (AMR) genes (16 [4 to 22] versus 6 [2 to 15]; < 0.001) than did -negative isolates. Pangenome tests confirmed a significant association of detection with mobile genetic element and heavy metal resistance genes. Overall, the presence of was not associated with significant changes in colistin resistance or clinical outcomes, but continued genomic surveillance to monitor for emergence of AMR genes is warranted. Colistin is a last-resort antibiotic for multidrug-resistant Gram-negative infections. A recently described allele of the mobile colistin resistance () gene family, designated has been widely reported among species. However, its clinical and public health significance remains unclear. We compared characteristics and outcomes of -positive and -negative CRE cases. All cases were acquired in the health care setting and associated with a high rate of mortality. The presence of was not associated with significant changes in colistin resistance, heteroresistance, or inducible resistance but was associated with resistance to other antimicrobials and antimicrobial resistance (AMR), virulence, and heavy metal resistance (HMR) genes. Overall, the presence of was not associated with significant phenotypic changes or clinical outcomes. However, given the increase in AMR and HMR gene content and potential clinical impact, continued genomic surveillance of multidrug-resistant organisms to monitor for emergence of AMR genes is warranted.