Molecular Epidemiology of Carbapenem-Resistant Acinetobacter baumannii in the United States, 2013-2017

Abstract

Healthcare-associated carbapenem-resistant Acinetobacter baumannii (CRAB) infections are a serious threat associated with global epidemic clones and a variety of carbapenemase gene classes. In this study, we describe the molecular epidemiology, including whole-genome sequencing analysis and antimicrobial susceptibility profiles of 92 selected, nonredundant CRAB collected through public health efforts in the United States from 2013 to 2017. Among the 92 isolates, the Oxford (OX) multilocus sequence typing scheme identified 30 sequence types (STs); the majority of isolates (n = 59, 64%) represented STs belonging to the international clonal complex 92 (CC92_OX). Among these, ST208_OX (n = 21) and ST281_OX (n = 20) were the most common. All isolates carried an OXA-type carbapenemase gene, comprising 20 alleles. Ninety isolates (98%) encoded an intrinsic OXA-51-like enzyme; 67 (73%) harbored an additional acquired bla_OXA gene, most commonly bla_OXA-23 (n = 45; 49%). Compared with isolates harboring only intrinsic oxacillinase genes, acquired bla_OXA gene presence was associated with higher prevalence of resistance and a higher median minimum inhibitory concentration to the carbapenem imipenem (64 μg/mL vs. 8 μg/mL), and antibiotics from other drug classes, including penicillin, aminoglycosides, cephalosporins, and polymyxins. These data illustrate the wide distribution of CC92_OX and high prevalence of acquired bla_OXA carbapenemase genes among CRAB in the United States.

Keywords: Acinetobacter baumannii; MLST; WGS; carbapenem resistance; healthcare-associated infections; molecular epidemiology.

FIG. 1.

Oxford MLST for CRAB isolates by geographic region. ST belonging to CC92_OX are in blue. Non-CC92_OX STs in gray. STs reported as the proportion of total isolates from that region. Total number of isolates for each region indicated in parentheses. “Other” category comprises STs with ≤3 total isolates and are listed, by CC, to the right of the graph. CC92, clonal complex 92; CRAB, carbapenem-resistant Acinetobacter baumannii; MLST, multilocus sequence typing; ST, sequence type.

FIG. 2.

Phylogenetic classification of 92 CRAB isolates based on whole-genome MLST reveals genetic diversity within CC92_OX. Clonal complexes defined by BURST are indicated by colored circles. Numbers indicate Oxford MLST designations. Geographic region, acquired OXA, and drug resistance patterns are indicated by colored blocks. MDR, multidrug-resistant; PDR, pan drug-resistant; XDR, extensively drug-resistant.