636. Genome Epidemiology of Carbapenem-Resistant Acinetobacter baumannii (CRAb) in the United States

BACKGROUND: CRAb is a major cause of healthcare-associated infections and is associated with high mortality due to the lack of reliable treatment options. We aimed to elucidate the contemporary population structure of CRAb isolates circulating in US hospitals using whole-genome sequencing (WGS). METHODS: A total of 131 CRAb isolates were identified at four tertiary care medical centers located in Ohio, Pennsylvania, Texas and North Carolina between 2017 and 2018. The genomes were sequenced with Illumina NextSeq and de novo assembled. Sequence types (STs) were identified using the Pasteur Institute MLST scheme. β-Lactamase genes were identified by ResFinder and manually curated. RESULTS: The 131 isolates belonged to 10 different ST types, including 8 known and 2 novel ones. In this collection, 101 isolates (77.1%) belonged to ST2, the dominant drug-resistant clone in the United States and Europe; 20 isolates belonged to ST499, a less common, but also globally distributed clone. Two isolates each belonged to ST46 and ST79, both common in South America. For the chromosomally encoded bla(OXA-51)-group genes, 11 variants were identified with bla(OXA-66), bla(OXA-82), and bla(OXA-95) being predominant. For the chromosomally encoded bla(ADC)-group genes, 26 variants were identified, with bla(ADC-161), bla(ADC-181), and bla(ADC-30) being the most common. The most frequent acquired carbapenemase gene was bla(OXA-23), which was present in 89 isolates (67.9%). Other acquired bla(OXA) carbapenemase genes were identified much less frequently and included bla(OXA-24), bla(OXA-72), bla(OXA-207), and bla(OXA-237). 17 isolates (13.0%) did not contain any known acquired carbapenemase genes despite resistance to carbapenems. CONCLUSION: ST2 is the most prevalent ST type among contemporary CRAb isolates identified in US hospitals, however, new STs are emerging, most notably ST499. Significant diversity was seen among chromosomal bla(OXA-51)-group carbapenemase, intrinsic bla(ADC)-group cephalosporinase and plasmid-mediated bla(OXA)-group carbapenemase genes, which likely represented diversification within the STs. Correlations between clinical presentation and outcomes and the genomic features of the infecting isolates are being investigated DISCLOSURES: All authors: No reported disclosures.