154. comparative Genomics Reveals Extra-hospital Transmission Networks of Carbapenem-resistant acinetobacter Baumannii sustained over Multiple Years in a US Midwest City

Abstract Background The transmission dynamics of Acinetobacter baumannii (Ab) outside the setting of hospital outbreaks is underinvestigated. The BJC Healthcare System in St. Louis, MO has not experienced an Ab hospital outbreak since 2012. Despite this, nearly 60% of all BJC Ab isolates are carbapenem-resistant Ab (CrAb). Methods We acquired whole genome sequences (WGSs) of 110 Ab isolates identified in five BJC hospitals from July 2017 to May 2019. We performed multilocus sequence typing, core genome alignment and pairwise average nucleotide identity analysis to compare WGSs from BJC isolates and GenBank-available WGSs of Ab isolates from other US hospitals. Further epidemiologic characterization was performed using BJC electronic medical records and detailed chart review. Results Though the majority of CrAb isolates in other US studies belonged to globally-prevalent sequence type 2 (ST2 [Pasteur scheme]), 62% and 26% of BJC CrAb index isolates belonged to the unrelated ST499 and ST406, respectively. BJC ST499 and ST406 isolates were phylogenetically distinct compared to corresponding isolates from other US hospitals. Under the assumption that Ab transmission occurs primarily through nosocomial spread, we expected BJC isolates from the same hospital and timespan to share the highest degree of homogeneity. However, geotemporal proximity between ST499 or ST406 BJC isolates was a poor predictor of their genetic relatedness, according to multiple comparative methods. Review of patient metadata did not identify epidemiological links between BJC isolates within phylogenetic subgroups. Conclusion We combined comparative genomics and detailed clinical chart review to characterize the transmission dynamics of two emerging US CrAb sequence types, ST499 and ST406. Though these highly homogeneous Ab isolates were identified over two years in multiple BJC hospitals, we found no evidence of robust intra-hospital transmission networks. Instead, it appears that these CrAb isolates independently emerged from yet-to-be-identified regional, extra-hospital Ab populations. To neutralize the threat of drug-resistant infections in the US, it is essential to identify, characterize and disrupt emergent CrAb transmission networks that exist outside of hospital environments. Disclosures All Authors: No reported disclosures