628. Characterizing a Sulbactam-Durlobactam Challenge Set of Acinetobacter baumannii Surveillance Isolates with Rapid Genotypic Testing (Carba-R) and Whole Genome Sequencing

Abstract Background Acinetobacter baumannii is emerging as an increasingly important opportunistic human pathogen. A 2023-2024 nationwide surveillance study focused on A. baumannii-calcoaceticus complex isolates among hospitalized patients (n=523) revealed that the majority of isolates were susceptible to sulbactam-durlobactam (SUD) (MIC50/90, 2/4 mg/L; 96.9% susceptible) with 3.1% of isolates (n=16) identified as non-susceptible. Comparative whole genome sequencing (WGS) and rapid genotypic analysis were performed to evaluate resistance mechanisms in this subset of non-susceptible isolates.Table 1.Isolate phenotypic and genotypic profiles Methods Of the 16 non-susceptible isolates, 12 were available for genotypic testing from contributing hospitals. WGS was performed, and data were analyzed using open-source tools while Xpert CarbaR was performed according to manufacturer instructions to detect blaKPC, blaVIM, blaOXA48, blaNDM and blaIMP carbapenemase genes. Isolates were epidemiologically typed through multi-locus sequence typing using the Institute Pasteur scheme. Results SUD non-susceptible isolates were all carbapenem-resistant with meropenem MICs of ≥8 mg/L (Table 1). The globally dominant clone ST2 was the most prevalent sequence type identified. Emerging clones ST10 and ST499 were also observed. All sequenced strains harbored genes for Acinetobacter-derived cephalosporinase (ADC) Ambler class C β-lactamase such as ADC-73, as well as various Ambler class D OXA β-lactamases (OXA-23, OXA-58, OXA-66 carbapenemases). A metallo-β-lactamase (NDM) was identified in 4 isolates. Target site alterations in penicillin-binding protein 3 (PBP3) and mutations in the AdeIJK efflux system and its transcriptional regulator AdeN were noted (Table 1). Carba-R concordance with WGS was noted, with identification of 4 blaNDM-positive isolates. Conclusion All isolates exhibited mutations in efflux genes adeNIJK (which durlobactam is a substrate for) in addition to a mutation in PBP3 (n=8) or presence of an NDM gene (n=4). This study contributes to our understanding of clonality as well as the interplay of NDM production and mutations in efflux pumps and PBP present in SUD non-susceptible A. baumannii isolates. Further experiments are needed to identify the contribution of specific efflux gene variants to SUD non-susceptibility. Disclosures All Authors: No reported disclosures