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Background Antibiotic resistance is a global health crisis. While persistent drug discovery of novel antibiotics has previously been relied upon to thwart resistance, evolution inevitably perseveres. While genes conferring antibiotic resistance have previously been characterized, it is unclear how varying genetic contexts can change the antibiotic resistance phenotype a given gene confers. Methods The DH10B strain of E. coli was transformed with a blaADC-7 plasmid. In 12 evolutionary replicates, the modified E. coli strain and a clinical strain of A. baumannii containing the same resistance gene were passaged daily for 10 days on cefepime gradient agar plates with gradually increasing concentrations of cefepime. MICs of cefepime and a diverse set of 15 other drugs were determined for the parental strains and after the final passage passage. MIC of cefepime after intermediary passages were determined for select replicates. Lastly the blaADC-7 gene after the final passage was sequenced. Results At the end of 10 passages, collateral sensitivity in A. baumannii was observed to tigecycline and fosfomycin in 5 and 6 replicates respectively, out of 12 total. 4 out of 12 E. coli replicates displayed collateral sensitivity to minocycline (Figure 1). In the third E. coli replicate, Sanger sequencing revealed a novel S286R mutation in blaADC-7 appearing in passage seven which preceded a several log fold increase in the MIC of cefepime (Figures 2 and 3). No additional mutations were found in the other evolutionary replicates. Conclusion Patterns of resistance varied among antibiotics of the same class, (e.g., tetracyclines, fourth-generation cephalosporins) in both E. coli and A. baumannii; however, A. baumannii expressed less widespread collateral resistance than E. coli. A previously undiscovered S286R mutation in blaADC-7 coincided with a pronounced increased in resistance to cefepime. Further studies are required to determine whether this mutation gives rise to a structural change in the protein product. Given that no other mutations were found, resistance to cefepime and subsequent collateral resistance to other antibiotics may have developed due to epigenetic changes or mutations outside the blaADC-7 genes. Indeed, future experiments with whole-genome sequencing may reveal such changes. Disclosures All authors: No reported disclosures.

open forum infectious diseases

609. Differing Genotypic Contexts Between E. coli and A. baumannii Modulate the Role of blaADC-7 in the Development of Antibiotic Collateral Sensitivity