146. Heteroresistance to cefiderocol among Pseudomonas aeruginosa clinical isolates harboring changes in TonB-dependent receptor pathways and AmpC

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Abstract Background Cefiderocol (FDC) is an iron siderophore cephalosporin with activity against multi-drug resistant (MDR) Gram-negative bacteria. Despite high rates of in vitro susceptibility, treatment failure with the emergence of resistance is still a concern. Heteroresistance, or the presence of a resistant sub-population in a strain that tests susceptible by standard methods, has been suggested as a potential mechanism for this observation. Previously, we showed that MDR Pseudomonas aeruginosa (PA) with changes in the iron siderophore transporters (TBDR) responsible for FDC uptake had decreased zone diameters on Kirby-Bauer (KB) testing as compared to MDR-PA with wild-type TBDRs. Here, we investigated the prevalence of heteroresistance (HR) among TBDR mutants using population analysis profile-area under the curve assay (PAP). Table 1.Pseudomonas aeruginosa strain characteristicsFigure 1.PAP-AUC results of Pseudomonas aeruginosa TBDR mutants v. non-mutant controls Methods Previously sequenced MDR-PA isolates were selected for the presence of mutations in TBDR genes within the primary FDC import pathways (pirA, pirRS, piuA, piuD). Isolates harboring pre-existing TBDR mutations (n=12) were compared with wild-type P. aeruginosa strain PA01 and 5 MDR-clinical isolates harboring wild-type TBDRs. FDC MICs were performed by broth microdilution (BMD) in iron depleted media (IDM). For PAP, isolates were grown overnight in IDM before serial dilution and plating on agar with increasing concentrations of FDC. Agreement between BMD, KB, and PAP-AUC HR detection was assessed. Results On BMD, 10 of 12 TBDR isolates tested susceptible to FDC (MIC range ≤0.5-2 µg/mL), and all control isolates had an MIC ≤0.5 µg/mL (Table 1). None of the control strains were HR per PAP (Fig 1). However, in the TBDR group, 50% of isolates showed either a resistant (n=2) or HR (n=4) phenotype. Resistance occurred in the setting of both TBDR mutations and either an exogenous β-lactamase (OXA-15 variant) or AmpC variant (PDC-205). Of the HR isolates, one had a change in AmpC Ω-loop (E221K), while the other three had only the PirR frameshift mutation. BMD was not able to identify any of the HR isolates. Conclusion A greater number of MDR-PA with TBDR mutations showed resistance or HR to FDC than MDR-PA controls with wild-type TBDRs. Mutations in TBDRs could be a marker for FDC non-susceptibility and potential therapeutic failure. Disclosures Cesar A. Arias, MD, PhD, Entasis Phramceuticals: Grant/Research Support|MeMed Diagnostics: Grant/Research Support|Merck: Grant/Research Support William R. Miller, MD, Entasis Therapeutics: Grant/Research Support|UpToDate: Royalties - Topic Contributor.

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