Impact of the Recent Clinical and Laboratory Standards Institute Breakpoint Changes on the Antimicrobial Spectrum of Aminoglycosides and the Activity of Plazomicin Against Multidrug-Resistant and Carbapenem-Resistant Enterobacterales From United States Medical Centers

BACKGROUND: The Clinical and Laboratory Standards Institute (CLSI) lowered the Enterobacterales-susceptible/-resistant breakpoints for amikacin in 2023 from ≤16/≥64 mg/L to ≤4/≥16 mg/L and the breakpoints for gentamicin and tobramycin from ≤4/≥16 mg/L to ≤2/≥8 mg/L. Because aminoglycosides are frequently used to treat infections caused by multidrug-resistant (MDR) and carbapenem-resistant Enterobacterales (CRE), we evaluated the impact of these changes on the susceptibility rates (%S) of Enterobacterales collected from US medical centers. METHODS: A total of 9809 Enterobacterales isolates were consecutively collected (1/patient) from 37 US medical centers in 2017–2021 and susceptibility was tested by broth microdilution. Susceptibility rates were calculated using CLSI 2022, CLSI 2023, and US Food and Drug Administration 2022 criteria. Aminoglycoside-nonsusceptible isolates were screened for genes encoding aminoglycoside-modifying enzymes (AMEs) and 16S rRNA methyltransferases (16RMT). RESULTS: The CLSI breakpoint changes mostly affected amikacin, especially against MDR (94.0%S to 71.0%S), extended-spectrum β-lactamase (ESBL)-producing (96.9%S to 79.7%S), and CRE (75.2%S to 59.0%S) isolates. Plazomicin was active against 96.4% of isolates and retained potent activity against CRE (94.0%S), ESBL-producing (98.9%S), and MDR (94.8%S) isolates. Gentamicin and tobramycin showed limited activity against resistant subsets of Enterobacterales. The AME-encoding genes and 16RMT were observed in 801 (8.2%) and 11 (0.1%) isolates, respectively. Plazomicin was active against 97.3% of the AME producers. CONCLUSIONS: The spectrum of activity of amikacin against resistant subsets of Enterobacterales was drastically reduced when interpretative criteria based on pharmacokinetic/pharmacodynamic parameters that are currently used to establish breakpoints for other antimicrobials were applied. Plazomicin was markedly more active than amikacin, gentamicin, or tobramycin against antimicrobial-resistant Enterobacterales.