Comparison of Agar Dilution to Broth Microdilution for Testing In Vitro Activity of Cefiderocol against Gram-Negative Bacilli.

Mariana Albano,Melissa J Karau,Robin Patel,Audrey N Schuetz,Carey-Ann D Burnham

doi:10.1128/jcm.00966-20

Abstract

Cefiderocol (CFDC) is a siderophore cephalosporin with activity against Gram-negative bacterial species that are resistant to carbapenems and other drugs. The MICs of CFDC were determined for 610 Gram-negative bacilli, including 302 multinational Enterobacterales isolates with characterized mechanisms of beta-lactam resistance, 180 clinical isolates from the Mayo Clinic and Mayo Clinic Laboratories not characterized for specific resistance mechanisms, and 128 isolates with CFDC MICs of ≥8 μg/ml obtained from International Health Management Associates, Inc. (IHMA, Schaumburg, IL). Broth microdilution using standard cation-adjusted Mueller-Hinton broth (BMD) and iron-depleted cation-adjusted Mueller-Hinton broth (ID-BMD), and agar dilution (AD) using standard Mueller-Hinton agar were performed according to Clinical and Laboratory Standards Institute (CLSI) guidelines. MICs were interpreted according to the investigational CLSI, FDA, and EUCAST breakpoints, and results were compared. MICs inhibiting 50 and 90% of organisms (MIC50 and MIC90, respectively), essential agreement (EA), categorical agreement (CA), and error of different types were determined. Results showed considerable discordance between AD and ID-BMD. CFDC showed low EA and CA rates and high error rates for AD in comparison to ID-BMD. Overall, this study does not support use of standard AD for determining CFDC MICs.

Highlights

Over the last few decades, infections caused by drug-resistant Gram-negative bacilli (GNB) have become a public health problem worldwide because of their prevalence and the proliferation of resistance mechanisms (1, 2)
For the 610 isolates tested, regardless of species, MICs obtained by broth microdilution (BMD) with standard cation-adjusted Mueller-Hinton broth (CAMHB) were, as expected, higher than those obtained by ID-BMD (Table S1), so we focused on comparison between agar dilution (AD) and ID-BMD
By AD, 83, 48, and 70% of P. aeruginosa isolates were susceptible to CFDC using investigational Clinical and Laboratory Standards Institute (CLSI), Food and Drug Administration (FDA), and European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, respectively; by ID-BMD, 97, 89, and 97%, respectively, were susceptible to CFDC

Summary

Introduction

Over the last few decades, infections caused by drug-resistant Gram-negative bacilli (GNB) have become a public health problem worldwide because of their prevalence and the proliferation of resistance mechanisms (1, 2). Of particular challenge are the multidrug-resistant (MDR) Enterobacterales, which have developed ␤-lactam resistance via (i) production of ␤-lactamases, including ESBLs, carbapenemases, and plasmid-mediated AmpC (4, 5); (ii) mutations in outer membrane porins, leading to loss of porins, decreased porin expression, or porins with narrow channels; and/or (iii) upregulation of efflux pumps, which pump antibiotics out of bacterial cells (6, 7). Cefiderocol (CFDC) is a novel siderophore cephalosporin which is actively transported into the periplasmic space along with ferric iron and binds mainly to penicillinbinding protein 3 (PBP3) of GNB, inhibiting bacterial cell wall synthesis (10). We evaluated AD in comparison to BMD (with and without iron-depleted medium) for testing the in vitro antimicrobial activity of CFDC against a diverse collection of GNB, including subsets enriched for CFDC-resistant and drug-resistant organisms

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