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Comparative evaluation of practical in-vitro methods for testing synergy between ceftazidime-avibactam and aztreonam against metallo-β-lactamase and serine carbapenemase-producing Enterobacterales species

Bawankar NS, Sayed SEH, Bhise SM, Gaind R

Indian journal of medical microbiology, 2026

Abstract

BACKGROUND: The emergence of multidrug-resistant (MDR) strains, including those producing metallo-β-lactamases (MBL), necessitates the development of effective therapeutic strategies. The combination of ceftazidime-avibactam (CZA) and aztreonam (ATM) has shown promise as a potential treatment option. This study aimed to evaluate practical and accessible antimicrobial susceptibility testing (AST) methods for assessing the synergy of CZA and ATM in carbapenemase-producing Enterobacterales species, particularly in resource-limited settings with high patient volumes.

METHODS: This cross-sectional study was conducted at a tertiary care hospital in Central India from October 2023 to July 2024. It compared different synergy testing methods to the gold-standard broth microdilution (BMD) method in genotypically confirmed carbapenemase-producing isolates. The study evaluated various synergy methods, such as the E-strip, disk replacement, inverse D-zone, and disk approximation.

RESULTS: Of the 60 Enterobacterales isolates, most carried the blaNDM gene (88.33%, 53/60). Using the BMD method, 73.33% (44/60) of the isolates showed synergy between CZA and ATM. The E-strip method was found to have almost perfect agreement (κ = 1) with BMD, making it highly reliable. However, it may be impractical for high-volume laboratories due to cost and complexity. On the other hand, the disk replacement and inverse D-zone methods demonstrated high sensitivity and specificity, with perfect (κ = 0.874)…

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Evidence Hub reads this as a cross-sectional, done in people. Its evidence level is 4 of 5 (Low · observational). Observational designs can show an association but not prove cause.

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