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Updated: Jun 25, 2026

Antimicrobial Synergy Testing by the Inkjet Printer-assisted Automated Checkerboard Array and the Manual Time-kill Method
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Antimicrobial Synergy Testing by the Inkjet Printer-assisted Automated Checkerboard Array and the Manual Time-kill Method

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Ceftazidime-Avibactam/Colistin Combination Against Carbapenem-Resistant Pseudomonas aeruginosa: In Vitro Synergy

Gulsah Altan1, Devrim Dundar1

  • 1Department of Medical Microbiology, Faculty of Medicine, Kocaeli University, İzmit, Turkiye.

Microbial Drug Resistance (Larchmont, N.Y.)
|June 24, 2026
PubMed
Abstract

Insights

The combination of ceftazidime-avibactam (CZA) and colistin (COL) showed partial synergy against multidrug-resistant Pseudomonas aeruginosa. This combination may be beneficial for treating CZA-resistant carbapenem-resistant P. aeruginosa (CRPA) infections.

Area of Science:

  • Microbiology
  • Infectious Diseases
  • Pharmacology

Background:

  • Rising multidrug-resistant Pseudomonas aeruginosa necessitates novel antimicrobial strategies.
  • Emerging resistance to newer agents like ceftazidime-avibactam (CZA) underscores the importance of combination therapies.
  • This study investigates the in vitro synergy of CZA and colistin (COL) against carbapenem-resistant P. aeruginosa (CRPA).

Purpose of the Study:

  • To evaluate the in vitro synergistic activity of the CZA-COL combination against clinical CRPA isolates.
  • To determine the frequency and characteristics of synergy in CRPA strains, including those resistant to CZA.

Main Methods:

  • Twelve clinical CRPA isolates were analyzed.
  • Minimum inhibitory concentrations (MICs) were determined using broth microdilution.
  • Synergy was assessed via checkerboard method, with synergy defined by fractional inhibitory concentration index (FICI).

Main Results:

  • All isolates were susceptible to colistin; four were resistant to CZA.
  • Partial synergy was observed in 75% of isolates (9/12).
  • Partial synergy was more prevalent in CZA-resistant isolates (100%) and associated with blaNDM and blaOXA-48 genes.

Conclusions:

  • The CZA-COL combination demonstrates potential for partial synergy, particularly against CZA-resistant CRPA.
  • Further in vivo and prospective clinical studies are warranted to validate the efficacy of this combination therapy.
Keywords:
carbapenem-resistant Pseudomonas aeruginosaceftazidime–avibactamcheckerboard assaycolistinsynergy

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