Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 18 5 2026 10 05 670 676 Siavosh Salmanzadeh-Ahrabi Women Research Center, Alzahra University, Tehran, Iran Farzaneh Salehinejad Department of Biotechnology, Faculty of Basic Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran Masoud Sokhanvar-Mahani Department of Microbiology and Microbial Biotechnology, Faculty of Life Sciences and Biotechnology, Shahid Beheshti University, Tehran, Iran 2026 02 20 2026 09 20 Background and Objectives: Urinary tract infections (UTIs) are among the most prevalent bacterial infections in women, with Escherichia coli and Klebsiella pneumoniae as leading causative pathogens. The emergence of multidrug-resistant (MDR) and carbapenem-resistant strains poses a significant therapeutic challenge. Ceftazidime–avibactam (CZA) has shown activity against β-lactamase-producing Gram-negative bacteria; however, data on its activity against urinary carbapenem-resistant isolates remain limited. This study aimed to determine CZA activity against urinary carbapenem-resistant E. coli and K. pneumoniae isolates. Materials and Methods: In this cross-sectional study, antimicrobial susceptibility testing was performed using the disk diffusion method according to CLSI guidelines. The tested antibiotics included carbapenems, third-generation cephalosporins, aminoglycosides, levofloxacin, fosfomycin, and CZA. Extended-spectrum β-lactamase (ESBL) production was assessed using the combined-disk method. Results: A total of 76 previously collected urinary bacterial isolates, including 44 (57.9%) E. coli and 32 (42.1%) K. pneumoniae isolates were included in this study. Carbapenem resistance was detected in 25.0% (11/44) of E. coli and 28.1% (9/32) of K. pneumoniae isolates. ESBL production was observed in 36.4% and 37.5%, respectively. Overall, CZA susceptibility was 68.2% for E. coli and 68.7% for K. pneumoniae, decreasing to 27.3% and 22.2%, respectively, among carbapenem-resistant isolates. Overall, 25.0% of carbapenem-resistant isolates remained susceptible to CZA. Fosfomycin susceptibility was 81.8% and 46.9% among E. coli and K. pneumoniae, respectively. Conclusion: CZA demonstrated substantial in vitro activity against urinary E. coli and K. pneumoniae isolates; however, its activity was markedly reduced among carbapenem-resistant strains. These findings highlight the importance of susceptibility-guided use of CZA and continued surveillance of antimicrobial resistance. https://ijm.tums.ac.ir/index.php/ijm/article/view/6079 https://ijm.tums.ac.ir/index.php/ijm/article/download/6079/1927