Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 18 5 2026 10 05 718 728 Conny Tjampakasari Department of Microbiology, Faculty of Medicine, Universitas Indonesia, Jakarta, Indonesia Reyhan Afif Sucahyo Department of Medical Education, Faculty of Medicine, Universitas Indonesia, Jakarta, Indonesia Dimas Seto Prasetyo Department of Microbiology, Faculty of Medicine, Universitas Indonesia, Jakarta, Indonesia 2026 05 13 2026 08 07 Background and Objectives: Inappropriate antibiotic use can lead to resistance, particularly in Gram-negative bacteria. The aimd of this study was to identify the distribution of Gram-negative bacteria responsible for lower respiratory tract infections and to assess their susceptibility profiles to beta-lactam antibiotics in the periods before and during the COVID-19 pandemic. Materials and Methods: This retrospective cross-sectional study utilized secondary data from WHONET 2022, drawing on a database of Gram-negative bacteria isolated from lower respiratory tract specimens processed at the Clinical Microbiology Laboratory, Faculty of Medicine, Universitas Indonesia, during the period of 2018-2021. Susceptibility to ampicillin-sulbactam, ceftazidime, cefepime, meropenem, and piperacillin-tazobactam was assessed. Comparisons between the pre-pandemic and pandemic periods were conducted using the chi-square test. Results: A total of 569 samples were analyzed, comprising 383 samples from the pre-pandemic period (2018-2019) and 186 samples from the COVID-19 pandemic period (2020-2021). Twenty-nine Gram-negative bacterial species were identified before the pandemic, compared with 19 species during the COVID-19 pandemic. Klebsiella pneumoniae emerged as the most common pathogen, with a prevalance of 30.29% before and 27.96% during the COVID-19 pandemic. Statistical analysis revealed significant differences in the resistance patterns of Acinetobacter baumannii to cefepime (p= 0.036), ceftazidime (p= 0.018), and piperacillin-tazobactam (p= 0.002). Conversely, no significant differences were observed for K. pneumoniae and Pseudomonas aeruginosa (p > 0.05). Conclusion: K. Pneumoniae remains the predominant Gram-negative pathogen causing lower respiratory tract infections, whereas A. baumannii shows an increased disease burden and resistance during the COVID-19 pandemic. This underscores the importance of continuous surveillance of antimicrobial resistance. https://ijm.tums.ac.ir/index.php/ijm/article/view/6191 https://ijm.tums.ac.ir/index.php/ijm/article/download/6191/1933