Epidemiology of Antimicrobial Resistance in Complicated Acute Pyelonephritis—A Romanian Cohort Study

Antimicrobial resistance represents a major global challenge for healthcare systems, particularly in urinary tract infections (UTIs), where empirical antibiotic therapy is frequently required. Acute pyelonephritis (AP) remains a severe condition, requiring prompt diagnosis and treatment. Local epidemiological data are essential for optimizing therapeutic strategies. The aim of this study was to analyze the pathogen distribution and antimicrobial resistance (AMR) patterns in patients with complicated AP. An observational, analytical study on community-acquired and hospital-acquired AP was conducted on patients admitted with complicated AP between January 2021 and December 2025. After applying the inclusions and exclusions criteria, 553 urinary isolates with complicated AP were analyzed to determine pathogen distribution and phenotypic AMR patterns derived from antimicrobial susceptibility testing. A total of 109 (19.7%) AMR isolates presented resistance phenotype. Resistant phenotypes were more frequently observed among male gender; age did not reach statistical significance. This study highlights the continued predominance of Escherichia coli in complicated AP while demonstrating a significant AMR burden among non-Escherichia coli pathogens, particularly Klebsiella and Pseudomonas species. These findings emphasize the importance of local epidemiological surveillance and culture-guided antibiotic therapy in the management of complicated UTIs.