Antibiotic Resistance Profiles of Invasive Bacterial Isolates from Hospitalized Pediatric Patients in Serbia: A Multicenter Surveillance Study, 2020–2024

Background: Antimicrobial resistance (AMR) threatens empirical therapy for invasive pediatric infections, but contemporary data from Serbia and the Western Balkans remain limited for blood and cerebrospinal fluid isolates. Methods: We performed a retrospective CAESAR-linked multicenter microbiological surveillance analysis of first eligible invasive bacterial isolates from hospitalized children in Serbia from 1 January 2020 to 31 December 2024. Organism distribution, ESBL production, carbapenemase marker results, and selected resistance outcomes were analyzed descriptively and with temporal trend and multivariable models. Results: Among 907 isolates, 879 (96.9%) were recovered from blood and 28 (3.1%) from cerebrospinal fluid; neonates and infants accounted for 57.9% of isolates. K. pneumoniae was the predominant organism (318/907, 35.1%), followed by E. coli (172/907, 19.0%), S. aureus (137/907, 15.1%), and Acinetobacter spp. (96/907, 10.6%). Among K. pneumoniae and E. coli isolates with available ESBL results, ESBL production was frequent, whereas carbapenemase testing was restricted to a selected subset and the carbapenemase type was not recorded in the national surveillance extract. In K. pneumoniae, meropenem resistance increased from 19.7% in 2020 to 48.6% in 2024, and later study years remained independently associated with meropenem resistance after adjustment. Conclusions: This microbiological surveillance study identifies K. pneumoniae predominance, neonatal concentration, frequent ESBL production, and a rising carbapenem-resistance signal as the principal findings among invasive pediatric isolates in Serbia. Resistance estimates, particularly carbapenemase production, should be interpreted as surveillance indicators rather than clinical prevalence estimates. These data support strengthened pediatric invasive-isolate surveillance, center- and age-specific antibiograms, molecular characterization, and coordinated antimicrobial stewardship and infection prevention and control efforts within the wider European AMR context.