Temporal Trends in Antimicrobial Resistance of Acinetobacter baumannii: A Mixed-Effects Analysis of Clinical Isolates Collected at Palermo University Hospital (2013–2025)

Abstract

Introduction

Acinetobacter baumannii is recognized by the World Health Organization (WHO) as one of the ESKAPE priority pathogens because of its environmental persistence and ability to acquire antimicrobial resistance. Carbapenem-resistant A. baumannii (CRAb) has been included in the WHO Global Priority List of Antibiotic-Resistant Bacteria since 2017 and remains a critical threat. In Europe, CRAb is responsible for a substantial burden of healthcare-associated infections and mortality, particularly among critically ill patients.

Objectives

To evaluate temporal trends in antimicrobial resistance among clinical isolates of Acinetobacter baumannii collected at the Microbiology Laboratory of the University Hospital “Paolo Giaccone” of Palermo over a 13-year period (2013–2025).

Methods

A retrospective analysis was conducted on all clinical isolates of A. baumannii collected between 2013 and 2025 (N = 3,010). One isolate per patient was included. Data collected comprised year of isolation, patient demographics (age and sex), hospital ward (Intensive Care Unit/Emergency, Medical Area, Surgical Area), and biological sample type. Antimicrobial susceptibility testing was performed against polymyxins, fluoroquinolones, aminoglycosides, and carbapenems. Isolates were classified as multidrug-resistant (MDR) when resistant to at least three antimicrobial classes.

Comparisons between MDR and non-MDR isolates were performed using chi-square or Fisher’s exact tests for categorical variables and the Kruskal–Wallis test for age. To identify independent predictors of MDR isolation and evaluate temporal variability, a mixed-effects logistic regression model was applied, with isolates nested within years. Age, sex, clinical area, and sample type were included as fixed effects, while year of isolation was modelled as a random effect. Results were reported as adjusted odds ratios (ORs) with 95% confidence intervals (95% CIs).

Results

Among the 3,010 isolates analyzed, 2,727 (90.6%) were classified as MDR. Patients were predominantly male (60.3%), with a mean age of 71.0 ± 15.7 years; 71.4% were aged 65 years or older. Most isolates originated from Intensive Care/Emergency Units (43.8%), followed by Medical (33.9%) and Surgical Wards (21.0%).

Resistance rates remained consistently high throughout the study period, reaching 95.8% for carbapenems, 95.1% for fluoroquinolones, and 91.0% for aminoglycosides. Polymyxin resistance remained low (1.6%). Univariable analyses did not identify significant associations between patient or clinical characteristics and MDR status. In the mixed-effects model, age emerged as the only independent predictor of MDR isolation (OR 1.01; 95% CI 1.00–1.02; p = 0.019). Significant between-year variability was observed (random intercept SD = 0.404; 95% CI 0.046–0.526). Year-specific analyses showed a significantly increased probability of MDR isolation in 2020, whereas significantly lower-than-expected odds were observed in 2022 and 2023.

Conclusions

The observed temporal variability suggests that healthcare-system factors, beyond individual patient characteristics, influenced MDR A. baumannii circulation. The increase observed in 2020 coincided with the COVID-19 pandemic, characterized by increased intensive care admissions, broad-spectrum antibiotic use, prolonged mechanical ventilation, and critical care overcrowding. Conversely, the reduction observed in 2022–2023 may reflect strengthened infection prevention and control measures and reduced antibiotic selective pressure associated with declining COVID-19-related critical care admissions.