Pandemic-associated Shifts in Microbial Ecology and Antimicrobial Resistance in a High-Volume Liver Transplant Intensive Care Unit: a 10-Year Surveillance Study

Background:Healthcare-associated infections (HAIs) remain a major source of morbidity in solid organ transplant recipients, particularly in liver transplant (LT) intensive care units (ICUs), where profound immunosuppression and extensive antimicrobial exposure shape local microbial ecology. While reductions in overall infection incidence have been reported in structured surveillance programs, longitudinal changes in pathogen distribution and antimicrobial resistance (AMR) in transplant-specific ICUs remain insufficiently characterized. We aimed to evaluate pandemic-associated shifts in microbial ecology and AMR patterns across a 10-year surveillance period in a high-volume LT ICU. Methods:This retrospective analysis was based on prospectively collected surveillance data from adult LT recipients admitted between January 2015 and December 2024. The study period was categorized into prepandemic (2015-2019), pandemic (2020-2021), and postpandemic (2022-2024) phases. All analyses were performed at the HAI-episode level. Isolation density was calculated per 1000 patient-days. Incidence rate ratios were estimated using Poisson regression models with patient-days included as an offset. Period-related differences in AMR were evaluated using logistic regression. A two-sided P value <0.05 was considered statistically significant. Results:Among 7717 patients corresponding to 48,001 patient-days, 380 clinically significant isolates were analysed. Gram-negative organisms remained predominant throughout the study period, while Gram-positive isolates demonstrated a significant decreasing trend over time. Acinetobacter spp. (31.3%), Klebsiella spp. (25.8%), and Pseudomonas spp. (15.0%) were the leading pathogens. Isolation densities of Acinetobacter spp. and Escherichia coli were significantly higher in the prepandemic period than in the postpandemic period. In contrast, the pandemic and postpandemic phases were associated with significant increases in resistance probabilities among major Gram-negative pathogens. Acinetobacter spp. showed higher resistance to meropenem and amikacin, Klebsiella spp. demonstrated increased aminoglycoside and fluoroquinolone resistance, and Pseudomonas spp. exhibited increased ciprofloxacin resistance in 2020-2024 compared with 2015-2019. Conclusions:In this transplant-specific ICU, microbial ecology and resistance trajectories evolved independently of overall infection incidence trends. Pandemic-associated healthcare disruptions were accompanied by organism-specific resistance shifts rather than uniform ecological changes. Continuous surveillance and locally tailored antimicrobial stewardship strategies are essential to preserve therapeutic efficacy and mitigate the growing threat of multi-drug-resistant (MDR) pathogens in transplant ICUs.