Pandemic-Associated Shifts in Microbial Ecology and Antimicrobial Resistance in a High-Volume Liver Transplant ICU: 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 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 liver transplant ICU. Methods This retrospective analysis was based on prospectively collected surveillance data from adult liver transplant recipients admitted between January 2015 and December 2024. The study period was categorized into pre-pandemic (2015–2019), pandemic (2020–2021), and post-pandemic (2022–2024) phases. Analyses were performed at the isolate level. Isolation density was calculated per 1,000 patient-days. Incidence rate ratios were estimated using Poisson regression models with patient-days included as an offset. Period-related differences in antimicrobial resistance were evaluated using logistic regression. A two-sided p value <0.05 was considered statistically significant. Results Among 7,717 patients corresponding to 48,001 patient-days, 380 clinically significant isolates were analyzed. 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 pre-pandemic period compared with the post-pandemic period. In contrast, the pandemic and post-pandemic 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, high-resolution surveillance and locally tailored antimicrobial stewardship strategies are essential to preserve therapeutic efficacy and mitigate the growing threat of multidrug-resistant pathogens in specialized transplant ICUs.