Pathogen Spectrum, Antimicrobial Resistance Profiles, and Clinical Outcomes of Polymicrobial Versus Monomicrobial Infections in Children After Cardiac Surgery: A Retrospective Single-Center Cohort Study

Objective: Hospital-acquired postoperative infections and rising antimicrobial resistance (AMR) pose critical threats to children receiving cardiopulmonary bypass (CPB) cardiac surgery in pediatric intensive care units (PICUs). Polymicrobial infections further complicate antimicrobial selection and are associated with worse clinical outcomes; yet stratified resistance data comparing monomicrobial and polymicrobial infections remain scarce in this vulnerable population. This single-center retrospective cohort aimed to characterize pathogen distribution and stratified AMR profiles and to explore associations between polymicrobial infection and clinical outcomes after pediatric cardiac surgery. Methods: Clinical and microbiological data of 523 screened children, 497 children with postoperative infections (January 2025–March 2026), were analyzed. Among these 497 patients, 435 patients with cultivable bacterial pathogens could be reliably categorized into monomicrobial (n = 297) and polymicrobial (n = 138) infection groups for subgroup comparison. The remaining 62 patients within the 497 cohort had clinical postoperative infection but no recoverable bacterial isolates and were excluded from this bacterial-focused subgroup analysis. Pathogen identification and antimicrobial susceptibility testing followed 2025 CLSI standards. Multidrug-resistant (MDR) organisms were defined as isolates resistant to ≥3 antimicrobial classes. We compared pathogen-level resistance profiles and patient-level clinical endpoints between monomicrobial and polymicrobial subgroups. Multivariable linear regression was used for continuous outcomes (mechanical-ventilation duration, PICU and total hospital length of stay). Regression diagnostic procedures, including log-transformation for skewed variables, residual assessment and homoscedasticity check were performed. Owing to model-fitting concerns for the binary continuous renal replacement therapy (CRRT) endpoint under the present sample size, Firth’s penalized logistic regression was attempted but showed unstable estimates due to quasi-complete separation; thus, only univariate group-difference was presented. Analyses adjusted for perioperative confounders to evaluate observed associations with polymicrobial infection. This study was reported in accordance with the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) statement. Results: A total of 435 pathogenic isolates were recovered from the 435 patients with cultivable bacterial pathogens among 497 children with postoperative infection. Antimicrobial resistance rates presented in this manuscript are isolate-level descriptive statistics, whereas clinical outcome comparisons are patient-level analyses. Key species included Escherichia coli (15.6%), Klebsiella pneumoniae (14.0%), and Pseudomonas aeruginosa (11.0%). Gram-positive pathogens accounted for 30.6%, primarily Streptococcus pneumoniae (12.6%). Severe resistance heterogeneity was observed: P. aeruginosa exhibited 40.0% imipenem resistance, while all E. coli isolates remained carbapenem-susceptible; 88.2% of S. pneumoniae strains were penicillin-resistant, with all Gram-positive isolates fully vancomycin-susceptible. Polymicrobial-infection patients had a higher proportion of patients with at least one MDR pathogen, longer CPB time, prolonged mechanical ventilation, higher unadjusted CRRT utilization rate, and extended ICU/hospital stays. After confounder adjustment, polymicrobial infection was associated with prolonged ventilatory support (β = 286.4, p < 0.001), ICU length of stay (β = 12.7, p < 0.001), and total hospitalization (β = 15.3, p < 0.001). Model diagnostics, including adjusted-R2, variance-inflation factor (VIF), and residual assessment, supported model validity for linear-regression models of continuous endpoints. Conclusions: In this cohort, postoperative infections in children receiving CPB-assisted cardiac surgery are predominantly caused by Gram-negative bacteria with stark interspecies differences in carbapenem susceptibility. Polymicrobial infection is associated with elevated MDR prevalence and greater organ-support demands as well as prolonged hospitalization. Local antibiogram data and early recognition of polymicrobial infection may help optimize empirical antimicrobial regimens and strengthen PICU AMR control.