To determine whether preoperative intestinal microbiome characteristics are associated with the development of systemic inflammatory response syndrome (SIRS) after cardiac surgery.

Single-center prospective observational cohort study.

Tertiary university hospital.

A total of 196 adults undergoing elective cardiac surgery with extracorporeal circulation between 2018 and 2019.

No microbiome-targeted intervention was performed.

Shotgun metagenomic sequencing was used to assess microbial diversity (inverse Simpson index, gene richness, dominance), taxonomic composition, and functional potential. The primary outcome was development of SIRS within 24 hours postoperatively. Associations were evaluated using Wilcoxon rank-sum tests, χ² tests, and logistic regression adjusted for age and sex. Forty-four patients (22%) developed SIRS. Microbiome diversity did not differ significantly between patients with and without SIRS (median inverse Simpson index 20.4 v 19.3, p = 0.12; gene richness, p = 0.30; dominance, p = 0.61). In adjusted analyses, diversity was not associated with SIRS risk (odds ratio, 1.04; 95% confidence interval, 0.99-1.07). Descriptive analyses of taxonomic composition and functional potential similarly revealed no significant differences between SIRS and non-SIRS groups.

In this cohort of elective cardiac surgery patients, preoperative gut microbiome diversity, composition, and functional potential were not associated with the development of postoperative SIRS. These findings do not support a strong causal or predictive role of the presurgical gut microbiome in postoperative inflammatory responses after cardiac surgery.