Background <p>The gut-kidney axis is critical in acute kidney injury (AKI) pathophysiology, but its specific characteristics in intensive care unit (ICU) patients remain unclear. This study aimed to investigate compositional and functional alterations of gut microbiota in AKI patients.</p> Methods <p>Patients were divided into AKI and non_AKI groups. Rectal swabs were collected within 24&#xa0;h of ICU admission. 16SrDNA sequencing was performed to assess taxonomic composition. Alpha diversity, beta diversity, and Linear discriminant effect size analysis (LEfSe) analyses were conducted. Redundancy analysis (RDA) examined relationships between microbial communities and environmental factors. PICRUSt2 inferred metabolic functional differences. Average causal mediation effect (ACME) analysis explored relationships among dysbiosis, metabolic function, and clinical factors.</p> Results <p>A total of 193 individuals comprising 142 AKI and 51 non_AKI were enrolled in the study. Compared to non_AKI patients, the AKI group exhibited significantly higher gut microbiota species richness as measured by the ace index (<i>p</i> &lt; 0.05), indicating an increase in moderately abundant species potentially including opportunistic pathogens. However, no significant differences were observed in chao1 index (another measure of richness), nor in the shannon and simpson indices (measures of overall diversity and evenness), suggesting that while certain bacterial populations expanded, the fundamental structure and balance of the gut ecosystem remained preserved. Significant differences in overall community composition (beta diversity, <i>p</i> &lt; 0.05) were observed between groups. LEfSe identified AKI-associated dysbiosis characterized by proliferation of opportunistic pathogens (e.g., <i>s__Staphylococcus haemolyticus</i>, <i>s__Ruminococcus bromii</i>, and <i>g__Veillonella</i>) and extraintestinal bacterial intrusion (e.g., <i>s__Sneathia amrii</i>,<i> s__Atopobium vaginae</i>). RDA showed that serum creatinine (SCr, r² = 0.079, <i>p</i> = 0.006) and blood urea nitrogen (BUN, r² = 0.066, <i>p</i> = 0.007) were the only clinical factors significantly impacting microbial community structure. <i>PICRUSt2</i> revealed significant functional alterations between groups, with multiple metabolic pathways (including URSIN-PWY, PWY-5420, PWY-5415, PWY-7854, PWY-5367, TYRFUMCAT-PWY, PWY-5181, PWY-7480, and PWY-5430) showing differential abundance. Exploratory mediation analysis constructed a potential association network linking gut microbiota dysbiosis, altered metabolic pathways, and SCr levels.</p> Conclusions <p>AKI patients exhibited gut microbiota dysbiosis characterized by opportunistic pathogen proliferation and extraintestinal bacterial intrusion. These alterations were observed in association with disease severity and could potentially relate to SCr levels, possibly via modulation of specific metabolic pathways.</p> Trial registration <p>This research was based on the analysis of hospital-acquired infection surveillance data and was not registered in a clinical trial registry.</p>