Intestinal Bacterial Overgrowth in Severe Traumatic Brain Injury Is Associated with Late In-Hospital Mortality

Background: Severe traumatic brain injury (TBI) is associated with high mortality. There is increasing evidence suggesting that intestinal dysbiosis may influence clinical outcomes; however, its prognostic significance in patients with severe TBI remains poorly defined. Methods: This retrospective cohort study included 235 patients with severe TBI (Glasgow Coma Scale [GCS] score ≤ 8) admitted to the intensive care units of three participating hospitals between September 2023 and December 2024. Quantitative microbiological analysis of jejunal aspirates was performed at admission and repeated on days 12–14 of the ICU stay. Small intestinal bacterial overgrowth (SIBO) was defined as a bacterial load of ≥105 colony-forming units per milliliter (CFU/mL). Multivariable logistic regression and receiver operating characteristic (ROC) curve analyses were performed. Results: At admission, SIBO was uncommon (3.8%) but increased to 27.9% among patients who underwent repeated assessment. Within the day-14 landmark cohort (n = 179), post-day-14 mortality was higher in patients with SIBO than in those without (34.0% vs. 23.3%). In multivariable logistic regression analysis, SIBO was independently associated with post-day-14 in-hospital mortality after adjustment for age, body mass index, GCS score, PaO2/FiO2 ratio, and blood glucose (OR 2.83, 95% CI 1.06–7.58; p = 0.039). The ROC model demonstrated good discriminative performance, with an apparent area under the curve (AUC) of 0.846 and an optimism-corrected AUC of 0.823 after bootstrap validation. Conclusions: In patients with severe TBI who survived and remained hospitalized through the day-14 landmark, SIBO detected at days 12–14 was independently associated with increased subsequent in-hospital mortality. However, its incremental contribution to discrimination beyond established clinical predictors was modest. Therefore, late SIBO should be interpreted primarily as a dynamic marker associated with prolonged critical illness and adverse outcome rather than as a demonstrated cause of mortality. Prospective validation is required.