Beyond the Lungs: ICU Oral Care and Gut Resilience—A Hypothesis-Generating Narrative Review

The oral and gut microbiomes form an interconnected ecosystem that may influence host defense. In the intensive care unit (ICU), endotracheal intubation, broad-spectrum antibiotics, and proton pump inhibitors can disrupt physiological barriers and may facilitate ectopic gut colonization by oral pathobionts. Such translocation could contribute to intestinal inflammation, barrier dysfunction, and systemic immune dysregulation; however, direct clinical evidence in critically ill patients remains limited and largely associative, and much of the mechanistic evidence derives from animal models or non-ICU populations. This hypothesis-generating narrative review synthesizes evidence across oral microbiology, gastroenterology, immunology, and critical care, while explicitly distinguishing clinically observed associations from preclinical mechanisms and proposed ICU pathways. Current evidence supports mechanical toothbrushing as a core component of oral care, whereas routine non-selective chlorhexidine use warrants caution. Saline irrigation and selective oropharyngeal decontamination may be considered in context, but evidence for protecting the gut through oral care remains insufficient for definitive practice recommendations. We also examine probiotics, prebiotics, postbiotics, and fecal microbiota transplantation as investigational approaches and propose standardized prospective multicenter studies to test the oral–gut axis framework.