Beyond Pneumonia: The Oral-Gut Axis in Critical Illness

The oral-gut microbiome axis represents a critical, interdependent ecosystem whose stability is essential for systemic immune homeostasis and barrier defense. In the intensive care unit (ICU), this axis undergoes a profound and targeted assault from life-sustaining interventions. Mechanical ventilation, broad-spectrum antibiotics, and proton pump inhibitors (PPIs) act synergistically to dismantle the physiological barriers separating the oral and gut microbiomes, driving dysbiosis at both sites and facilitating the ectopic translocation and colonization of oral pathobionts in the gastrointestinal tract. This process transcends passive microbial spillover, actively fueling intestinal inflammation, compromising epithelial integrity, and exacerbating systemic immune dysregulation—key pathways in the pathogenesis of ventilator-associated pneumonia (VAP), enterogenic sepsis, and multiple organ dysfunction. This review synthesizes contemporary evidence to posit that the integrity of the oral-gut axis is a modifiable determinant of outcome in critical illness. Consequently, we advocate for a paradigm shift in infection prevention from external disinfection towards the active stewardship of endogenous microbial ecology. We critically evaluate evolving oral care strategies, deconstruct the limitations of non-selective antiseptics like chlorhexidine, and highlight the evidence supporting ecological approaches such as mechanical hygiene, physiological irrigation, and selective oropharyngeal decontamination. Future research must prioritize the standardization of axis-preserving protocols and rigorously evaluate microbiome-targeted interventions, including the refinement of selective decontamination strategies for targeted populations, to restore ecological resilience and improve patient survival.