The leaky gut and microbiome in critical illness: emerging insights into microbial "translocation".

PURPOSE OF REVIEW: Microbial translocation has long been viewed as a nonspecific consequence of gut barrier failure in critical illness, with downstream effects attributed primarily to the host immune response. This review examines emerging evidence that the gut microbiome plays a more active and specific role in this process than previously appreciated.

RECENT FINDINGS: Dysbiosis during critical illness directly contributes to barrier dysfunction through depletion of metabolites that sustain epithelial integrity. Culture-independent approaches have revealed that gut-derived organisms are a major reservoir for secondary infection, with translocation governed by microbial virulence, community dynamics, and immune cell-mediated transport rather than barrier permeability alone. In parallel, organism-specific microbial components - including structurally diverse forms of lipopolysaccharide and bacterial DNA detected across multiple blood fractions - enter the circulation and differentially modulate host immune responses. Recent studies link circulating microbial DNA composition to distinct inflammatory phenotypes in sepsis and acute respiratory distress syndrome, suggesting these signals contribute to clinical heterogeneity.

SUMMARY: These findings support a revised framework in which translocation reflects the composition of the dysbiotic gut, not barrier integrity alone. Integrating microbial data with host phenotyping may enable more precise risk stratification and microbiome-informed therapeutic strategies in critical illness.