Abstract: The gut microbiota is the second genome of the human body, which plays a crucial role in maintaining host health through its diversity and functional homeostasis. Elderly populations are more prone to gut microbiota imbalance due to physiological decline, diseases such as sepsis and medication use, which can affect immune regulation, metabolic homeostasis, and inflammatory responses, significantly increasing the incidence and mortality of critical illnesses. This article systematically reviews the core functions of the gut microbiota in immune modulation, metabolic balance, and barrier protection, analyzes the epidemiological characteristics of microbiota dysbiosis in the elderly and its association with critical illnesses, and focuses on the research progress of how microbial metabolites (such as, short-chain fatty acids, bile acids, and tryptophan derivatives) regulate the pathological processes of severe condition via the “gut-organ axis.” Finally, it looks forward to the clinical potential of targeted microbiota interventions for preventing and managing critical illnesses in the elderly.

Key words: aged, critical illness, gut microbiota, gut-organ axis, microbiota intervention