重症(critical illness) 包括敗血症、急性肺傷害及多重器官失能(multiple-organ dysfunction) 等，是一個需投入大量資源醫治的課題。許多研究支持腸道是多重器官失能症候群的驅動者 (motor)，而微菌叢恆定受破壞是其間變化的關鍵之一。健康的腸道微菌叢(microbiota) 協助人 類維持腸道功能及免疫。當微菌叢出現不良改變，即微生態失調(dysbiosis) 時，易引發許多 代謝性及發炎疾病。近年來應用新技術，非經培養(culture-independent) 去氧核醣核酸(DNA) 定序方法，對微菌叢變化如何影響疾病發展有大量發現。重症病人因生理變化及治療介入， 使胃腸蠕動變慢，積聚大量微菌；也影響腸道免疫、損傷腸道防禦。於此環境，腸道微菌 叢從共生模式(commensal lifestyle) 轉變成毒性強的致病模式(pathogenic lifestyle)。困難梭菌 (Clostridium difficile) 造成偽膜性大腸炎，而Pseudomonas、Enterobacteria 等致病菌經呼吸道 嗆入及經淋巴循環，導致嚴重肺炎、急性肺傷害，也影響腦等多處器官，使重症惡化，嚴重 者造成多重器官衰竭。治療重症病人時，應減少藥物或介入措施對有益微菌叢的破壞、謹慎 使用抗生素。專一性清除口咽及消化道病菌法可減少加護病房病原菌，但會提升細菌對抗生 素的抗藥性。益生菌(probiotics) 可減少感染、呼吸器相關肺炎。早期腸道營養可降低死亡 率。糞便微菌移植(fecal microbiota transplantation) 可治療困難梭菌感染造成的腸炎甚至進一 步治療微生態失調所致之嚴重腹瀉及多重器官失能症候群。本文為綜論。希望將來經更多研 究，掌握微生態失調在重症的角色並發展對應的治療，以利人類健康。

Critical illness, including sepsis, acute lung injury and multiple-organ dysfunction, is an issue consuming great amount of medical resource. Many studies supported that the gut is the “motor” of multiple-organ dysfunction and the disruption of the homeostasis of microbiota is one of hinges in the process. Healthy microbiota maintains the function and immunity of human gut. Dysbiosis (adversely alteration of microbiota) is related to many metabolic and inflammatory diseases. In recent years, with culture-independent microbiome DNA sequencing methods, many studies were developed to explore the influence of microbiota to diseases. Due to the physical changing and interventions of treatment, the patients with critical illness have slow gastrointestinal peristalsis that accumulates huge amount of bacteria and impaired gut immune/ barrier. The commensal lifestyle of the microbiota can be shifted to a pathogenic one during critical illness. Infection with Clostridium difficile induces diarrhea and pseudomembrane colitis. Pathogens such as Pseudomonas, Enterobacteriaceae induce or worsen pneumonia, acute lung injury, dysfunction of brain and multiple organs via aspiration and lymph circulation. We should avoid the adversely alteration of microbiota when managing the patients with critical illness and be cautious with choosing antibiotics. Selective decolonization of the digestive tract decreases pathogens in ICU but increases bacterial resistance to antibiotics. Probiotics decreases infection and ventilator-associated pneumonia. Early enteral nutrition decreases mortality in ICU. Fecal microbiota transplantation reduces Clostridium difficile colitis and may alleviate MODS and diarrhea by correcting the dysbiosis among patients in the ICU. This article is a review. With more studies to clarify the role of dysbiosis in critical illness and to develop the associated management in future will benefit human health. (J Intern Med Taiwan 2017; 28: 232-242)