The intestinal microbiome is comprised of diverse, robust microbial communities within the intestine, modified by the host’s interaction with the environment (Amon and Sanderson 2017). Increasingly, research links alterations in the microbiome to maintenance of health and pathophysiology of disease. Critical illness is no exception. Given the lack of therapies aimed at the host response in critical illness combined with pathologic alterations in the microbiome seen in the ICU, treatment directly targeting the intestinal microbiome serves as a potential avenue for therapy in critically ill patients.
The Intestinal Microbiome in Health
The intestinal microbiome is composed of all microbes (bacteria, fungi, and viruses) that occupy the intestinal lumen. The number of bacteria present in the gut lumen equals the number of the cells in the human body, yet with nearly 100 times the number of genes (Li et al. 2014; Sender et al. 2016). Starting even before birth, the microbiome begins to be established (Younge et al. 2019) and is shaped by every host interaction with its environment. As the host develops, a baseline proportion and diversity of bacteria takes hold for each human, predominantly in the phylogenic families of Bacteroidetes and Firmicutes (Dethlefsen et al. 2006). The vast majority of these bacteria act as commensal organisms aiding the host in nutrient absorption and vitamin production (Eckburg et al. 2005; Qin et al. 2010), while assisting in enterocyte maintenance and immune function and diversity (Morrison and Preston 2016; Zegarra-Ruiz et al. 2021). Bacterial products can have profound influence on human health. A group of metabolites of particular interest are short chain fatty acids (SCFA) (Pickard et al. 2017). These bacterial fermentation products are found within the lumen of the intestinal tract and are used for signalling between the microbiota and the host and can serve as an energy source for intestinal epithelial cells (Corrêa-Oliveira et al. 2016).