The Importance of the Microbiome in Critically Ill Patients: Role of Nutrition

Abstract

Critically ill patients have an alteration in the microbiome in which it becomes a disease-promoting pathobiome. It is characterized by lower bacterial diversity, loss of commensal phyla, like Firmicutes and Bacteroidetes, and a domination of pathogens belonging to the Proteobacteria phylum. Although these alterations are multicausal, many of the treatments administered to these patients, like antibiotics, play a significant role. Critically ill patients also have a hyperpermeable gut barrier and dysregulation of the inflammatory response that favor the development of the pathobiome, translocation of pathogens, and facilitate the emergence of sepsis. In order to restore the homeostasis of the microbiome, several nutritional strategies have been evaluated with the aim to improve the management of critically ill patients. Importantly, enteral nutrition has proven to be more efficient in promoting the homeostasis of the gut microbiome compared to parenteral nutrition. Several nutritional therapies, including prebiotics, probiotics, synbiotics, and fecal microbiota transplantation, are currently being used, showing variable results, possibly due to the unevenness of clinical trial conditions and the fact that the beneficial effects of probiotics are specific to particular species or even strains. Thus, it is of great importance to better understand the mechanisms by which nutrition and supplement therapies can heal the microbiome in critically ill patients in order to finally implement them in clinical practice with optimal safety and efficacy.

Keywords: critically ill patient; fecal microbiota transplantation; microbiome; nutrition; prebiotics; probiotics; synbiotics.

Figure 1

Composition and functions of the intestinal microbiome in critically ill patients compared to healthy individuals. Critically ill patients exhibit an intestinal disease-promoting microbiome or pathobiome. This pathobiome is characterized by a lower prevalence of the Firmicutes and Bacterioidetes phyla, and a higher prevalence of the Proteobacteria phyla, in contrast to healthy individuals. Furthermore, the intestinal epithelium is altered in critically ill patients, showing reduced reperfusion, that could lessen the hydrophobicity of the mucus layer and favor the translocation of pathogens through gaps between the epithelial cells, and epithelial apoptosis, resulting in poor absorption of nutrients, diarrhea, loss of fecal energy, and lower production of short chain fatty acids (SCFA).

Figure 2

Factors that may alter the microbiome in critically ill patients in the ICU. The treatments administered to patients in the ICU, including antibiotics, proton pump inhibitors, vasopressors, and opioids, produce harmful effects outside their target organ, which directly affect the microbiome. The nutritional components (carbohydrates, lipids, and proteins) and the route of administration (enteral/parenteral) might also alter the health of the microbiome. The pharmacological interventions can modify the specific conditions of the body site (for example, skin decontamination with chlorhexidine) and invasive procedures may impair the natural barrier mechanisms (e.g., endotracheal intubation and intravascular catheters), facilitating the access and proliferation of microbes.