Influence of the intestinal microbiota on the critically ill patient

The gastrointestinal (GI) tract, from mouth to anus, represents the most diverse, complex and fragile microenvironment in the mammalian body. The human host and microbiome within are so closely related that many consider it a single “super-organism” (Gill et al. 2006). It has now been well validated that the intestinal microbiota influences the physiology, nutritional status, immune function, and overall health status of the host (Morowitz et al. 2011; Bengmark 2013). The intensive care unit (ICU) is an extremely “artificial” environment. Invasive techniques (which eliminate or dramatically alter natural barriers) and the use of multiple medications (which affect GI motility, luminal pH, blood flow, and mucosal oxygenation) radically influence gut microbiota. The widespread use of broad spectrum antibiotics and the need to provide artificial nutrition therapy make the changes even more complex to understand. Within hours of the insult or injury which leads to admission to an ICU, the microbiome is dramatically and rapidly altered (Morowitz et al. 2011; Alverdy et al. 2005). These changes in the gut microbiota accompany the often major changes in intra-vascular volume and blood flow to the GI tract which commonly occurs in the ICU population. Therapeutic measures involved with critical care can cause dramatic changes in the mucosal redox potential and pH within the epithelial cell. Numerous secretory products of the GI tract (such as saliva, bile, mucous, immunoglobulins, and pancreatic enzymes) are all significantly altered in the ICU setting, changes which in turn will secondarily alter the gut microbiome.