Probiotic and synbiotic therapy in the critically ill: State of the art

•Commensal microorganisms play vital roles in human physiology in nutrition, vitamin synthesis, drug metabolism, protection against infection, and recovery from illness.•Data from our ongoing intensive care unit (ICU) microbiome research revealed a rapid and marked change from a “healthy” microbiome to disrupted microbiota (dysbiosis) in ICU and surgical patients. The loss of “health-promoting” bacteria and overgrowth of pathogenic bacteria (dysbiosis) in the ICU is believed to contribute to nosocomial infections, sepsis, and organ failure.•A wide range of experimental studies have demonstrated that probiotics and synbiotics (probiotic and prebiotic combinations) may provide clinical benefit by maintaining gut epithelial barrier, increasing generation of nutritional substrate for host epithelial cells, changing the host metabolic transcriptional landscape, and optimizing immune system function.•Probiotics and synbiotics show promise in reducing community infections, sepsis, ICU infections, and Clostridium difficile colitis; however larger, targeted trials are needed to define how and who to best implement “dybiosis-therapy” with probiotics and synbiotics.•Future probiotic and symbiotic studies using microbiome signatures to characterize actual ICU and patient illness-related dysbiosis and to determine, and perhaps even to personalize, ideal probiotic and symbiotic therapies are needed. Recent medical history has largely viewed our bacterial symbionts as pathogens to be eradicated rather than as essential partners in optimal health. However, one of the most exciting scientific advances in recent years has been the realization that commensal microorganisms (our microbiome) play vital roles in human physiology in nutrition, vitamin synthesis, drug metabolism, protection against infection, and recovery from illness. Recent data show that loss of “health-promoting” microbes and overgrowth of pathogenic bacteria (dysbiosis) in patients in the intensive care unit (ICU) appears to contribute to nosocomial infections, sepsis, and poor outcomes. Dysbiosis results from many factors, including ubiquitous antibiotic use and altered nutrition delivery in illness. Despite modern antibiotic therapy, infections and mortality from often multidrug-resistant organisms are increasing. This raises the question of whether restoration of a healthy microbiome via probiotics or synbiotics (probiotic and prebiotic combinations) to intervene on ubiquitous ICU dysbiosis would be an op