Gut–lung crosstalk during critical illness

Purpose of review Study of organ crosstalk in critical illness has uncovered complex biological communication between different organ systems, but the role of microbiota in organ crosstalk has received limited attention. We highlight the emerging understanding of the gut–lung axis, and how the largest biomass of the human body in the gut may affect lung physiology in critical illness. Recent findings Disruption of healthy gut microbial communities and replacement by disease-promoting pathogens (pathobiome) generates a maladaptive transmitter of messages from the gut to the lungs, connected via the portal venous and the mesenteric lymphatic systems. Gut barrier impairment allows for microbial translocation (living organisms or cellular fragments) to the lungs. Host-microbiota interactions in the gut mucosa can also impact lung physiology through microbial metabolite secretion or host-derived messengers (hormones, cytokines or immune cells). Clinical examples like the prevention of ventilator-associated pneumonia by selective decontamination of the digestive tract show that the gut–lung axis can be manipulated therapeutically. Summary A growing body of evidence supports the pathophysiological relevance of the gut–lung axis, yet we are only at the brink of understanding the therapeutic and prognostic relevance of the gut microbiome, metabolites and host-microbe interactions in critical illness.