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Background: Virtually no studies on the dynamics of the intestinal microbiota in patients admitted to the intensive care unit (ICU) are published, despite the increasingly recognized important role of microbiota on human physiology. Critical care patients undergo treatments that are known to influence the microbiota. However, dynamics and extent of such changes are not yet fully understood. To address this topic, we analyzed the microbiota before, during and after planned major cardio surgery that, for the first time, allowed us to follow the microbial dynamics of critical care patients. In this prospective, observational, longitudinal, single center study, we analyzed the fecal microbiota using 16S rRNA gene sequencing. Results: Samples of 97 patients admitted between April 2015 and November 2016 were included. In 32 patients, data of all three time points (before, during and after admission) were available for analysis. We found a large intra-individual variation in composition of gut microbiota. During admission, a significant change in microbial composition occurred in most patients, with a significant increase in pathobionts combined with a decrease in strictly anaerobic gut bacteria, typically beneficial for health. A lower bacterial diversity during admission was associated with longer hospitalization. In most patients analyzed at all three time points, the change in microbiota during hospital stay reverted to the original composition post-discharge. Conclusions: Our study shows that, even with a short ICU stay, patients present a significant change in microbial composition shortly after admission. The unique longitudinal setup of this study displayed a restoration of the microbiota in most patients to baseline composition post-discharge, which demonstrated its great restorative capacity. A relative decrease in benign or even beneficial bacteria and increase of pathobionts shifts the microbial balance in the gut, which could have clinical relevance. In future studies, the microbiota of ICU patients should be considered a good target for optimisation.

Background Although the important role of intestinal microbiota on human physiology is increasingly recognised, few studies on its dynamics in patients admitted to the intensive care unit (ICU) have been published. Disturbance of microbiota (dysbiosis) has been reported, but clinical consequences are unclear. We assessed the dynamics of intestinal microbiota before, during and after hospital admission in planned cardio-surgical patients admitted to the ICU. Methods In this prospective, observational, longitudinal, single centre study, we analysed the microbiota at three time points in patients scheduled for surgery using 16SrRNA-gene sequencing. Findings Samples of 97 patients admitted between April 2015 and November 2016 to our institution were included. We found a large inter-individual variation in gut microbiota. At baseline, some patients' microbiota appeared to be in a state of dysbiosis, others showed a profile consistent with that of healthy individuals. Most displayed a significant change from baseline towards a composition with a relative preponderance of potentially pathogenic species, to restore post-discharge. Interpretation Our study shows that even patients with a short ICU stay present a marked change in microbial composition shortly after admission, to restore post-discharge, while some patients' microbiota are in a state of dysbiosis before any intervention. Consequently, a relative decrease in benign or even beneficial bacteria and increase of pathobionts shifts the microbial balance in the gut, which could have clinical relevance. In future studies, the microbiota of ICU patients should be considered a good target for optimisation. Funding University Medical Center Groningen Healthy Ageing Committee, Marie SkA‚odowska-Curie Actions. Conflict of Interest: All authors declare no competing interests. Ethical Approval Statement: Ethical approval by the Medical Ethics Committee of our institution was received (approval number METc2014/206) and all research was conducted in accordance with the Declaration of Helsinki.