Our bodies harbour trillions of bacterial cells called the microbiota. In healthy people, distinct communities of microbiota are found at different body sites. These organisms confer beneficial advantages to our bodies such as metabolism (breaking down complex polysaccharides and vitamins synthesis), resistance to infection and immune homeostasis. In return we provide an ecological niche and nutrients to the microbes.

Perturbations of these ecosystems have been associated with a broad range of illnesses including asthma and obesity to sepsis and multiple organ dysfunction syndrome.

During critical illness, numerous aspects of the patient’s physiology are perturbed. Patients can be admitted to the intensive care unit (ICU) for various reasons, including infections, trauma and recent surgery. The focus of our study was to determine the impact of critical illness on the microbial ecosystem and investigate the influence of microbial disturbances on patient outcomes.

We found that in ICU patients there is a loss in distinction of microbiota between the anatomic sites.

While most studies focus on the microbiota in fecal samples, we also sampled the stomach and lung microbiota. In a healthy individual, the microbiota from different organ systems can usually be readily differentiated. However, we found that in ICU patients there is a loss in distinction of microbiota between the anatomic sites. The drastic microbial perturbation seen during critical illness could have significant impact on host physiology.

Another way to assess the microbiota is looking into how complex the community is within one patient sample, known as α-diversity. In stool samples, a higher α-diversity has been associated with health but little is known about other sample sites.

In our study, we found that a lower α-diversity value in the respiratory tract in the first few days of ICU admission was associated with a greater risk of mortality compared to those with higher α-diversity. This mortality difference between groups was not due to a different bacterial burden or different clinical interventions (e.g. antimicrobials, acid suppressant).

Microbiome research is a rapidly evolving field, and its clinical applications are only expanding. Our work raises the question of whether a microbial signature at the time of admission to the ICU could be used as a prognostic tool for decision making within the ICU.

Ultimately, we anticipate that investigating these microbial signatures could help stratify and identify high-risk patients in the ICU, where even slight improvements in care can improve patient outcomes.

Our study has just set the foundation for investigating the relationship between microbial diversity and patient’s outcomes in the ICU setting and larger studies are needed to further confirm our findings.