One of the most notable findings to arise from this analysis was the importance of the ratios between specific bacterial taxa in predicting mortality risk. Previous studies of the gut microbiome in critical illness showed both a loss of bacterial diversity [8–9, 2–13, 25] and a rise in the presence of pathologic bacteria [7, 11–12, 15, 26,] among critically ill patients. One recent study of twelve patients found that differences in relative abundance levels of Bacteroidetes and Firmicutes phyla corresponded to differences in subsequent mortality [10]. Similar to earlier studies, the current study confirmed that patients with increased mortality had a lower level of bacterial diversity and increased levels of pathogenic taxa. However, the current goes a step further by establishing the importance of ratios between pathogenic bacteria and commensal organisms in the gut in patients with increased mortality.
In developing our MMI, we found that an increased risk of mortality was characterized by an increase in GPAC and Enterobacteriaceae at the expense of commensal taxa. GPACs are anaerobic cocci that typically colonize the skin and mucosal surface of the human body [27]. When patients become immunocompromised, or develop skin wounds, GPACs can cause invasive infections [28]. The presence of GPACs as a key component of our MMI predictor suggests that invasion of these organisms is a driver of mortality in critically ill patients. Gram negative rods, such as the Enterobacteriaceae present as a component of our MMI, are a well-known pathogen in critically ill ICU patients, present in many nosocomial infections [29]. For our analysis, we grouped these organisms as a family because we because we found that taxa within the Enterobacteriaceae family could not be reliably distinguished from one another in our dataset. Enterobacteriaceae infections are significant in critically ill patients because they are associated with widespread antimicrobial resistance [30].
Components of the MMI that were inversely associated with mortality were the Parasutterella and Campylobacter taxa. Parasutterella is a gram negative anaerobic organism, and is understood to be a core member of the healthy gut microbiome [31]. The presence of Parasutterella in the denominator of the MMI suggests that decreased commensal organisms are associated with increased mortality in the ICU. The presence of Campylobacter in the denominator of the MMI is somewhat more difficult to explain. Campylobacter is a commensal organism in many animals, but is typically a mediator of food-borne illness in humans [32]. Its role in the MMI is unclear. One explanation for its role in the MMI involves its interaction with carbohydrate and amino acid metabolism, which are converted to SCFAs in states of symbiosis. Campylobacter was recently shown to have enhanced growth in the presence of Bacteroides vulgatus, possibly due to a scavenging interaction [32]. We observed a positive correlation between Campylobacter and butyrogenic genera such as Odoribacter, Lachnoclostridium, and Butyricoccus, as well as the protective Akkermansia genus and potentially butyrogenic Colidextribacter. Notably, Parasutterella had negative correlations with the latter two, suggesting that Campylobacter signals the presence of certain commensals that Parasutterella does not. In other words, if Campylobacter are abundant, something may be feeding them.
Another notable finding arising from this analysis with the effect of the MMI as a predictor of mortality. Consistent with earlier studies, we found that the presence of ARDS as well as a high APACHE II score were significant predictors of 28-day mortality in critically ill patients [33–34]. However, we also found that MMI was a significant predictor of 28-day mortality and outperformed both ARDS and APACHE II, with a multivariate model containing MMI, ARDS, and APACHE II a particularly strong predictor of mortality. This result suggests that MMI and specifically disruption of homeostasis in the intestinal tract of critically ill patients is predictive of increased mortality. Implicit in this finding is that correction of this dysbiosis may lead to decreased mortality.
Our study had several limitations. First, the development of MMI was limited by our small sample size. We attempted to minimize the impact of small sample size in the development of our model using cross validation to minimize overfitting of the model to a small dataset. We were also limited by our lack of an independent validation cohort. This will need to be addressed in future studies.
In summary, we have developed the MMI, a biomarker based upon ratios of different taxa from the gut. The MMI is an accurate predictor of 28-day mortality that augments previous mortality predictors in the ICU, including the presence of ARDS and APACHE II score. Further studies will be necessary to confirm these findings, including confirmation in an independent validation cohort.