Decreased Intestinal Microbiome Diversity in Pediatric Sepsis: A Conceptual Framework for Intestinal Dysbiosis to Influence Immunometabolic Function

The enteroendocrine axis and its effect on gastrointestinal function, nutrition, and inflammation

PURPOSE OF REVIEW

Gastrointestinal (GI) dysfunction limits enteral nutrition (EN) delivery in critical illness and contributes to systemic inflammation. The enteroendocrine (EE) axis plays an integral role in this interface between nutrition, inflammation, and GI function in critical illness. In this review, we present an overview of the EE system with a focus on its role in GI inflammation and function.

RECENT FINDINGS

Enteroendocrine cells have been primarily described in their role in macronutrient digestion and absorption. Recent research has expanded on the diverse functions of EE cells including their ability to sense microbial peptides and metabolites and regulate immune function and inflammation. Therefore, EE cells may be both affected by and contribute to many pathophysiologic states and interventions of critical illness such as dysbiosis , inflammation, and alternative EN strategies. In this review, we present an overview of EE cells including their growing role in nonnutrient functions and integrate this understanding into relevant aspects of critical illness with a focus on EN.

SUMMARY

The EE system is key in maintaining GI homeostasis in critical illness, and how it is impacted and contributes to outcomes in the setting of dysbiosis , inflammation and different feeding strategies in critical illness should be considered.

The Microbiome and Metabolome of the Gut of Children with Sepsis and Septic Shock

BACKGROUND

There is limited understanding of alteration of gut microbiota and metabolome in children with sepsis/septic shock.

METHODS

In this prospective observational study carried out in a pediatric intensive care unit of a tertiary care center from 2020 to 2022, patients aged <17 years with sepsis/septic shock and healthy children (HC) were enrolled. We characterized the gut bacterial compositions by metagenome sequencing and metabolomes by untargeted gas chromatography-mass spectrometry. The primary outcome was to compare the gut microbiota and metabolome of children with sepsis/septic shock with that of HC. The Firmicutes/Bacteroidetes (F/B) ratio was compared between children with sepsis/septic shock and HC. Key secondary outcomes were to evaluate association of factors associated with a low F/B ratio in children with sepsis/septic shock.

RESULTS

A total of 40 children (63% boys) (15 children with sepsis and septic shock and 10 healthy children) with a median (IQR) age of 5.5 (1.5, 10) years were enrolled. In the fecal microbiota, the α-diversity index including Shannon and Simpson indices of the sepsis/septic shock groups was significantly lower than that of the HC. The samples lacked beneficial Bifidobacterium spp. and were dominated by Bacteroides, Enterobacteriaceae, and Enterococcaceae. There was reduction in short-chain fatty acids (SCFAs) in patients with sepsis/septic shock as compared to healthy children. A lower F/B ratio (≤1.57) of the gut microbiota discriminated well between children with sepsis/septic shock and HC. Factors associated with lower F/B ratio were male gender, clinical GI dysfunction, elevated inflammatory markers, and higher organ failure scores.

CONCLUSION

There were significant alterations in the gut microbiota and metabolome in children with sepsis/septic shock as compared to healthy children. Larger study is needed to confirm these exploratory findings and develop potential therapeutic targets that will improve outcomes in children with sepsis/septic shock.

Biomarkers and Functional Assays of Epithelial Barrier Disruption and Gastrointestinal Dysmotility in Critical Illness-A Narrative Review

Enteral nutrition in critically ill children has been associated with improved clinical outcomes. Gastrointestinal dysfunction often impedes the timely initiation and advancement of enteral nutrition and can contribute to immune dysregulation and systemic inflammation. Therefore, assessing gastrointestinal function, at a cellular and functional level, is important to provide optimal enteral nutrition therapy and reduce the gastrointestinal tract's contribution to the inflammatory cascade of critical illness. In this narrative review, we present an overview of biomarker and functional assays for gastrointestinal dysfunction, including epithelial barrier disruption and gastrointestinal dysmotility, that have been considered for critically ill patients.

Sodium butyrate reverses lipopolysaccharide-induced mitochondrial dysfunction in lymphoblasts

Butyrate is a short-chain fatty acid that is produced by commensal microbes within the intestinal microbiome through fermentation of dietary fibre. Microbial-derived butyrate has been shown to promote immunologic and metabolic homeostasis, in part through its beneficial effects on mitochondrial function, and thus has been proposed as a possible anti-inflammatory therapy. We tested the hypothesis that butyrate could mitigate the decrease in mitochondrial respiration in immune cells under septic conditions as a preliminary step towards better understanding the potential for butyrate as a novel therapy in sepsis. Mitochondrial respiration and content (measured as citrate synthase activity) were compared within four Epstein-Barr virus-transformed lymphoblast (LB) cell lines exposed to either control media or lipopolysaccharide (LPS) 100 ng/ml. Both co-incubation of LBs with LPS + butyrate and treatment with butyrate after LPS stimulation reversed the decrease in mitochondrial respiration observed in LBs exposed to LPS without butyrate. Neither LPS nor butyrate led to significant changes in citrate synthase activity. The preliminary findings support further investigation of a potential mitochondrial-based mechanism through which butyrate may help to mitigate the immuno-inflammatory response in sepsis.

The Prebiotic Effect of Australian Seaweeds on Commensal Bacteria and Short Chain Fatty Acid Production in a Simulated Gut Model

Diet is known to affect the composition and metabolite production of the human gut microbial community, which in turn is linked with the health and immune status of the host. Whole seaweeds (WH) and their extracts contain prebiotic components such as polysaccharides (PS) and polyphenols (PP). In this study, the Australian seaweeds, Phyllospora comosa, Ecklonia radiata, Ulva ohnoi, and their PS and PP extracts were assessed for potential prebiotic activities using an in vitro gut model that included fresh human faecal inoculum. 16S rRNA sequencing post gut simulation treatment revealed that the abundance of several taxa of commensal bacteria within the phylum Firmicutes linked with short chain fatty acid (SCFA) production, and gut and immune function, including the lactic acid producing order Lactobacillales and the chief butyrate-producing genera Faecalibacteria, Roseburia, Blautia, and Butyricicoccus were significantly enhanced by the inclusion of WH, PS and PP extracts. After 24 h fermentation, the abundance of total Firmicutes ranged from 57.35−81.55% in the WH, PS and PP samples, which was significantly greater (p ≤ 0.01) than the inulin (INU) polysaccharide control (32.50%) and the epigallocatechingallate (EGCG) polyphenol control (67.13%); with the exception of P. comosa PP (57.35%), which was significantly greater than INU only. However, all WH, PS and PP samples also increased the abundance of the phylum Proteobacteria; while the abundance of the phylum Actinobacteria was decreased by WH and PS samples. After 24 h incubation, the total and individual SCFAs present, including butyric, acetic and propionic acids produced by bacteria fermented with E. radiata and U. ohnoi, were significantly greater than the SCFAs identified in the INU and EGCG controls. Most notably, total SCFAs in the E. radiata PS and U. ohnoi WH samples were 227.53 and 208.68 µmol/mL, respectively, compared to only 71.05 µmol/mL in INU and 7.76 µmol/mL in the EGCG samples. This study demonstrates that whole seaweeds and their extracts have potential as functional food ingredients to support normal gut and immune function.

Tipping the Balance: Vitamin D Inadequacy in Children Impacts the Major Gut Bacterial Phyla

Vitamin D inadequacy appears to be on the rise globally, and it has been linked to an increased risk of osteoporosis, as well as metabolic, cardiovascular, and autoimmune diseases. Vitamin D concentrations are partially determined by genetic factors. Specific single nucleotide polymorphisms (SNPs) in genes involved in vitamin D transport, metabolism, or binding have been found to be associated with its serum concentration, and these SNPs differ among ethnicities. Vitamin D has also been suggested to be a regulator of the gut microbiota and vitamin D deficiency as the possible cause of gut microbial dysbiosis and inflammation. This pilot study aims to fill the gap in our understanding of the prevalence, cause, and implications of vitamin D inadequacy in a pediatric population residing in Qatar. Blood and fecal samples were collected from healthy subjects aged 4–14 years. Blood was used to measure serum metabolite of vitamin D, 25-hydroxycholecalciferol 25(OH)D. To evaluate the composition of the gut microbiota, fecal samples were subjected to 16S rRNA gene sequencing. High levels of vitamin D deficiency/insufficiency were observed in our cohort with 97% of the subjects falling into the inadequate category (with serum 25(OH)D < 75 nmol/L). The CT genotype in rs12512631, an SNP in the GC gene, was associated with low serum levels of vitamin D (ANOVA, p = 0.0356) and was abundant in deficient compared to non-deficient subjects. Overall gut microbial community structure was significantly different between the deficient (D) and non-deficient (ND) groups (Bray Curtis dissimilarity p = 0.049), with deficient subjects also displaying reduced gut microbial diversity. Significant differences were observed among the two major gut phyla, Firmicutes (F) and Bacteroidetes (B), where deficient subjects displayed a higher B/F ratio (p = 0.0097) compared to ND. Vitamin D deficient children also demonstrated gut enterotypes dominated by the genus Prevotella as opposed to Bacteroides. Our findings suggest that pediatric vitamin D inadequacy significantly impacts the gut microbiota. We also highlight the importance of considering host genetics and baseline gut microbiome composition in interpreting the clinical outcomes related to vitamin D deficiency as well as designing better personalized strategies for therapeutic interventions.