Panorganismal gut microbiome-host metabolic crosstalk

Role of gut microbiota in the control of energy and carbohydrate metabolism

PURPOSE OF REVIEW

To describe the recent developments and insights gained in the role played by the colonic microbiota in energy and carbohydrate metabolism related to obesity in humans.

RECENT FINDINGS

Previous findings that the ratio of Firmicutes and Bacteriodetes is important in energy harvesting and obesity have not been confirmed in recent studies. In fact, sometimes, the opposite results were obtained. Nevertheless, it is clear that the microbiota plays a role in energy extraction from nondigested carbohydrates in the form of production of short-chain fatty acids. Also, the microbiota plays a role in host metabolism by influencing and modulating host gene expression in various tissues.

SUMMARY

Despite numerous recent studies trying to link the composition of the microbiota to obesity, the picture is far from clear, and it remains to be seen whether changes in microbiota composition are the cause or the consequence of obesity. Molecular studies reveal the enzyme machineries used by individual members of the microbiota to break down and ferment polysaccharides. Also, the mechanisms of host-microbe mutualism are becoming unraveled. Using stable-isotope-labeled substrates, the exact microorganisms involved in fermentation of the substrates and the exact metabolites that are produced from the substrate can be deciphered.

Metabolic fate of polyphenols in the human superorganism

Dietary polyphenols are components of many foods such as tea, fruit, and vegetables and are associated with several beneficial health effects although, so far, largely based on epidemiological studies. The intact forms of complex dietary polyphenols have limited bioavailability, with low circulating levels in plasma. A major part of the polyphenols persists in the colon, where the resident microbiota produce metabolites that can undergo further metabolism upon entering systemic circulation. Unraveling the complex metabolic fate of polyphenols in this human superorganism requires joint deployment of in vitro and humanized mouse models and human intervention trials. Within these systems, the variation in diversity and functionality of the colonic microbiota can increasingly be captured by rapidly developing microbiomics and metabolomics technologies. Furthermore, metabolomics is coming to grips with the large biological variation superimposed on relatively subtle effects of dietary interventions. In particular when metabolomics is deployed in conjunction with a longitudinal study design, quantitative nutrikinetic signatures can be obtained. These signatures can be used to define nutritional phenotypes with different kinetic characteristics for the bioconversion capacity for polyphenols. Bottom-up as well as top-down approaches need to be pursued to link gut microbial diversity to functionality in nutritional phenotypes and, ultimately, to bioactivity of polyphenols. This approach will pave the way for personalization of nutrition based on gut microbial functionality of individuals or populations.

Gut microbiota in obesity and metabolic disorders

Obesity is a major public health issue as it is causally related to several chronic disorders, including type-2 diabetes, CVD and cancer. Novel research shows that the gut microbiota is involved in obesity and metabolic disorders, revealing that obese animal and human subjects have alterations in the composition of the gut microbiota compared to their lean counterparts. Moreover, transplantation of the microbiota of either obese or lean mice influences body weight in the germ-free recipient mice, suggesting that the gut ecosystem is a relevant target for weight management. Indigenous gut microbes may regulate body weight by influencing the host's metabolic, neuroendocrine and immune functions. The intestinal microbiota, as a whole, provides additional metabolic functions and regulates the host's gene expression, improving the ability to extract and store energy from the diet and contributing to body-weight gain. Imbalances in the gut microbiota and increases in plasma lipopolysaccharide may also act as inflammatory factors related to the development of atherosclerosis, insulin resistance and body-weight gain. In contrast, specific probiotics, prebiotics and related metabolites might exert beneficial effects on lipid and glucose metabolism, the production of satiety peptides and the inflammatory tone related to obesity and associated metabolic disorders. This knowledge is contributing to our understanding of how environmental factors influence obesity and associated diseases, providing new opportunities to design improved dietary intervention strategies to manage these disorders.

Determination of thein vivoprebiotic potential of a maize-based whole grain breakfast cereal: a human feeding study

Epidemiological studies have shown an inverse relationship between risk of CVD and intake of whole grain (WG)-rich food. Regular consumption of breakfast cereals can provide not only an increase in dietary WG but also improvements to cardiovascular health. Various mechanisms have been proposed, including prebiotic modulation of the colonic microbiota. In the present study, the prebiotic activity of a maize-derived WG cereal (WGM) was evaluated in a double-blind, placebo-controlled human feeding study (n32). For a period of 21 d, healthy men and women, mean age 32 (sd8) years and BMI 23·3 (sd0·58) kg/m2, consumed either 48 g/d WG cereal (WGM) or 48 g placebo cereal (non-whole grain (NWG)) in a crossover fashion. Faecal samples were collected at five points during the study on days 0, 21, 42, 63 and 84 (representing at baseline, after both treatments and both wash-out periods). Faecal bacteriology was assessed using fluorescencein situhybridisation with 16S rRNA oligonucleotide probes specific forBacteroidesspp.,Bifidobacteriumspp.,Clostridium histolyticum/perfringenssubgroup,Lactobacillus–Enterococcussubgroup and total bacteria. After 21 d consumption of WGM, mean group levels of faecal bifidobacteria increased significantly compared with the control cereal (P = 0·001). After a 3-week wash-out period, bifidobacterial levels returned to pre-intervention levels. No statistically significant changes were observed in serum lipids, glucose or measures of faecal output. In conclusion, this WG maize-enriched breakfast cereal mediated a bifidogenic modulation of the gut microbiota, indicating a possible prebiotic mode of action.

Utilization of amino acids by bacteria from the pig small intestine

This study determined the utilization of amino acids (AA) by bacteria from the lumen of the pig small intestine. Digesta samples from different segments of the small intestine were inoculated into media containing 10 mmol/L each of select AA (L-lysine, L-threonine, L-arginine, L-glutamate, L-histidine, L-leucine, L-isoleucine, L-valine, L-proline, L-methionine, L-phenylalanine or L-tryptophan) and incubated for 24 h. The previous 24-h culture served as an inoculum for a subsequent 24-h subculture during each of 30 subcultures. Results of the in vitro cultivation experiment indicated that the 24-h disappearance rates for lysine, arginine, threonine, glutamate, leucine, isoleucine, valine or histidine were 50-90% in the duodenum, jejunum or ileum groups. After 30 subcultures, the 24-h disappearance rates for lysine, threonine, arginine or glutamate remained greater than 50%. The denaturing gradient gel electrophoresis analysis showed that Streptococcus sp., Mitsuokella sp., and Megasphaera elsdenii-like bacteria were predominant in subcultures for utilizing lysine, threonine, arginine and glutamate. In contrast, Klebsiella sp. was not a major user of arginine or glutamate. Furthermore, analysis of AA composition and the incorporation of AA into polypeptides indicated that protein synthesis was a major pathway for AA metabolism in all the bacteria studied. The current work identified the possible predominant bacterial species responsible for AA metabolism in the pig small intestine. The findings provide a new framework for future studies to characterize the metabolic fate of AA in intestinal microbes and define their nutritional significance for both animals and humans.

An optimised sample preparation method for NMR-based faecal metabonomic analysis

Faecal metabonomic NMR analysis plays an essential role in investigating the interactions between mammalian metabolism and symbiotic gut microbiota. However, the faecal metabolite extraction method remains to be optimised and standardised to take into consideration signal-to-noise ratios, pH and chemical shift consistency. In the current investigation, we compared extraction consistency of three homogenisation methods including manual ultrasonication, automatic homogenization with tissuelyser and their combination, and systematically optimised faecal metabolite extraction parameters, including the faeces-to-buffer ratio (W(f) : V(b)), extraction repetition times and duration. We found that automatic homogenisation with tissuelyser was the choice of extraction method owning to its good metabolite extraction consistency and high throughput. We also recommend W(f) : V(b) of 1 : 10 (mg microl(-1)) and use of the combined first two extracts as the resultant samples to represent faecal metabolite composition. Such recommendation is based on considerations of maximisation of the spectral signal-to-noise ratio, pH and chemical shift consistency, completeness of metabolite extraction and sample preparation throughput so that the method is suitable for analysing a large number of samples especially in human population studies.

Probiotics and gut health: A special focus on liver diseases

Probiotic bacteria have well-established beneficial effects in the management of diarrhoeal diseases. Newer evidence suggests that probiotics have the potential to reduce the risk of developing inflammatory bowel diseases and intestinal bacterial overgrowth after gut surgery. In liver health, the main benefits of probiotics might occur through preventing the production and/or uptake of lipopolysaccharides in the gut, and therefore reducing levels of low-grade inflammation. Specific immune stimulation by probiotics through processes involving dendritic cells might also be beneficial to the host immunological status and help prevent pathogen translocation. Hepatic fat metabolism also seems to be influenced by the presence of commensal bacteria, and potentially by probiotics; although the mechanisms by which probiotic might act on the liver are still unclear. However, this might be of major importance in the future because low-grade inflammation, hepatic fat infiltration, and hepatitis might become more prevalent as a result of high fat intake and the increased prevalence of obesity.

Microecology, intestinal epithelial barrier and necrotizing enterocolitis

Soon after birth, the neonatal intestine is confronted with a massive antigenic challenge of microbial colonization. Microbial signals are required for maturation of several physiological, anatomical, and biochemical functions of intestinal epithelial barrier (IEB) after birth. Commensal bacteria regulate intestinal innate and adaptive immunity and provide stimuli for ongoing repair and restitution of IEB. Colonization by pathogenic bacteria and/or dysmature response to microbial stimuli can result in flagrant inflammatory response as seen in necrotizing enterocolitis (NEC). Characterized by inflammation and hemorrhagic-ischemic necrosis, NEC is a devastating complication of prematurity. Although there is evidence that both prematurity and presence of bacteria, are proven contributing factors to the pathogenesis of NEC, the molecular mechanisms involved in IEB dysfunction associated with NEC have begun to emerge only recently. The metagenomic advances in the field of intestinal microecology are providing insight into the factors that are required for establishment of commensal bacteria that appear to provide protection against intestinal inflammation and NEC. Perturbations in achieving colonization by commensal bacteria such as premature birth or hospitalization in intensive care nursery can result in dysfunction of IEB and NEC. In this article, microbial modulation of functions of IEB and its relationship with barrier dysfunction and NEC are described.

Bacterial infections: new and emerging enteric pathogens

PURPOSE OF REVIEW

The aim of this review is to highlight recent advances in knowledge of bacterial enteric infections. We focus on understanding of enterohemorrhagic Escherichia coli O157:H7 and Campylobacter jejuni infections, and to link these acute events with long-term consequences in a susceptible host, including irritable bowel syndrome and chronic inflammatory bowel diseases.

RECENT FINDINGS

Enterohemorrhagic E. coli and C. jejuni are zoonotic infections that are acquired from exposure to tainted food (undercooked hamburger and chicken, respectively) and contaminated drinking water. Noninvasive E. coli O157:H7 elaborates Shiga-like toxins and protein effectors that are injected, via a molecular syringe that is encoded by a bacterial type 3 secretion system, into infected eukaryotic cells. Less is known about the precise virulence properties of enteroinvasive Campylobacter strains, but both enteric pathogens are able to disrupt polarized epithelial monolayers resulting in increased uptake of macromolecules and antigens.

SUMMARY

An improved understanding of the epidemiology, pathobiology and mechanisms underlying infectious enterocolitides will provide the basis for developing new intervention strategies including, for example, the use of probiotics, to interrupt the infectious process.

Gut microbiology: fitting into the intestinal neighbourhood

Microbes inhabiting the gut affect our health in profound and unexpected ways: new studies now show that these effects depend on synergistic and competitive interactions between the bacteria, which are influenced by diet.

Monitoring healthy metabolic trajectories with nutritional metabonomics

Metabonomics is a well established analytical approach for the analysis of physiological regulatory processes via the metabolic profiling of biofluids and tissues in living organisms. Its potential is fully exploited in the field of “nutrimetabonomics” that aims at assessing the metabolic effects of active ingredients and foods in individuals. Yet, one of the greatest challenges in nutrition research is to decipher the critical interactions between mammalian organisms and environmental factors, including the gut microbiota. “Nutrimetabonomics” is today foreseen as a powerful approach for future nutritional programs tailored at health maintenance and disease prevention.

Metabolomics for assessment of nutritional status

PURPOSE OF REVIEW

The current rise in diet-related diseases continues to be one of the most significant health problems facing both the developed and the developing world. The use of metabolomics - the accurate and comprehensive measurement of a significant fraction of important metabolites in accessible biological fluids - for the assessment of nutritional status is a promising way forward. The basic toolset, targets and knowledge are all being developed in the emerging field of metabolomics, yet important knowledge and technology gaps will need to be addressed in order to bring such assessment to practice.

RECENT FINDINGS

Dysregulation within the principal metabolic organs (e.g. intestine, adipose, skeletal muscle and liver) are at the center of a diet-disease paradigm that includes metabolic syndrome, type 2 diabetes and obesity. The assessment of both essential nutrient status and the more comprehensive systemic metabolic response to dietary, lifestyle and environmental influences (e.g. metabolic phenotype) are necessary for the evaluation of status in individuals that can identify the multiple targets of intervention needed to address metabolic disease.

SUMMARY

The first proofs of principle building the knowledge to bring actionable metabolic diagnostics to practice through metabolomics are now appearing.

Intestinal dysbiosis: a possible mechanism of alcohol-induced endotoxemia and alcoholic steatohepatitis in rats

BACKGROUND

Clinical and animal data indicate that gut-derived endotoxin and other luminal bacterial products are necessary cofactors for development of alcoholic liver disease (ALD). Although gut leakiness is clearly an important cause of endotoxemia in ALD, it cannot fully explain endotoxemia in all ALD subjects and thus other factors may be involved. One possible factor is a change in gut microbiota composition (dysbiosis). Thus, the aim of our study was to interrogate the gut bacterial microbiota in alcohol-fed rats to see if chronic alcohol consumption affects gut bacteria composition.

METHOD

Male Sprague-Dawley rats were given either alcohol or dextrose intragastrically by gavage twice daily for up to 10 weeks. A subgroup of rats was also given either a probiotic (lactobacillus GG) or a prebiotic (oats) by gavage. Ileal and colonic mucosal-attached microbiota composition were interrogated by Length Heterogeneity PCR (LH-PCR) fingerprinting.

RESULTS

Bacterial microbiota composition in alcohol-fed rats is not different from dextrose-fed rats at weeks 4 and 6. Mucosa-associated microbiota composition in the colon is altered at 10 weeks of daily alcohol gavage. Both LGG and oats prevented alcohol-induced dysbiosis up to 10 weeks of alcohol treatment.

CONCLUSION

Daily alcohol consumption for 10 weeks alters colonic mucosa-associated bacterial microbiota composition in rats. Our data showed, for the first time, that daily alcohol consumption can affect colonic microbiome composition and suggest that dysbiosis may be an important mechanism of alcohol-induced endotoxemia. Further studies are needed to determine how dysbiotic microbiota contributes to development of ALD and whether therapeutic interventions targeted towards dysbiotic microbiota can prevent complications of alcoholism like ALD.

Intestinal dysbiosis: a possible mechanism of alcohol-induced endotoxemia and alcoholic steatohepatitis in rats

BACKGROUND

Clinical and animal data indicate that gut-derived endotoxin and other luminal bacterial products are necessary cofactors for development of alcoholic liver disease (ALD). Although gut leakiness is clearly an important cause of endotoxemia in ALD, it cannot fully explain endotoxemia in all ALD subjects and thus other factors may be involved. One possible factor is a change in gut microbiota composition (dysbiosis). Thus, the aim of our study was to interrogate the gut bacterial microbiota in alcohol-fed rats to see if chronic alcohol consumption affects gut bacteria composition.

METHOD

Male Sprague-Dawley rats were given either alcohol or dextrose intragastrically by gavage twice daily for up to 10 weeks. A subgroup of rats was also given either a probiotic (lactobacillus GG) or a prebiotic (oats) by gavage. Ileal and colonic mucosal-attached microbiota composition were interrogated by Length Heterogeneity PCR (LH-PCR) fingerprinting.

RESULTS

Bacterial microbiota composition in alcohol-fed rats is not different from dextrose-fed rats at weeks 4 and 6. Mucosa-associated microbiota composition in the colon is altered at 10 weeks of daily alcohol gavage. Both LGG and oats prevented alcohol-induced dysbiosis up to 10 weeks of alcohol treatment.

CONCLUSION

Daily alcohol consumption for 10 weeks alters colonic mucosa-associated bacterial microbiota composition in rats. Our data showed, for the first time, that daily alcohol consumption can affect colonic microbiome composition and suggest that dysbiosis may be an important mechanism of alcohol-induced endotoxemia. Further studies are needed to determine how dysbiotic microbiota contributes to development of ALD and whether therapeutic interventions targeted towards dysbiotic microbiota can prevent complications of alcoholism like ALD.

Effects of probiotics and commensals on intestinal epithelial physiology: implications for nutrient handling

Eukaryotes and prokaryotes have developed mutually beneficial relationships over millennia of evolutionary adaptation. Bacteria in our gut rely on our diet and the protected environment of our bodies just as our health depends on byproducts of microbial metabolism. Microorganisms of the gut microbiota ferment carbohydrates into short-chain fatty acids, convert dietary and endogenous nitrogenous compounds into ammonia and microbial protein, and synthesize and activate B vitamins and vitamin K. The benefit from their activity is multiplex and translates into increased energy for the gut epithelial cells, balanced absorption of salt and water, nitrogen recycling, breakdown of complex lipids and cholesterol, and detoxification of waste compounds.