The first part of this review focuses on the role of microbiota in obesity.
2.1. The Role and Composition of Gut Microbiota in Developing Elements of the Metabolic Syndrome with Special Regard to Obesity
Early studies aimed to clarify whether there are significant differences in the gut microbiome in lean and in obese subjects in animal models as well as in humans and investigated the possible relevance of these differences. The first studies were performed using laboratory mouse models, which have the advantage that many confounding factors such as the environment, diet and genotype are known or can be controlled.
Backhed et al. in 2004 [8
] investigated adult GF C57BL/6 mice. These animals are protected against developing obesity caused by consuming a high-fat, high-sugar diet. It was demonstrated that colonizing the mice’s GI with normal microbiota harvested from the distal intestine (cecum) of conventionally raised obese animals produced a 60% increase in body fat content and insulin resistance within 14 days despite reduced food intake.
Furthermore, the authors could demonstrate that obesity affects the diversity of the gut microbiota [9
]. They analysed 5088 bacterial 16S rRNA gene sequences from the cecal microbiota in genetically obese ob/ob mice, lean ob/+ and wild-type siblings, and their ob/+ mothers, all fed the same polysaccharide-rich diet. The authors could demonstrate that, compared with lean mice and regardless of kinship, ob/ob animals have a 50% reduction in the abundance of Bacteroidetes
and a proportional increase in Firmicutes
. These changes indicate that, in this model, obesity affects the diversity of gut microbiota in mice.
In an attempt to explain the impact of the different microbiota between lean and obese animals, the authors [10
] speculated that the microbiota of the obese animals might be more efficient in extracting energy from their diet than the microbiota of the lean animals. By shotgun sequencing [3
], the authors compared the microbiota of obese mice (ob/ob) with those of lean mice (ob/+). They could demonstrate that the ob/ob genome was enriched with environmental gene tags encoding enzymes involved in the initial steps in breaking down dietary polysaccharides that are otherwise indigestible. The enrichment concerns eight glycoside hydrolase families, which are capable of degrading dietary polysaccharides and starch. Furthermore, genes for encoding proteins importing products of these glycoside hydrolases, the so-called ATP-binding cassette (ABC) transporters, the metabolization (e.g., alpha and beta-galactosidases), and generating the end products of fermentation (butyrate and acetate) are significantly enriched in the microbiota of the ob/ob mice. That said, the authors concluded that these mice have a gut microbiome with an increased capacity for energy harvest.
In a landmark study, Ridaura et al. [12
] were the first to transplant human faeces into GF mice. It was demonstrated that culture collections generated from human microbiota samples can transmit the donor phenotype of mice: they transplanted faecal microbiota from human adult female twin pairs discordant for obesity into germ-free mice fed low-fat mouse feed, as well as diets representing different levels of saturated fat and fruit and vegetable consumption. The “human obesity” could be transferred and the mice developed increased total body and fat mass, as well as obesity-associated metabolic phenotypes. In mice containing the lean co-twin’s microbiota, this prevented the development of increased body mass and obesity-associated metabolic phenotypes.
To define the effects of diet on lean (ln) and obese (ob) microbiota-mediated transmission of body composition and metabolic phenotypes, they constructed a diet made with foods that characterize diets representing the lower tertile of consumption of saturated fats and the upper tertile of consumption of fruits and vegetables. Significant differences in body composition were documented between ob/ob and ln/ln mice consuming this diet.
Additionally, the authors used co-housing to determine whether exposure of a mouse harbouring a culture collection from the lean twin could prevent development of the increased adiposity phenotype and microbiome-associated metabolic profile of a cagemate colonized with the culture collection from its obese co-twin, or vice versa (mice are coprophagic). Interestingly, ob mice exhibited a significantly lower increase in adiposity compared to control ob animals that had never been exposed to mice harbouring the lean co-twin’s culture collection.
These findings demonstrate that a donor phenotype can be transmitted into the phenotype of a recipient. However, diet still plays a central role in developing an obese or a lean phenotype [12
The possible diversity of human gut microbiota and its impact on metabolic syndrome with special regard to lean and obese individuals had also been addressed by Le Chatelier et al. in 2013 [13
]. The authors studied the human gut microbial composition in a population sample of 123 non-obese and 169 obese Danish individuals. Their approach was to compare the gene number across the total study sample of individuals, which showed a bimodal distribution of bacterial genes. They termed individuals with <480,000 genes ‘low gene count’ (LGC) and the others ‘high gene count’ (HGC). These had on average 380,000 and 640,000 genes, respectively, suggestive of a more “rich” microbiota and more diverse microbial communities. Forty-six genera differed significantly in abundance between the HGC and LGC individuals. At the phylum level, this shift resulted in a higher abundance of Proteobacteria
in LGC individuals versus increased populations of Verrucomicrobia, Actinobacteria
in HGC individuals. Interestingly, at the genera level, there is a contrast between the distribution of anti-inflammatory species, such as Faecalibacterium prausnitzii
, which are more prevalent in HGC individuals. Potentially pro-inflammatory bacteria such as Bacteroides
and R. gnavus
—which are also associated with inflammatory bowel disease [14
]—were found to be more frequent in LGC individuals. These findings are suggestive of the domination of potentially pro-inflammatory bacteria in obese subjects. The LGC subjects had more marked overall adiposity, insulin resistance and dyslipidaemia. The phenotype of the LGC subjects was also associated with increased serum leptin, decreased serum adiponectin, insulin resistance, increased levels of triglycerides, free fatty acids, decreased HDL-cholesterol and a more marked inflammatory phenotype (increased highly sensitive C-reactive protein). Obese individuals among the lower bacterial richness group also gained more weight over time. However, it needs to be pointed out that not all LGC subjects were obese and not all HGC subjects were lean. This reflects that there is still an impact of the “own” genome of the individual. The latter is the “human” part of the “metagenome”, a term used for the composite of the human genome and the genomes of the microbiota colonizing the human body.
In terms of the distribution of phylae associated with the obese phenotype, an increase in the phylum Firmicutes
and a decrease in Bacteroidetes
associated with obesity were also observed in some previous studies [16
]. However, Schwiertz et al. [18
] reported in a total of 98 subjects that the most abundant bacterial groups in the faeces of lean and obese subjects were the phyla Firmicutes
. The ratio of Firmicutes
changed in favour of the Bacteroidetes
in overweight and obese subjects.
Interestingly, in a subgroup of subjects from the Le Chantelier study [19
] (38 obese and 11 overweight individuals), dietary intervention (6-week energy restricted high-protein diet followed by a 6-week weight-maintenance diet) improved low gene richness and the clinical phenotypes, but seemed to be less efficient for inflammation parameters in individuals with lower gene richness.
2.2. The Role and Composition of Gut Microbiota in Developing Elements of the Metabolic Syndrome with Special Regard to Diabetes Mellitus Type 2
The above-mentioned studies demonstrated the possible significance of the composition of gut microbiota in obesity. As to be expected, the typically obesity-related T2 DM was soon another focus of research.
A first study by Larsen et al. [20
] included 36 male adults. 18 subjects were diagnosed with diabetes type 2; they had a mean age of 56 years, and a mean body mass index (BMI) of 30. The other 18 controls had a mean age of 59 years, and a mean BMI of 28. The total bacterial counts were similar in the diabetic and the control group. By characterization of the intestinal microbiota by tag-encoded pyrosequencing, the authors demonstrated that the proportion of Firmicutes
was significantly higher in the controls compared to the diabetic group. The phylum Bacteroidetes
were somewhat but not significantly enriched in the diabetic group compared to controls.
The authors concluded that T2 DM is associated with compositional changes in the intestinal microbiota mostly apparent at the phylum and class levels. These results are in agreement with the recent evidence obtained for overweight persons by Schwiertz and colleagues [18
] though they contradict other previously mentioned studies [13
]. Given the a.m. findings, a positive correlation between ratios of Bacteroidetes
and BMI could be expected. The reverse tendency was observed, which was regarded as indicative that overweight and T2 DM are associated with different groups of the intestinal microbiota.
However, the study group was small and no information was given on the therapy of the patients with diabetes, a factor also likely to have an impact on the microbiota.
Qin et al. [21
] performed a metagenome-wide association study in 345 Chinese T2 DM subjects. It was designed as a case-control study with non-diabetic controls. Using a shotgun-sequencing approach, the authors detected a “moderate” degree of dysbiosis in the diabetic subjects. This meant—in comparison to the controls—a decrease in the abundance of various butyrate producing bacteria, including Clostridiales
sp. SS3/4, Eubacterium rectale
, Faecalibacterium prausnitzii
, Roseburia intestinalis
and Roseburia inulinivorans
(all these bacteria belong to the phyla Firmicutes
). Furthermore, they identified more opportunistic pathogens such as Bacteroides caccae
, Clostridium hathewayi
, Clostridium ramosum
, Clostridium symbiosum
, Eggerthella lenta
and Escherichia coli
. At the pathway level, the gut microbiota of T2 DM patients was functionally characterized by an enrichment in the membrane transport of sugars, branched-chain amino acid (BCAA) transport, methane metabolism, xenobiotics degradation and metabolism, and sulphate reduction. This does to some degree support the concept of an increased capacity for energy harvest proposed by Turnbaugh [10
]. By contrast, there was a decrease in the level of bacterial chemotaxis, flagellar assembly, butyrate biosynthesis and metabolism of cofactors and vitamins. Comparable changes in intestinal bacteria composition have recently been reported for ageing people [22
]. The findings may also suggest a protective role for butyrate-producing bacteria against T2 DM [23
In a comparable approach, Karlsson et al. [24
] investigated the faecal microbiota in 145 70-year old women—53 women had T2 DM, impaired glucose tolerance was present in 49 women, and 43 had normal glucose tolerance (NGT). In contrast to previous reports on observations between lean and obese people, the faecal microbiota of non-diabetic, impaired glucose tolerance (IGT) and T2 DM women contained similar numbers of genes. In the diabetic group, the authors observed an increase in the abundance of four Lactobacillus
species and decreases in the abundance of five Clostridium
species. As for metabolic control, Lactobacillus
species correlated positively with fasting glucose and glycated hemoglobin (HbA1c), whereas Clostridium
species correlated negatively with fasting glucose, HbA1c and other metabolic markers. Both species were in no correlation with the BMI of the test subjects. In a metagenomic cluster model, they identified Roseburia
and Faecali bacterium prausnitzii
as highly discriminant for T2 DM. These bacteria are known as human gut colonizers and butyrate producers. Such bacteria have been reported to improve diabetic control and insulin sensitivity [23
], aspects that will be discussed later in this review.
In accordance with the study of Qin [21
] and Karlsson [24
] that supports the concept of an “increased capacity for energy harvest”, the non-diabetic and the T2 DM bacterial communities had different functional compositions and several pathways were differentially abundant in T2 DM and NGT women. These pathways which showed the highest scores for enrichment in T2 DM metagenomes included starch and glucose metabolism, fructose and mannose metabolism and ABC transporters for amino acids, ions and simple sugars.
As to be expected in different ethnicities and in those with different nutritional habits, many metagenomic markers for T2 DM are different between the Chinese and the European cohort; however, in agreement with the previous study, they observed that Clostridium clostridioforme metagenomic clusters were increased whereas Roseburia_272 was decreased in T2 DM metagenomes. As for medication possibly confounding the results, the authors report that only two of the species included in their statistical model were affected by the use of metformin (Clostridium botulinum Bstr.Eklund 17B and Clostridium sp. 7_2_43FAA).
Shortcomings of the above-mentioned studies include the relatively low number of patients analysed, the lack of a gender balance and the matching of the cohorts (e.g., age, BMI). The use of proton pump inhibitors and previous antibiotic therapy was not reported. Possible effects of antidiabetic therapy are not addressed in the Qin study [21
] and the “insignificant influence” of metformin in the Karlsson [24
] study raises doubts, as several effects of metformin, as well as of acarbose, on gut microbiota are reported and will be discussed later [25
]. It can be assumed that in the European cohort there was a widespread use of metformin. On the other hand, acarbose is extensively used with Chinese patients [25
However, the observation that a certain composition of gut microbiota may have an important impact on the aetiology of T2 DM, as well as on its control, definitely represents a major upgrade in our understanding.