At birth, the gut microbiota of an infant is sterile but rapidly assembles over days or months. Mode of delivery (natural delivery versus caesarean section) and feeding method (breast feeding versus bottle feeding) have an early impact on the development of a child’s gut microbiome. At the age of four, the gut microbiota is fully mature. Eventually, each person develops a unique gut microbiota which is stable over time in healthy adults.
In this cross-sectional study, the obese gut microbiota composition was compared with that of a lean one. We focused on two major phyla Bacteroidetes and Firmicutes, next to the Bacteroides fragilis group, Bifidobacterium, Clostridium, Staphylococcus and Lactobacillus. Different bacterial groups were selected according to the frequency to which they have been described in relevant literature[17–19, 24] and the ease of detection by the techniques used. On the one hand, quantitative plating was used as the ‘gold standard’ technique to isolate and characterise the selected bacterial groups. However, only 10 to 50% of all bacteria associated with the human body can be cultivated successfully[23, 25]. Subsequently, high-throughput culture-independent techniques, which use DNA sequences encoding the 16S ribosomal RNA subunit, were applied in order to assign an organism to a phylogenetic classification more accurately.
To our knowledge, our study was the first to perform an in-depth analysis of species belonging to the Bacteroides fragilis group by means of MALDI-TOF MS. Overall, our results reveal a high Firmicutes-to-Bacteroidetes ratio in faeces of obese children including alterations at species level.
Selective media have been used successfully to identify and enumerate Bacteroides fragilis group from human faeces. For the first time, a further in-depth analysis of species of the Bacteroides fragilis group revealed reduced relative proportions of B. vulgatus in obese children and adolescents. One study reported decreased relative proportions of B. vulgatus in the faeces of type 2 diabetic subjects using species specific PCR-denaturing gradient gel electrophoresis (DGGE). B. vulgatus was found to constitute a part of the core gut microbiota in healthy humans and is generally considered to be a beneficial gut commensal. These findings point towards a possible role for B. vulgatus in the pathophysiology of Western diseases, such as obesity and diabetes.
Moreover, the qPCR method that was used in this study to detect and quantify Bacteroidetes (Bacteroides-Prevotella-Porphyromonas spp.), Firmicutes (Clostridium coccoides-Eubacterium rectale group, Clostridium leptum group, Staphylococcus spp. and Lactobacillus spp.), and Bifidobacterium spp. in human faeces has already been thoroughly evaluated and validated[29–31]. In agreement with the findings of previous studies[32, 33], we describe higher concentrations of Lactobacillus spp. in the obese gut microbiota. However, the use of quantitative plating did not permit the detection of a significantly higher concentration of Lactobacillus spp. in faeces of obese children, which we did see using qPCR. A possible explanation is that L. gasseri and L. acidophilus could not be identified in culture due to the presence of vancomycin in the LAMVAB medium. Nevertheless, both quantitative culturing and qPCR resulted in a similar proportion of Lactobacillus spp. in the obese gut microbiota. A study conducted by Million et al., demonstrated that Lactobacillus reuteri was associated with obesity in adults. By contrast, Santacruz et al. showed that BMI SDS reduction in obese adolescents led to a concomitant increase in the concentrations of Lactobacillus spp. These findings thus suggest a possible role of Lactobacillus at species level in body weight and obesity. Additionally, we showed that the concentration of Lactobacillus spp. is positively correlated to plasma hs-CRP levels in obese children and adolescents. An increased prevalence of positive Firmicutes to higher levels of plasma hs-CRP was also seen in a study conducted in 51 obese and 28 normal-weight children and adults. These results seems therefore to suggest a possible role for Lactobacillus spp. in “low-grade” inflammation, a major pathophysiological process of obesity.
Interestingly, we detected an elevated Firmicutes-to-Bacteroidetes ratio in the gut microbiota of obese children and adolescents. Previous investigators also showed significant associations between this ratio and obesity in mice and humans[11–14]. The results of our study are similar to a study in Spanish children, demonstrating increased concentrations of Firmicutes and decreased concentrations of Bacteroidetes in the obese gut. Contrary to these findings, other studies described no or even opposite differences in the Firmicutes-to-Bacteroidetes ratio between obese and lean subjects[15, 16]. Possibly, these variations in study outcome are related to the fact that different methodologies were applied in these studies.
To further elucidate the complex role of gut microbiota in host physiology, a more thorough examination of the influence of diet on gut microbiota is recommended. In order to do so, we analysed the relationship between the presence of certain gut bacterial species with dietary compounds and energy intake. Here, we demonstrate that, independent of the BMI status, children and adolescents with a high energy intake (expressed in kcal/d) possess high faecal concentrations of Staphylococcus spp. analysed by quantitative culture. Note that the regression coefficient β of energy intake is low in all cases. This is due to the fact that values of energy intake are expressed in kcal/d. Given the range of energy intake (1635.53 to 2669.64 kcal), results in effect on mean concentration of Staphylococcus spp. of 1.27 to 2.08 are obtained. These results are not negligible and a real significant association has been detected. However, caution must be taken when translating these findings into a biological meaningful interpretation. Hence, more detailed research on this topic is necessary. Nevertheless, the importance of Staphylococcus spp. in childhood obesity has already been demonstrated by Kalliomaki et al. who showed that a greater faecal concentration of Staphylococcus spp. during infancy predicted the development of overweight during childhood. A possible role of Staphylococcus spp. in energy harvesting during childhood is thus suggested.
One major limitation of the current study is the small sample size and therefore these results should be interpreted with caution. In addition, pregnancy related factors, social status, and the period of being obese prior to inclusion were not taken into account.
Further longitudinal research on the cause-effect relationship between gut microbiota and obesity is highly justified, since different bacterial species could play a significant role in the human energy harvest and weight regulation. Moreover, consideration of lifestyle factors in gut microbiota studies is highly recommended, since changes in dietary pattern and physical activity could influence gut microbiota composition and the development of obesity. Finally, we suggest to focus future research not only on the elucidation of gut microbiota composition in obese subjects, but also on the study of gut metabolites, i.e. “metabolomics”. This suggestion for future research aims at expanding our knowledge on the complex interplay between gut microbiota, energy homeostasis and obesity.
In the future, modification of the gut microbiota composition by the administration of pro-, pre- or synbiotics in early childhood could offer an opportunity to prevent and/or treat obesity. However, additional research is required.