# A Review of the Relationship between Gut Microbiome and Obesity
## 4 The Role of Certain Bacterial Phylum and Species  
In the context of energy balance, the diversity and microbial stability are important factors for gut health, the changes in which lead to dysbiosis [5,16,25]. Dysbiosis has been associated with three different phenomena that can occur together: (1) loss of beneficial microbiota, (2) overgrowth of potentially harmful bacteria, and (3) a decrease in overall microbial diversity [26]. Diseases that were found to have possible connection to microbial alterations involve autoimmune and allergic diseases, inflammatory bowel diseases, obesity, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Population** & **Dietary Pattern** & **Microbiome Diversity** & **Obesity Prevalence** & **Reference** \\ \hline \multirow{3}{*}{Hadza tribe} & Predominantly & \(\uparrow\)_Prevetella_ & & \\  & plant-based diet & \(\uparrow\)_Bacteroides_ & \(<\)5\% & [18] \\  & & \(\uparrow\)_Treponema_ & & \\ \hline \multirow{3}{*}{Inuit} & High in animal fat and protein, & \(\downarrow\)_Prevetella_ & & \\  & low in dietary fibre & \(\downarrow\)_Akkermansia_ & 20.6\% & [19,20] \\  & & _mucintibria_ & & \\ \hline Western population & Western diet (high in fat, sugar, & & \\ US & sodium, animal protein, & & 38.2\% (US) & \\ Netherlands & processed food; low in fruits, & \(\uparrow\)_Bacteroides_ & 12.8\% (Netherlands) & \\ Italy & vegetables, whole grains, and & & \(\downarrow\)_Prevetella_ & 9.8\% (Italy) & \\ Spain & dietary fibre) & & 16.7\% (Spain) & \\ \hline Non-western populations & Agricultural diets, & & \\ parts of central and & predominantly containing & & \(\uparrow\)_Prevetella_ & 5\% (India) & \\ northern India & plant-based components with & & \(\downarrow\)_Bacteroides_ & 26.3\% (Peru) & [22, 23, 24] \\ Peru & the presence of animal-based & & 4\% ( Madagascar) & \\ Madagascar & components & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Obesity prevalence and microbiome diversity characteristics in specific population group \(\uparrow\): increased; \(\downarrow\): decreased. and central nervous system diseases [26; 27]. About 90% of the bacterial species within the microbiome community belong to the _Firmicutes_ (i.e., _Bacillus_ spp.) and _Bacteroidetes_ (_Bacteroides_ spp.) phyla [7; 27] with other important phyla including _Actinobacteria_ (Bifidobacterium spp. ), _Proteobacteria_ (_Escherichia_, _Helicobacter_), and _Verrucomicobia_ (_Akkermansia_ spp.) [5; 7]. Nevertheless, a wide range of individual species could be found, resulting in an increased amount of individual variability. The _Firmicutes/Bacteroidetes_ ratio has often been considered as a possible predictor of obesity risk [28]. In a mouse model, the gut microbiota of obese subjects was found to have higher _Firmicutes_ and lower _Bacteroidetes_ ratios compared to lean subjects, but after 1 year of dietary therapy, a reversed profile was found [4; 5]. A metagenomic study comparing the gut microbiome of obese and lean twins found lower bacterial diversity and _Bacteroidetes_ ratios, but higher _Actinobacteria_ ratios in obese individuals compared to lean individuals, but no significant difference in _Firmicutes_ ratios [29]. However, further studies and meta-analyses have not found a certain relationship between _Firmicutes_ and _Bacteroidetes_ ratios and obesity [2], suggesting a more complex role for the gut microbiome in the regulation of obesity than a simple imbalance of these phylum. A 6 week randomised controlled clinical trial investigated the difference in body weight change in healthy people based on the abundance of _Prevotella_ in participants who consumed an ad libitum diet containing whole grain or refined wheat. They found that _Prevotella_ abundances were inversely correlated with body weight change. Subjects with high _Prevotella_ abundance spontaneously lost more body weight on a diet containing whole grain wheat than on a diet containing refined wheat, whereas the weight of subjects with low _Prevotella_ abundance remained stable.