**2. The role of bacteria homeostasis in gastrointestinal**

Dietary patterns and quantity of food intake have been described to influence the microbiome in the gut [21]. *Bacteriodetes, Firmicutes, Actinobacteria,* 

*Dietary Patterns for Immunity Support and Systemic Inflammation against Infections… DOI: http://dx.doi.org/10.5772/intechopen.96610*

*Fusobacteria, Proteobacteria and Verrucomicrobium* are predominant phyla of microbiota in the human gastrointestinal tract [22]. Among these phyla, more than 90% of the microbiome colonies in the colon are *Bacteriodetes* (*Bacteroides, Prevotella*) and *Firmicutes* (*Eubacterium, Lactobacillus*) [23]. In Western countries, it has been shown that *Firmicutes* phylum becomes blooming and *Bacteroidetes* phylum population decrease due to prominent animal product consumption [24]. On the other hand, it has been demonstrated that high content fiber in the diet resulted in more *Bacteroidetes* phylum bacteria dominance and an increased amount of concentration of short-chain fatty acids (SCFAs) among children from Africa compared to children of European origin [25]. Short-chain fatty acids are fatty acids with fewer than six carbon atoms (acetate, propionate, and butyrate) derived from intestinal microbial fermentation of dietary fibers and resistant starch [26]. The concentration of short-chain fatty acids in the colon and systemic blood is crucial for immune response regulation. The fermentation of dietary fiber by gut microbes, resulting in the establishment of SCFAs, has been proposed to regulate anti-inflammatory pathways through numerous receptors such as G-protein coupled receptors [27]. Additionally, fermented foods and beverages are found to produce beneficial improvements in intestinal barrier function and permeability [28].

It has been suggested that vegan or vegetarian diets may stimulate intestinal microbiota that promotes anti-inflammatory response and lead to be more varied and steadier microbiota systems [29]. Contrary to this, particular food items such as red meat, gluten in wheat, and alcohol can induce dysbiosis which might cause a heightened pro-inflammatory response triggered by viral infections such as COVID-19 from underlying diet-derived chronic inflammation [21]. This intestinal mucosal chronic inflammation is characterized by the presence of cytokines (TNF-α and IFN-γ) which are produced by macrophages, T-cells and natural killer. Besides cytokines, various proteases are also released into the mucosa that has been reported to cause leaky gut due to degradation of tight junctions [30]. Thus, leaky gut allows translocation of microbial products such as lipopolysaccharides (LPS) from the gut into the blood circulation. This condition may transform the existing state of gut inflammation into chronic systemic inflammation during infections such as HIV [31]. This chronic inflammation may remain undetected as a predisposing risk factor and can develop any time into serious morbidity including infectious diseases [23].

Several studies have shown the association between the change of intestinal microbiomes with infectious diseases. It has been reported that intestinal bacterial diversity significantly decreases inversely associated with the severity in patients of chronic viral hepatitis C compared to healthy individuals. The gut microbiome could be a biological indicator and a novel potentially therapeutical approach to reduce the complications of chronic liver disease [32]. Another study has identified *Lachnospiraceae*, *Ruminococcaceae*, and butyrate-producing anaerobic bacteria can be significantly decreased in diarrhea caused by *Clostridium difficile* infection [33]. A short-term nutritional intervention study has reported the positive effect of the supplement on HIV-associated dysbiosis, which was most apparent among untreated individuals but less so in subjects with anti-retroviral therapy, whose gut microbiota was found more resilient [34].

In contrast to short-term supplement intake, long-term dietary patterns and habitual diet are key factors that influence the composition of the gut microbiota. It reflects the potential for therapeutic dietary approaches to modulate microbiome variety, formation, and stability. Besides diet, the intestinal bacteria are formed by a composition of extrinsic (e.g., lifestyle and medication) and intrinsic (e.g., host genetics, immune and metabolic regulations) factors [35]. Changes in dietary patterns following the western diet, along with modifications in dietary components,

result in significant changes in the intestinal microbial configuration and function. As an example, changing from a low-fat, high-fiber diet to a high-fat, high-protein, low-fiber diet leads to reduced α-diversity (intra-individual gut microbiota richness), increased β-diversity (inter-individual gut microbiota diversity) and deteriorated richness or even the extermination of *Prevotella* and *Treponema* species, with lower butyrate levels [36].
