**2. Intestinal microbes**

With around a hundred billion bacteria, the microorganisms in the human gastrointestinal system are large and varied. This colon is expected to have a bacterial cell density of 1011 to 1012 per ml, making it one of the world's most densely inhabited microbial ecosystems [28]. The gut flora contains around 3 million genetic materials and hundreds of compounds, but the genome sequence only contains approximately 23,000 genes [29]. The host intestinal flora contains 10–100 trillion germs, making it challenging for biologists to characterize the whole microbiota, particularly with the classic Sanger method.

Bacteria, fungi, and viruses are among the species that make up gut bacteria. Bacteria are divided into phylums, classes, groups, families, species, and individuals. Even though only a few phyla are included, there are over 160 species [30]. The most prevalent gut microbe phyla include Species, Acidobacteria, Lactobacillus, Lactobacillus, Actinobacteria, Microbacterium, and Verrucomicrobia, alongside

Taxa and Eubacterium [31]. accounting for 90% of a microbial population. The Firmicutes phylum has about 200 genera, including Escherichia, Staphylococcus, Coli, Enterobacter, and Ruminicoccus. The Clostridium genus makes up roughly 95% of the Firmicutes phylum. Bacteroidetes is a bacterial family that contains well-known bacteria which including Bacteroides and Prevotella. There are fewer bacteria in the Actinobacteria phylum, with the Bifidobacterium genus dominating [31].

The Firmicutes phylum, including comprises % of the gut bacteria, encompasses more than 200 genera, Escherichia, Pseudomonas, Vibrio, Enterobacter, and Ruminicoccus are a few examples. Almost the whole Genera class is represented by the Clostridium genus. Bacteroidetes is a bacterial family that includes well-known bacteria such as Bifidobacteria and Prevotella. The Lactobacillus genus dominates the Lactobacilli phylum, which contains a smaller amount of microbes.

### **2.1 Individual differences**

Each section of the Gastrointestinal system has a different taxonomy and functional flora, which fluctuates throughout time as a result of perinatal changes, aging, and external conditions such as antimicrobial usage.

### **2.2 Anatomy of the intestine**

Physiological factors such as acidity and high oxygen tension, digestion flow that is quick inside the lips but slows down afterward food supply, and finally human fluids all have an impact on the microbiota [32]. The gut provides a more difficult habitat for microbes due to its short transit times (3–5 hours) and high bile concentration. The largest microbial population is located in the large intestine, which has a slow mass flow and a normal to slightly acid pH, with obligate anaerobic bacteria dominating.
