**2. The avian GIT microbiota**

There are many different types of microorganisms in the animal microbiota, including those that are beneficial and those that are harmful [12]. The term "microbiota" is used to describe this microbial community. It includes commensal, symbiotic, and pathogenic microorganisms, as well as those that are beneficial or harmful to the host [13]. The microbiome refers to all of these symbionts' genetic material as a whole [14]. When an organism consists of both host and microbial components, it is referred to as "supraorganisms" because of the important role it plays in the health and development of the host [5].

In the chicken intestinal tract, there is a diverse and ever-changing community of microorganisms [15]. When the gut microbiota is first established, it's mostly anaerobic bacteria that take over [16]. The intestinal microbiota of newly hatched chicks is heavily influenced by the surrounding environment, and this is especially true for chicks that have only a small number of bacteria in their system [17]. As animals older, the GIT microbiota evolves from simple to complex and obligate anaerobes, reaching a relatively stable dynamic equilibrium [18]. A variety of functions and microbial compositions are found throughout the chicken GIT, which is divided into numerous sections [19].

The digestion and absorption of nutrients are dependent on the proper functioning of each section of the digestive tract. In chickens, there are two paired ceca, both of which are home to a similar bacterial community [20].

According to Wei *et al.* [21], a total of 915 operational taxonomic units (OTUs), or species (defined as having a phylogenetic distance of 3%), were found in 13 phyla, with *Firmicutes* (70 percent), *Bacteroidetes* (11.3%), and *Proteobacteria* (9.3%) accounting for more than 90% of all sequences analyzed. *Clostridium*, *Ruminococcus*, *Lactobacillus* and *Bacteroides* dominated the 117 genera described in total. *Ethanoligenes* (*Firmicutes*), a genus of bacteria that produces ethanol, was found to be prevalent. The most common *Proteobacterium* was *Desulfohalobium.* The genus *Bifidobacterium* was found in only 1% of the *Actinobacteria* sequences. *Cyanobacteria*, *Spirochaetes, Synergisteles, Fusobacteria, Tenericutes,* and *Verrucomicrobia* were among

#### *The Impact of Heavy Metals on the Chicken Gut Microbiota and Their Health and Diseases DOI: http://dx.doi.org/10.5772/intechopen.105581*

the lesser-known phyla. The phylum *Euryarchaeota* was the only Archaea phylum to be found in the chicken GIT, with only 11 out of a total of 3184 sequences. This supports the lack of methanogens in the chicken GIT [22]. There are fewer species of bacteria in chicken GIT than in the GIT of other animals, which may be due to the rapid transit and short retention times of food in the digestive system [5]. For example, a 29-day-old broiler chicken's typical retention time is between 4 and 5 hours, compared to humans' average of 20 hours [23]. *Firmicutes*, *Bacteroidetes, Proteobacteria,* and *Actinobacteria* are said to be the most common phyla found in the ceca [24]. The presence of Firmicutes and Bacteroidetes in the ceca suggests that the microbiota present is important in the production of essential amino acids, the digestion of non-starch polysaccharides, which stimulates the production of shortchain fatty acids (SCFAs), and nitrogen recycling using uric acid [5]. *Bacteroides* are the most common species in the Bacteroidetes phylum (40%). Others in this family include *Prevotella* (the genus), *Tannerella* (the genus), and Ri*emerella*, *Desulfohalobium*, *Escherichia*, *Shigella*, and *Neisseria* are the most common genera of *Proteobacteria* [21].

Clostridium, Bacteroides, and Ruminococcus are among the obligate anaerobes found in the cecum [25]. The small intestine, which includes the duodenum, jejunum, and ileum and is where nutrients are primarily digested and absorbed, has fewer microorganisms and is primarily colonized by acid-tolerant and facultative anaerobes such as Lactobacillus, Enterococcus, and Streptococcus [17]. The feacal microbiota composition varies greatly depending on the contributions of microbiota from different gut segments [12].

### **3. Importance of GIT microbiota to broilers**

Microorganisms are primarily found in the gastrointestinal tract. Broilers and their intestinal microbiota interact in a variety of ways, with an emphasis on nutrient exchange, immune modulation, digestive system physiology, and pathogen exclusion being the most important [5, 26]. These functions are summarized in the following sections.

#### **3.1 Nutrient exchange**

Chickens benefit from the nutrients provided by commensal bacteria in their digestive systems, both directly and indirectly [5, 27]. SCFAs, ammonium, amino acids, and vitamins [12, 15] are all included in this category. Polysaccharides, oligosaccharides, and disaccharides can all be hydrolyzed to primary sugars by the majority of intestinal bacteria [28]. SCFAs such as acetate, propionate, and butyrate are produced by the fermentation of these sugars by intestinal bacteria [12, 15]. Passive diffusion in the ceca allows SCFAs to be absorbed and enter a number of metabolic pathways [29]. SCFAs are a carbon and energy source for chickens [15]. To further enhance their ability to modulate intestinal immune response, they regulate blood flow and stimulate the proliferation of enterocytes [29].

Nitrogen metabolism is also aided by bacteria in the intestines [30]. When uric acid is broken down into ammonium by bacteria in the urinary tract, it can travel from the cloaca to the cecum, affecting the metabolism in the latter and allowing ammonium to be absorbed by the host [29, 31]. This allows the host to use ammonium to synthesize amino acids. However, the same intestinal bacteria can also be a source

of amino acids and vitamins [29], Despite the fact that most of the proteins and vitamins produced by these bacteria are excreted, because most intestinal bacteria are found in the cecum, which cannot digest or absorb proteins [5]. Chickens, on the other hand, may be able to provide nutrients to the intestinal bacteria in a reciprocal manner.
