**3.2 Immunological modulation**

Chickens' immune systems include both innate and acquired immune responses [32]. The microbiota plays an important role in modulating the regulation and activation of both elements [33]. In terms of the innate immune response, the intestinal mucosa is thought to be the first line of defense against infection and a barrier that prevents commensal bacteria from penetrating the intestinal epithelium [32]. The interior surface of the avian intestine is covered in a mucous layer composed of the glycoprotein mucin, which is secreted by calceiform epithelial cells [34]. Mucins containing sialic acid have been found to be more abundant in conventionally reared chickens than mucins containing sulfate, which are found in birds with low bacterial loads. These differences are visible as early as day four (4) after birth, implying that the intestinal microbiota is involved in regulating the establishment of the mucous layer [35]. The intestinal microbiota also regulates the production of antimicrobial peptides on the surface of the intestinal epithelium, which are capable of rapidly killing or suppressing the activity [36]. Some of these peptides are expressed naturally, while others are induced in host cells by bacteria.

Regarding the acquired immune system, it appears that commensal bacteria protect the mucosa membrane by modulating the immune response, controlling the amount of mediators secreted by acquired immune system cells, and stimulating helper T cells [37]. Using germ-free chickens, it was demonstrated that microbiota has a dramatic effect on the repertoire of intestinal T cells and their cytokine expression [38].

#### **3.3 The physiology of the digestive system**

After hatching from the egg, the chicks must transition from a yolk-based diet to one rich in carbohydrates and proteins, which is critical to their development and health [39]. So, in this stage of development, the digestive system's organs go through anatomical and physiological changes. An ideal environment for microorganisms to colonize is the rapidly developing digestive tract, and the microbiota also plays an important role in the development of this organ. Compared to conventionally reared chickens, germ-free chickens have smaller intestines and cecas that weigh less and have thinner wall thickness [38]. There is some evidence to suggest that SCFAs increase enterocyte proliferation and growth, which could explain some of the discrepancy [29]. Intestinal microbiota may also influence the enzyme activity in chicken intestines [5]. Compare germ-free and conventionally raised chicken alkaline phosphatase enzyme activity and you'll see that the latter has higher levels of activity [38]. Bifidobacterium and Lactobacillus, which increase the activity of proteases, trypsin and lipases, can be induced by diet as well [40]. Morphological changes can be caused by pathogenic bacteria as well [35]. Co-infection with Eimeria and Clostridium perfringens has been shown to reduce the length of the intestinal villi [41]. Chickens infected with Salmonella typhimurium were also shown to exhibit these symptoms [35].

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

#### **3.4 Competitive exclusion**

Ecologically speaking, two species that compete for the same resources cannot coexist indefinitely [42]. A single competitor will always win out, leading to an evolutionary change, shift to another niche or even the complete demise of the other [5]. To reduce pathogen adhesion and colonization, the intestinal microbiota competes with colonizing pathogenic bacteria [43]. There are a variety of mechanisms that could lead to this reduction, including the physical occupation of space, competition for resources in a specific niche, and even direct physical or chemical confrontation with a potential colonizer [5]. Bacteriocins, for example, have been linked to a reduction in the ability of pathogens to invade the body [44]. No mechanism has been discovered yet to explain the protective effects of the competitive exclusion process on Salmonella colonization in broiler chickens' intestinal tracts [5]. It has been shown that the pathogen can be controlled using a variety of products ranging from probiotics to inoculation of bedding with cultures drawn from the fecal matter produced in more productive sheds with better intestinal health [5, 17].

### **4. Factors affecting the GI tract microbiota**

Intestinal microbiota, intestinal environment, and dietary compounds all work together to maintain a delicate equilibrium [45]. Disease can occur if this relationship is out of place [5]. Environmental factors, host age and health, and dietary habits all have the potential to influence microbial populations in either a positive or negative way [5, 45]. Aside from promoting growth and preventing the spread of endemic diseases, the use of low-dose antibiotics in livestock feed is a common practice in intensive farming [46]. Drug-resistant bacteria and public pressure to reduce the use of drugs in food-producing animals have created a need for 'natural' alternatives to boost performance and prevent disease spread [47]. However, these natural alternatives are not without their drawbacks. Intestinal microbiota can be influenced through the use of prebiotics and probiotics [48]. Specific changes in the composition and/or activity of the intestinal microflora, made possible by selective fermentation, that benefit the health and well-being of humans. "Live microorganisms that when administered in adequate amounts confer a health benefit on the host" is defined as [49]. Probiotics, prebiotics, or a combination of the two have been shown to improve the health of broilers in numerous studies [48, 49]. However promising probiotic supplements appear to be in the labs, their effects on commercial broilers vary widely [49]. There are many factors that can affect the intestinal microbiota's composition and must be taken into consideration when trying to manipulate the intestinal microbiota, including the complex relationship between the host and the microbiota [50].

Food is a major source of energy for intestinal bacteria, and as a result, diet has a significant impact on the population of bacteria in the digestive tract [29]. Since different bacterial species have different dietary requirements and preferred substrates, changing one's diet can have an impact on one's gut microbiome [51]. It has been found that when wheat was added to the diet of birds, it promotes the growth of bacteria with 50–55% Guanidine to Cytosine (GC) content and suppressed the growth of those bacteria with 60–79% content [52]. In contrast to diets based on maize, it has been revealed that populations of *lactobacilli* and *coliforms* increased in response to wheat and barley diets [53]. The *Lactobacilli* population and *C. perfringens* are the most affected by changes in the ileal microbiota due to dietary fat source [54].

When soy oil was substituted for tallow as a source of dietary fat, there was a rise in anaerobes in the intestinal microbiota and an increase in gut transit time [55]. This allows for the manipulation of chicken microbiota through dietary changes and the inclusion of specific components (essential oils, oligosaccharides, enzymes, and specific carbohydrate sources) aimed at enhancing growth and improving intestinal tract conditions for specific commensal bacterial groups [12].

Poultry living conditions and the management that go along with them have a significant impact on their intestinal microbiota as well [56]. As a result of poor hygiene, there will be an increase in food-borne illness and wet litter issues [57]. Since farm litter is a source of bacteria for the birds and a potential source of pathogenic bacteria, proper litter management is essential [56, 57].

Age has been shown to influence the composition of the intestinal microbiota, along with host genotype [58]. The diversity and complexity of the bacterial populations in the intestinal microbiota of older and younger birds are shown to increase as the birds age [59], according to culture-independent molecular profiling techniques [45]. According to Wickramasuriya *et al.* [60], broiler chickens' ileal and caecal microbiota had remarkably similar microbiotas at 3 days of age, but after 2 weeks, these subpopulations had diverged significantly. Many factors are likely to play a role in age-related GIT microbiota changes, including changes in diet, maturation of immune systems, changes in environmental influences, and increased interplay with other animals that expose individuals to a wider range of bacteria [61].

Birds raised in xenobiotic-rich environments are more likely to have a diverse and beneficial GUT microbiota [62]. Heavy metals, plastics, and agrochemicals are just a few of the potentially harmful substances on this list. HMs and the gut microbiota interact in a variety of ways. Exposure alters the normal gut microbiota's metabolism [62].
