**2. Inheritance of microbiome**

## **2.1 Microbiome**

A microbiome is the community of microbes dwelling collectively in a selected habitat. Humans, animals, vegetation, soils, oceans or even buildings have their own specific microbiome [15].

#### **2.2 Host genetics and gut microbiome**

The human gut environment is extremely complex with a unique ecology which comprises of trillion of microbiota with approximately 1.5 kg in mass. By using genetic techniques like 16S sequencing, 1000 microorganisms have been identified within the gut, with approx. 200 (0.5%) defining the core of the intestine microbiome [16]. These bacteria protect the gut epithelial cells against external pathogens. They also help the breakdown of indigestible dietary polysaccharides in the gut and

**177**

**2.3 Inherited microbiomes**

**Figure 1.**

*Core human microbiome.*

these microorganisms in health.

*Gut Microbiome: A New Organ System in Body DOI: http://dx.doi.org/10.5772/intechopen.89634*

thus supply a quick chain of fatty acids, including acetate, butyrate, and propionate, which serve as vital metabolites for direct energy source of intestinal epithelial cells, prevention of insulin resistance and modulators of insulin secretion [17] (**Figure 1**). The genetic makeup of humans is virtually identical, yet the small differences in DNA give rise to remarkable phenotypic assortment across the human population. The trillions of microbes inhabit our bodies and create complex, body-habitat-specific, adaptive ecosystems that are finely tuned to frequently changing host physiology [18]. A healthy "functional core" is actually a complement of metabolic and other molecular functions that are performed by the microbiome within a particular habitat but are not necessarily provided by the same organisms in different people [19].

The gastrointestinal tract (GIT) of humans is colonized by a vast variety of microbial population that can be understood as a complex and polygenetic trait which has been interacting and co-evolved with their host genetic environment [20–22]. It was previously considered that fetus lives in a germ free environment in the mother womb and the gut microbiota are transferred to the baby from mother's birth canal and body via horizontal transmission only [23]. But advanced researches have revealed that microbiota are also vertically transmitted to the infants from their mothers [24]. Presence of microbes in the meconium of the babies born by cesarean section clearly demonstrates that the gut microbes are not only derived after the birth [25, 26]. Moreover, presence of many microbes in the umbilical cord blood of the preterm babies and in the amniotic fluid substantiate the findings that the fetus in the mother womb is not totally sterile [27, 28]. Many gut bacterial genera are shared among the mammal species. The microbiomes of mice show strong fidelity throughout the generations and reiterate the intrinsic significance of

*Parasitology and Microbiology Research*

via several mechanisms including direct killing of pathogen, competition for limited nutrients, and enhancement of immune responses [2]. The intestinal microorganisms also co-evolved and have strong affiliations and association towards each other. In this evolutionary process, the persistent and enduring members of this microflora become more competent during unsettling influences and thereby become essential for human health [3]. Definite composition of human microbiome varies between individuals [4] particularly among lean and obese people. The microbiome is also affected by the dietary modifications adapted for the weight loss [5]. Examination of metabolic profiles of human infant microbiota revealed that ingestion, storage and digestion of dietary lipids were explicitly regulated by the microbiome [6, 7]. The human gut microbial communities are a mixture of microorganisms. The classes of microbes that constitute the gut microbiome communities differ between hosts. The difference is attributed to factors such as, inability of a microorganism to migrate between different hosts, intense environmental conditions inside and outside host's gut and host inconsistency in terms of genotype, diet, and colonization history [8]. The co-evolution of humans and their symbiotic microorganism has created bilateral interactions which are important for the health of humans, and any genetic or ecological change in this bilateral interaction can result in pathological conditions like infection [8]. Gut microbial communities are important for diverse host functions, including metabolism, fertility, development, immunity, and even antioxidant activities which promote health and fitness of the host [9–12]. The gut microbiome has a much larger genetic variety compared to the genome of the host, e.g., human genome is comprised of 20-25,000 genes whereas microbiome inhabiting the body is estimated to be in trillions. Almost 1010 microorganisms enter the human body daily and with the progress of co-evolution of gut microbes in humans, the capability of microbes to exchange their genes and associated functions with the environment are some of the main factors leading to host adaptation. Therefore, the "hologenome" model appraises the host and its microbes genomes as one unit under assortment [13, 14]. It is acknowledged that host-symbiont co-evolution is accountable for basic biological aspects. In this chapter we aim to discuss the importance of gut microbiomes as a new organ system because of its association with the genetics and its role in the disease and health condition of the host. Moreover, the involvement of these microbiomes in shaping the overall health and constructing a symbiotic relationship with their host species is

discussed as well as the co-evolution of gut microbes with the human body.

A microbiome is the community of microbes dwelling collectively in a selected habitat. Humans, animals, vegetation, soils, oceans or even buildings have their

The human gut environment is extremely complex with a unique ecology which

comprises of trillion of microbiota with approximately 1.5 kg in mass. By using genetic techniques like 16S sequencing, 1000 microorganisms have been identified within the gut, with approx. 200 (0.5%) defining the core of the intestine microbiome [16]. These bacteria protect the gut epithelial cells against external pathogens. They also help the breakdown of indigestible dietary polysaccharides in the gut and

**2. Inheritance of microbiome**

own specific microbiome [15].

**2.2 Host genetics and gut microbiome**

**2.1 Microbiome**

**176**

**Figure 1.** *Core human microbiome.*

thus supply a quick chain of fatty acids, including acetate, butyrate, and propionate, which serve as vital metabolites for direct energy source of intestinal epithelial cells, prevention of insulin resistance and modulators of insulin secretion [17] (**Figure 1**).

The genetic makeup of humans is virtually identical, yet the small differences in DNA give rise to remarkable phenotypic assortment across the human population. The trillions of microbes inhabit our bodies and create complex, body-habitat-specific, adaptive ecosystems that are finely tuned to frequently changing host physiology [18]. A healthy "functional core" is actually a complement of metabolic and other molecular functions that are performed by the microbiome within a particular habitat but are not necessarily provided by the same organisms in different people [19].
