**2. Gut microbiota and the gut-brain axis**

Gut microbiota is a collective name of ten of trillions of microorganisms living in our intestine, including more than 35,000 different species of known bacteria [74]. Microbiome refers to the collective genomes of more than 3 million genes of the microorganisms in a particular environment. Gut microbiota, like a virtual endocrine organ, reacts to various internal and external stimuli [5, 75–78]. Consequently, gut microbiota influences and regulates hosts' health and mood statue including aggression by integrating metabolic, immune, endocrine, and neural reactions through the bidirectional communication of the gut-brain axis (**Figure 1**) [7, 33, 79–83]. Gut microbiota in hosts, for example, regulates brain neurotransmitters [84–87] such as serotonin (5-HT) through releasing its precursor, tryptophan, an essential amino acid [75, 88–90]. Serotonin is a key neurotransmitter within the brain, contributing to the development of the central nervous system (CNS)

**33**

**Figure 1.**

*immunity (modified from [79]).*

*Gut-Brain Axis: Probiotic,* Bacillus subtilis*, Prevents Aggression via the Modification…*

**2.1 Social stress and the hypothalamic-pituitary-adrenal axis**

[91–94] and social behavior [95, 96], affecting mental health [97–100] and providing therapeutic strategy for treating or preventing stress reaction and related

The social world is filled with different types of interactions, such that social challenges (fight-or-flight) and social environmental changes (group instability), as stressors, can cause highly potent stress [101–103]. Some management practices used in the intensive farm animal production systems, for example, may cause stress in animals. Farm animals are often housed in large groups at high densities, and during the animals' lifetimes, they are repeatedly transported to new environments and mixed with unfamiliar individuals. Based on the social network theory, animal group disruption can lead to social stress and related aggression [104] when they are unable to cope with these management practices and/or to reset their dominance rank (a form of animal social structure hierarchy) [105, 106].

Aggression has been commonly defined as feeling of anger or antipathy during social interactions, leading to hostile or destructive behaviors or attitudes, provoking physical or verbal attack or confront toward another individual [1]. Aggressive encounters among conspecifics for obtaining or maintaining a socially dominant position or rank cause a negative or unstable social environment to activate the hypothalamic-pituitary-adrenal (HPA) axis which is known as the key stress response system [107–109]. Cortisol (or corticosterone, CORT, in rodents and birds), as one of the final hormones released from the adrenal cortex, has multifunctional functions in both normal and abnormal states, regulating behavioral styles, metabolic patterns, and endocrine and immune functions, and ensuring an adequate coping strategy and well-being [110–112]. In mammals, hypercortisolism in response to novel environment exposures has been evidenced to be of adrenal origin [113, 114]. The value of circulating cortisol (or CORT) has been used as a criterion of stress response in humans and various animals [115–117]. Corticotrophinreleasing factor facilitates aggression [118, 119], and related elevation of CORT (or cortisol) also affects neuroendocrine functions through binding to their receptors [120, 121], causing neuron loss in the hippocampus [122], dysfunction of the serotonergic system [123], and inhibition of immunity (increased pro-inflammatory cytokines as neuroinflammatory response) within stress-sensitive brain regions [124]. These changes finally lead to the development of psychological disorders such as aggression [109, 125]. Similar to that the HPA hyperactivity generates aggressive behavior, exogenous glucocorticoid treatments increase exacerbated aggressiveness in both humans and experimental animals [123, 126]. Taken together, emerging

*The microbiota-host interaction occurs at the level of the gastrointestinal mucosa via local neural, endocrine, and immune activities, influencing brain neurotransmitter expression, physiological homeostasis, and* 

*DOI: http://dx.doi.org/10.5772/intechopen.86775*

neuropsychiatric disorders [94, 97].

[91–94] and social behavior [95, 96], affecting mental health [97–100] and providing therapeutic strategy for treating or preventing stress reaction and related neuropsychiatric disorders [94, 97].
