**2. Autism spectrum disorder and the gut microbiota**

#### **2.1 Gut microbiota involved in the pathogenesis of autism spectrum disorders**

Microbiota role in health and disease is as crucial as is complex. Alterations in normal commensal GM (known as dysbiosis) have been widely reported as a key contributor to the etiology and/or pathogenesis of various diseases including several neurobehavioral conditions, such as Parkinson's disease, schizophrenia, Alzheimer's disease, depression, anxiety, and most compellingly autism spectrum disorder (ASD) [14].

ASD refers to a group of heterogeneous and complex neurodevelopmental disorders characterized by impaired social interactions and reciprocal communication skills as well as restricted, repetitive, and stereotyped patterns of behavior, interests, and activities [15]. Over the last decades, a steady increase of ASD prevalence has been reported worldwide. ASD is currently estimated to affect about 1 in every 68 children, with greater incidence found among boys (4:1) [14]. To date, the etiology of ASD remains elusive, and it is thought to involve both genetic predisposition and different environmental triggers. Although several genetic factors are known to influence the etiology of different types of ASD, these only apply to a minor part of the autistic population. By estimate, the heritability accounts approximately for only 35–40% of the contributing elements and the remaining 60–65% results from the combination of prenatal, perinatal, and postnatal environmental factors as well as related medical disorders [16]. Besides, along with significant psychiatric symptoms, ASD is often characterized by a number of medical comorbidities, the most prominent of which implicates the GI tract. Children with ASD experience significantly more GI symptoms than children without ASD occurring nearly at a fourfold greater rate [17]. Symptoms include constipation, diarrhea, abdominal pain, bloating, gastroesophageal reflux, and food selectivity and seem to be strongly associated with the severity of ASD behaviors. Clinical abnormalities such as altered GI motility and increased gut permeability have also been reported [18]. The cause of ASD-associated GI problems is difficult to ascertain, but it appears to partly relate to the excessive use of oral ATBs which can alter GM. Indeed, several studies report increased use of oral ATBs in children with ASD compared to neurotypical children. By eliminating beneficial indigenous GM, long-term ATB use destabilizes microbial community and creates favorable environment for colonization by potentially harmful (toxin-producing) microorganisms. Thus, considering the potential interactions between intestinal microbes and the CNS, loss of the

*Prebiotics and Probiotics - Potential Benefits in Nutrition and Health*

through stimulation of local and systemic immune responses.

psychiatric, neurobehavioral, neurodegenerative, and mental disorders.

**1.2 Colonization of intestinal ecosystem in early life and its evolution**

The composition of the GM varies widely from fetal life to adult age. Until recently, babies were believed to be born sterile and only populated by microbes on exposure to their first postdelivery environments. Recent research suggests that the process of microbial colonization of the GI tract could begin prenatally as acquisition of maternal microbiota might occur during intrauterine life via placenta [9]. However, these findings are questionable, and recent data strongly suggest that bacteria isolated in utero are rather a contamination linked to the sampling methods than a specific microbiota [10]. Neonates show unstable and highly dynamic intestinal microbiota with a low microbial diversity. First colonizers in healthy neonates are enterobacteria, *Staphylococcus*, and *Streptococcus*, followed by strict anaerobes

genome, the GM is now recognized as an environmental factor that affects normal

Lastly, very recently there is mounting evidence of the significant influence of GM in the modulation of brain activity and behavior across the so-called "microbiota-gut-brain axis (GBA)" [3]. The GBA consists of bidirectional communication between the central nervous system (CNS), the enteric nervous system, and the gut linking emotional and cognitive centers of the brain with peripheral intestinal functions [4]. The exact mechanisms of signal transmission within this network are not completely elucidated. The CNS asserts its role over the GM through influencing gut motility patterns, altering the equilibrium in the gut permeability, and modulating mucus secretion which are known to exert control over gut microbial composition [5]. Conversely, the GM claims its influence over the CNS by regulating the hypothalamic-pituitary-adrenal axis and the production and turnover of cytokines and neurotransmitters. In addition to their effects on development and maturation of the enteric nervous system, these neuroactive metabolites can signal beyond the local gastrointestinal (GI) tract to the distant CNS potentially through signaling pathways that include the vagus nerve. Catecholamines can modulate important processes, including neurogenesis, myelination, microglia activation, brain plasticity, and blood-brain barrier permeability [6]. Indeed, recent studies demonstrated that, under extreme conditions (e.g., in a germ-free environment or during antibiotic (ATB) treatment), GM absence is associated with several abnormalities in brain gene expression and neurophysiology [7]. Interestingly, these aberrations are reversed after colonization with a conventional GM [8] or even specific bacterial species [7]. Thus, the disruption of neural, endocrine, immune, and metabolic mechanisms that are involved in gut-CNS signaling seems to be involved in neuro-

During recent decades, the development of high-throughput sequencing technology has hugely contributed to increased understanding of host-microbe interactions in health and disease [2]. First, GM offers a barrier, protecting against external factors and proliferation of pathogenic microbes, through various mechanisms such as increasing mucus production, reinforcing intestinal epithelium permeability, and production of bacteriocins and antimicrobial peptides. GM is also involved in many fundamental metabolic functions such as synthesis of essential nutrients, hormones, vitamins, supply of energy from dietary sources otherwise unavailable to host and clearance of drugs, and toxins. Furthermore, GM is shown to be involved in the maturation of the host immune system, where specific strains such as *Bifidobacterium* and *Lactobacillus* produce anti-inflammatory cytokines, and others such as *Clostridium* and *Ruminococcus* produce pro-inflammatory cytokines. Hence, GM is essential for the development of innate and acquired immunity

host physiology, metabolism, immunity, brain function, and behavior.

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protective commensal microbiota along with the overgrowth of pathogenic microorganisms is hypothesized to cause or contribute not only to GI dysfunction but also to ASD-related behavioral symptoms. All these findings have gained an insight into the influence of GM in ASD as potential mediator of risk factors.
