Encyclopedia
Please note this is a comparison between Version 1 by Margherita Prosperi and Version 2 by Lindsay Dong.
The alteration of the microbiota–gut–brain axis has been recently recognized as a critical modulator of neuropsychiatric health and a possible factor in the etiopathogenesis of autism spectrum disorders (ASD).
- probiotics
- prebiotics
- fecal microbiota transplantation
- psychobiotics
- microbiota
- gastrointestinal
- ASD
1. Introduction
Autism spectrum disorders (ASD) are neurodevelopmental disorders characterized by persistent social communication difficulties with concurrent restricted interests, repetitive activities, and sensory abnormalities [1]. According to a recent Italian study, ASD have a prevalence of about one in 87 children aged between 7 and 9 years [2].
The high prevalence of some specific medical comorbidities, such as food selectivity and gastrointestinal (GI) disorders, in subjects with ASD compared to typical-development (TD) peers has led to a growing interest in organs and systems other than the central nervous system (CNS), but closely related to it. In recent years, research has focused on the role of bidirectional communication between the intestine and the brain (“the gut–brain axis”) in the etiopathogenesis of various stress-related psychopathological disturbances and neuropsychiatric conditions, including ASD, providing an essential contribution to understanding the diseases and proposing new therapeutic perspectives [3]. It has been hypothesized that the presence of alterations in the gut microbiota, a complex community of microorganisms living in the intestine and including anaerobic bacteria and viruses, protozoa, archaea, and fungi, could cause secondary effects at the level of the CNS [4].
Different scholars have hypothesized that GI disorders and alterations in the gut microbiota could contribute to the expression of the autistic phenotype or exacerbate the severity of symptoms in subjects genetically predisposed to ASD [5][6][7][8]. As emerges in studies on animal models, the microbiota is essential for developing social relationships. By re-establishing a condition of eubiosis in the intestinal microbiota during a specific developmental time window in germ-free mice, or in the maternal immune activation mouse model of ASD, it is possible not only to correct the defects of permeability and intestinal dysbiosis but also to act on ASD symptoms by reducing the production and absorption of toxins in the intestine [9][10]. In pioneering research [11], oral administration of minimal doses of vancomycin was associated with significant improvements in children’s behavior with regressive ASD. However, treatment had time-limited beneficial effects that ceased when therapy stopped; on the other hand, antibiotic treatment was not justifiable for prolonged periods. Therefore, it has been hypothesized that oral antibiotic therapy with vancomycin could temporarily improve chronic dysbiosis [11], indirectly reducing the increased intestinal permeability and indirectly acting on behavioral symptoms typical of ASD.
Different authors have hypothesized that GI disorders and alterations in the gut microbiota could contribute to the expression of the autistic phenotype or exacerbate the severity of symptoms in subjects genetically predisposed to ASD [5,6,7,8]. As emerges in studies on animal models, the microbiota is essential for developing social relationships. By re-establishing a condition of eubiosis in the intestinal microbiota during a specific developmental time window in germ-free mice, or in the maternal immune activation mouse model of ASD, it is possible not only to correct the defects of permeability and intestinal dysbiosis but also to act on ASD symptoms by reducing the production and absorption of toxins in the intestine [9,10]. In pioneering research [11], oral administration of minimal doses of vancomycin was associated with significant improvements in children’s behavior with regressive ASD. However, treatment had time-limited beneficial effects that ceased when therapy stopped; on the other hand, antibiotic treatment was not justifiable for prolonged periods. Therefore, it has been hypothesized that oral antibiotic therapy with vancomycin could temporarily improve chronic dysbiosis [11], indirectly reducing the increased intestinal permeability and indirectly acting on behavioral symptoms typical of ASD.
On the basis of these findings, treatments acting at the gut microbiota level, such as prebiotics, probiotics, and fecal microbiota transplantation (FMT), promise a reduction in GI symptoms and autistic symptoms in individuals with ASD, as already partially shown by other researches [12].
Probiotics are nonpathogenic living microorganisms considered beneficial to human health when administered in adequate quantities as a dietary supplement. They have recently been defined as “psychobiotics” because they are considered a therapeutic tool, influencing brain development and behavior through their activity in restoring the healthy balance of the intestinal microbiota, producing and/or modulating the levels of neurotransmitters [13]. Commonly used probiotics are Lactobacillus, Bifidobacterium, Saccharomyces cerevisiae, and some Escherichia coli and Bacillus species.
Prebiotics are nondigestible substances naturally contained in some foods (such as resistant starch, nonstarch polysaccharides, oligosaccharides, galacto-oligosaccharides and xylo-oligosaccharides), which selectively stimulate the growth of probiotics such as Lactobacilli in the intestine and Bifidobacteria [14]. Promising results derive from the studies on prebiotics [15], although their administration on children with ASD is still in the initial experimentation phase.
The symbiotic treatments, a combination of probiotics with prebiotics, also resulted in a positive modulation of the gut microbiota and metabolic activity of children with ASD [16].
FMT or fecal bacteriotherapy is a nondrug medical treatment in which fecal material from a donor is treated in the laboratory and placed orally in the recipient as capsules, through endoscopic procedures (i.e., colonoscopy, orogastric tube) or with enema. FMT has been proposed as a popular treatment for refractory Clostridium difficile infection, obesity, chronic inflammatory bowel diseases, and recently as a therapeutic strategy for autism [17]. A research concerning the duration of a single transplant observed that there is a significant impact on the microbiota up to 24 weeks [18]. Few clinical studies have evaluated the impact of the FMT or microbiota transfer therapy (MTT, i.e., a modified FMT protocol) on autistic symptoms in individuals with ASD [19][20][21].
2. Microbiota Interventions and Autism Spectrum Disorders
2.1. Clinical Studies on Probiotics
By exploring the possible applications of probiotic therapy, not only case reports [22][23][24] have identified a beneficial effect of specific probiotics on some of the behavioral characteristics specific to, or associated with, ASD in small populations [11][25][26][27][28][29][30][31][32][33], or larger samples [34][35]. Moreover, GI symptoms such as constipation, stool consistency, flatulence, and abdominal pain were improved [27][28][30][31][32][33][34]. In a cohort research [26] where oral supplementation of a strain of L. acidophilus was tested, there was an improvement in the ability of concentration and carrying out orders. From baseline, no difference in reacting to other people’s emotions or using eye contact was present. While intriguing, the small sample and open-label design of the research limit the relevance of the results. It is also of relevance that the total duration of the trial was double that of the intervention conventionally selected in clinical trials using probiotics on ASD [36]. In the placebo-controlled pilot research by Arnold et al. [28] about the use of a probiotic containing eight different bacterial species (mainly Lactobacilli and Bifidobacteria) in a sample of 13 children with ASD aged 3 to 12 years, no significant differences emerged as far as the improvement of the quality of life or the reduction of anxiety symptoms. Still, significant improvements in GI discomfort were seen during probiotic treatment compared to the placebo treatment period. It should be noted the small sample size, with a high dropout rate (10/13 completed the research), and the use of a new anxiety scale, not necessarily sensitive to highlight possible clinical changes.2.2. Clinical Studies on Prebiotics
Seven studies have been published to date that used different types of prebiotic compounds, including carrot powder [31], partially hydrolyzed guar gum [37], vitamin A [38][39], and galacto- [32][40] and fructo-oligosaccharides [33]. Some scholars have examined their effects when administered alone [37][38][39][40] or associated with probiotics [31][32][33]. Therefore, in the latter studies, it is not possible to determine whether the effects/benefits are due to the specific prebiotic or its function as a substrate for some probiotic strains. In the paper by Inoue et al. [37], a significant decrease in microbial alpha-diversity and some cytokines and chemokines (IL-1, IL-6, and TNF-α) were highlighted in a small sample of constipated ASD children after administering a prebiotic diet based on guar gum and β-endoglucanase produced by an Aspergillus strain. Prebiotic supplementation also increased the frequency of bowel movements with a consequent higher frequency of defecations per week. Grimaldi and colleagues [40] found a significant increase in the Lachnospiraceae family and significant changes in the fecal and urinary metabolites and antisocial behavior of 30 children with ASD after a prebiotic intervention with supplementation for six weeks with Bimuno galacto-oligosaccharides (B-GOS
Probiotics are nonpathogenic living microorganisms considered beneficial to human health when administered in adequate quantities as a dietary supplement. They have recently been defined as “psychobiotics” because they are considered a therapeutic tool, influencing brain development and behavior through their activity in restoring the healthy balance of the intestinal microbiota, producing and/or modulating the levels of neurotransmitters [13]. Commonly used probiotics are Lactobacillus, Bifidobacterium, Saccharomyces cerevisiae, and some Escherichia coli and Bacillus species.
Prebiotics are nondigestible substances naturally contained in some foods (such as resistant starch, nonstarch polysaccharides, oligosaccharides, galacto-oligosaccharides and xylo-oligosaccharides), which selectively stimulate the growth of probiotics such as Lactobacilli in the intestine and Bifidobacteria [14]. Promising results derive from the studies on prebiotics [15], although their administration on children with ASD is still in the initial experimentation phase.
The symbiotic treatments, a combination of probiotics with prebiotics, also resulted in a positive modulation of the gut microbiota and metabolic activity of children with ASD [16].
FMT or fecal bacteriotherapy is a nondrug medical treatment in which fecal material from a donor is treated in the laboratory and placed orally in the recipient as capsules, through endoscopic procedures (i.e., colonoscopy, orogastric tube) or with enema. FMT has been proposed as a popular treatment for refractory Clostridium difficile infection, obesity, chronic inflammatory bowel diseases, and recently as a therapeutic strategy for autism [17]. A study concerning the duration of a single transplant observed that there is a significant impact on the microbiota up to 24 weeks [18]. Few clinical studies have evaluated the impact of the FMT or microbiota transfer therapy (MTT, i.e., a modified FMT protocol) on autistic symptoms in individuals with ASD [19,20,21].
2. Microbiota Interventions and Autism Spectrum Disorders
2.1. Clinical Studies on Probiotics
By exploring the possible applications of probiotic therapy, not only case reports [53,54,55] have identified a beneficial effect of specific probiotics on some of the behavioral characteristics specific to, or associated with, ASD in small populations [11,25,26,27,28,29,30,37,38,39], or larger samples [31,32]. Moreover, GI symptoms such as constipation, stool consistency, flatulence, and abdominal pain were improved [27,28,30,31,37,38,39]. In a cohort study [26] where oral supplementation of a strain of L. acidophilus was tested, there was an improvement in the ability of concentration and carrying out orders. From baseline, no difference in reacting to other people’s emotions or using eye contact was present. While intriguing, the small sample and open-label design of the study limit the relevance of the results. It is also of relevance that the total duration of the trial was double that of the intervention conventionally selected in clinical trials using probiotics on ASD [56]. In the placebo-controlled pilot study by Arnold et al. [28] about the use of a probiotic containing eight different bacterial species (mainly Lactobacilli and Bifidobacteria) in a sample of 13 children with ASD aged 3 to 12 years, no significant differences emerged as far as the improvement of the quality of life or the reduction of anxiety symptoms. Still, significant improvements in GI discomfort were seen during probiotic treatment compared to the placebo treatment period. It should be noted the small sample size, with a high dropout rate (10/13 completed the study), and the use of a new anxiety scale, not necessarily sensitive to highlight possible clinical changes.2.2. Clinical Studies on Prebiotics
Seven studies have been published to date that used different types of prebiotic compounds, including carrot powder [37], partially hydrolyzed guar gum [35], vitamin A [34,36], and galacto- [33,38] and fructo-oligosaccharides [39]. Some authors have examined their effects when administered alone [33,34,35,36] or associated with probiotics [37,38,39]. Therefore, in the latter studies, it is not possible to determine whether the effects/benefits are due to the specific prebiotic or its function as a substrate for some probiotic strains. In the paper by Inoue et al. [35], a significant decrease in microbial alpha-diversity and some cytokines and chemokines (IL-1, IL-6, and TNF-α) were highlighted in a small sample of constipated ASD children after administering a prebiotic diet based on guar gum and β-endoglucanase produced by an Aspergillus strain. Prebiotic supplementation also increased the frequency of bowel movements with a consequent higher frequency of defecations per week. Grimaldi and colleagues [33] found a significant increase in the Lachnospiraceae family and significant changes in the fecal and urinary metabolites and antisocial behavior of 30 children with ASD after a prebiotic intervention with supplementation for six weeks with Bimuno galacto-oligosaccharides (B-GOS®: 80% galacto-oligosaccharides). Despite the research’s strength, which also considers the participants’ dietary habits assessed by 4-day food diaries as macronutrients and micronutrients intakes, there was a high dropout rate (63% completed the research). This could further limit the power of the results from an already relatively small sample (41 enrolled subjects).
Two pilot studies of the same research group [38][39] tested vitamin A supplementation in a sample of children with ASD, showing in one case [39] a significant increase in the Bacteroidetes/Firmicutes ratio without changes in autism severity and behavioral problems, while in the other [38] a reduction in the severity of autism and in serum levels of 5-hydroxytryptamine, which correlated positively with autistic symptoms. The somewhat conflicting results on the severity of autism and the lack of a placebo-controlled study design in both types of research limit these findings’ strength.
In conclusion, in terms of emotional–behavioral symptoms and related to ASD, some scholars have found an improvement after the administration of prebiotics [32][37][38][40]. In contrast, others have not shown a pre–post treatment change [39] in ASD subjects. The variability in the choice of prebiotics, the simultaneous administration with probiotic strains, and the few studies published to date do not allow us to draw definitive conclusions about their benefits in subjects with ASD.
2.3. Clinical Studies on Fecal Microbiota Transplantation
The first open-label research on MTT evaluated the impact of this technique on a sample of 18 autistic children aged 7 to 16 years with moderate-to-severe GI symptoms [19]. An approximately 80% reduction in GI symptoms (significant improvement in constipation, diarrhea, abdominal pain, digestive problems) and symptoms related to autism were identified. The improvement persisted after eight weeks since the end of the treatment. The protocol included preliminary therapy with antibiotics for two weeks, intestinal washing, and maintenance treatment with antacid drugs. Laboratory investigations revealed partial engraftment of the donor’s microbiota with consequent benefits at the level of the intestinal microenvironment (increase in Bifidobacteria, Prevotella, Desulfivibrio). In one case, there was an adverse dermatological reaction to vancomycin, and in 12 subjects, an increase in hyperactivity and aggression up to three days after the end of the treatment. Despite the relevance of side effects and the complex implant procedure, the scholars suggested the superiority of MTT over probiotic therapy due to the greater probability of engraftment as well as the presence of richer bacterial populations. The scholars then carried out a check on the same group of patients two years after the previous study [20], finding maintenance over time of both GI and autistic symptoms improvements and persistence of the increase of Bifidobacteria and Prevotella in the microbiota.3. Summary
ASD can be considered a relatively frequent disorder with a high longitudinal diagnostic stability [41], characterized by a significant individual, familial, and societal burden [42]. To date, evidence-based rehabilitative interventions can improve global outcome for some ASD people [43], but the possibility of boosting these with an easily administered supplementary treatment that acts on the gut–brain axis, with limited side effects and low costs, should be adequately explored. As shown, the studies published to date that have examined integrative treatment with probiotics and/or prebiotics, and FMT are few and show heterogeneous results. Most scholars, however, found a benefit of these therapies not only on GI disorders but also on behavioral problems and severity of autism symptoms, both in RCT and in nonrandomized studies. As already highlighted [44], not all these positive results reach statistical significance. Moreover, it is unclear if the period of supplementation that each research considers is long enough to expect behavioral changes. Although it should be noted that there is a good level of truthfulness from parents about their child’s emotional and behavioral problems [45], the placebo effect should be considered when the improvement has been rated through questionnaires [46][47]. Beyond the type and duration of treatment, probiotics positively modify the fecal microbiota. A reduction of Clostridia; an increase in Lactobacilli, Enterococci, and Bifidobacteria; a normalization of the Bacteroidetes/Firmicutes ratio; a reduction in Candida, as well as a decrease in intestinal inflammation and permeability in children with ASD have been shown [25][26][28][31][48]. Despite the discrepant results between various studies, a different intestinal microbiota in individuals with ASD than in individuals with TD emerges. Although it remains to be clarified whether the above identified microbiota alterations are implicated in the onset of ASD or occur subsequently, there is growing evidence that they can aggravate autistic symptoms. It could happen either through a mechanism mediated by their action on the GI system or through indirect pathways related to the microbiota–gut–brain axis. In recent years, several studies that have examined therapies acting on the microbiota–gut–brain axis as prebiotics, probiotics and FMT, have shown improvements in some gastrointestinal symptoms and some psychiatric symptoms in subjects with ASD. It should be noted that these treatments are easily administered, with limited side effects and low costs. However, considering the variability of the treatments, the samples size, the duration of treatment and the tools used to evaluate the outcome, these results are still partial and do not allow us to establish a conclusive beneficial effect of probiotics and other interventions on the symptoms of ASD [44]. In particular, the optimal species, subspecies, and dosages have yet to be identified. Considering ASD heterogeneity, it would be desirable that treatments should be selected on the basis of the specific characteristics of both subjects with ASD and the host’s microbiota, with the ultimate goal to individualize the therapy [44].: 80% galacto-oligosaccharides). Despite the study’s strength, which also considers the participants’ dietary habits assessed by 4-day food diaries as macronutrients and micronutrients intakes, there was a high dropout rate (63% completed the study). This could further limit the power of the results from an already relatively small sample (41 enrolled subjects).
Two pilot studies of the same research group [34,36] tested vitamin A supplementation in a sample of children with ASD, showing in one case [36] a significant increase in the Bacteroidetes/Firmicutes ratio without changes in autism severity and behavioral problems, while in the other [34] a reduction in the severity of autism and in serum levels of 5-hydroxytryptamine, which correlated positively with autistic symptoms. The somewhat conflicting results on the severity of autism and the lack of a placebo-controlled study design in both types of research limit these findings’ strength.
In conclusion, in terms of emotional–behavioral symptoms and related to ASD, some authors have found an improvement after the administration of prebiotics [33,34,35,38]. In contrast, others have not shown a pre–post treatment change [36] in ASD subjects. The variability in the choice of prebiotics, the simultaneous administration with probiotic strains, and the few studies published to date do not allow us to draw definitive conclusions about their benefits in subjects with ASD.
