Update on ASD and microbiota composition
Several research articles have been published over the last three years and since the previous meta-analyses. The Autism and Developmental Disabilities Monitoring Network in the United States can be used as a reference because 7,611 papers from the United States were published on childhood ASD from January 1, 2012, to December 31, 2021 [13].
In 2020, Ahmed et al. [14] reported a study including 41 children, in this study, the authors concluded that there was no correlation between the nature of the alteration of the gut microbiota and the severity of autism or GI symptoms [14].
Kandeel et al. [15] focused on Clostridium in ASD patients’ feces. In this study, the number of Clostridium spp. (Clostridium paraputri, Clostridium bolteae, and Clostridium perfringens) was greater in the feces of children compared to healthy controls. Furthermore, the gut microbiota of children with ASD exhibited two types of Clostridia (Clostridium difficile and Clostridium clostridioides) that were not found in healthy children. Healthy children possess in their microbiota Clostridium tertium, a bacterium not found in ASD children [15].
In the work published by Wang et al. [16] after probiotics and fructooligosaccharides (FOS) ingestion in individuals with ASD, they observed an increase in beneficial bacteria (Bifidobacteriales and Bifidobacterium longum (B. longum) in gut microbiota and a decrease in the number of suspected pathogenic bacteria (Clostridium). Concomitantly, they observed a significant reduction in the severity of autism and GI symptoms [16].
Tomova et al. [17] looked at the potential effects of alteration in the composition of the microbiota in individuals with ASD in relation to food intake and food preference [17]. The authors of this study reported that in the ASD group, the microbiota was characterized by the abundance of Dichelobacter, Nitriliruptor, and Constrictibacter, whereas in the control group, Diaphorobacter and Nitratireductor were more abundant. The study also pointed out that fresh fruit and vegetable intake was significantly higher in healthy children than in children with ASD. They also observed that increasing the consumption of fruits and vegetables increased microbiota diversity, but this effect was only observed in the control group. The authors recommended increasing fresh fruit and vegetable consumption for ASD patients, keeping in mind that this may be very difficult to achieve [17].
Zou et al. [18] looked at the composition of the microbiota at the yeast level. The authors reported that Saccharomyces and Aspergillus were more abundant in the feces of individuals with ASD than in controls (59% vs. 40%). A decreased abundance of Aspergillus versicolor has also been observed in patients with ASD [18].
The goal of the study by Khalil et al. [19] was to assess the presence of Clostridium difficile in the feces of children with ASD; however, in this study, no significant differences in both Clostridium difficile detection and toxins A and B production were observed between individuals with ASD and controls [19]. Toxins A and B of Clostridium difficile are proteins produced by the bacterium Clostridium difficile. These toxins play a crucial role in the pathogenesis of the disease. Both toxins cause damage to intestinal epithelial cells, but they act differently on cells. They are responsible for inducing diarrhea and other symptoms associated with Clostridium difficile infection.
Zhang et al. [20] examined the beneficial effect of traditional Chinese medicine on autistic symptoms through its action on the microbiota of patients with ASD [20]. Bu Yang Huan Wu Tang [21] was given to change the composition of the gut microbiota. In this study, it is proposed that (1) a difference in gut microbiota composition exists between control and ASD patients, and (2) the consumption of the herb restores the composition of the microbiota. These intriguing findings are weakened by the total absence of data on ASD symptoms and the low number of individuals enrolled in this study. Microbial fermentation of plant-based fibers can produce different types of short-chain fatty acids (SCFAs) that may have beneficial or detrimental effects on the gut and neurological development of individuals with autism [21].
Zou et al. [22] characterized the nature of the microbiota through the sequencing of the bacterial 16S ribosomal RNA (16S rRNA) gene. The authors reported a decrease in the number of Firmicutes, Proteobacteria, and Verrucomicrobia in the microbiota of children with ASD and an increase in the abundance of Bacteroidetes/Firmicutes [22].
Ding et al. [23], reported that children with ASD have a higher biomass and an increased richness of gut microbiota. They also reported an increase in the relative abundance of unidentified Lachnospiraceae, Clostridiales, Erysipelotrichaceae, Dorea, Collinsella, and Lachnoclostridium, whereas the abundance of Bacteroides, Faecalibacterium, Parasutterella, and Paraprevotella was lower in patients with ASD. The authors also reported that ASD severity may be correlated with the presence of unidentified Erysipelotrichaceae, Faecalibacterium, and Lachnospiraceae [23].
The overall goal of the paper published by Laue et al. [24], was to prospectively characterize the association between the gut microbiome and ASD-related behaviors at the age of three. Gut composition and function were determined by 16S rRNA gene analysis and shotgun metagenomic sequencing, respectively. ASD-related social behavior was assessed using Social Responsiveness Scale (SRS) T-scores. SRS-2 performance was associated with several taxa, including those in the Lachnospiraceae family. Conversely, a higher relative abundance of Adlercreutzia equolifaciens and Ruminococcus was related to poor SRS-2 performance. The authors also reported that two functional pathways, L-ornithine and vitamin B6 biosynthesis, were associated with better social skills at three years old [24].
Hua et al. [25] reported a study carried out on 120 children diagnosed with ASD with or without sleeping disorders. The authors concluded that ASD children with sleep disorders exhibit a decline in the abundance of Faecalibacterium and Agathobacter and a decreased level of melatonin in their gut microbiota [25].
Santocchi et al. [26] explored the effects of probiotics on autism over a 6-month period. The authors observed that in some children without GI symptoms treated with probiotics, there was a significant improvement in ASD symptoms, especially in the social-affect domain, which is not related to the effect of probiotics on GI symptoms [26].
Hazan et al. [27] analyze the gut microbiota profile of one child with ASD and GI disorders in comparison with her triplet siblings. They reported a high Bacteroidetes/Firmicutes ratio, increased relative abundance of Proteobacteria, and lower relative abundance of Actinobacteria. The major limitation of this study was that it included only one individual [27].
Chen et al. [28] addressed the interesting question of the possible impact of the maternal gut microbiota on offspring microbiota and ASD symptoms. The authors of this study reported that the gut microbiota of children with ASD differed from that of healthy children [28].
Needham et al. [29] took another approach to understanding the non-behavioral features of ASD in association with microbiota. These data suggest that oxidative stress is increased in ASD patients. The authors concluded that a connection exists between general metabolism, including GI physiology, and behavioral traits [29].
In 2021, Zhang et al. [30] published an article comparing the composition of fecal samples from 21 adults with ASD and 21 obese adults. They observed that the relative amount of Firmicutes/Bacteroidetes in the gut of ASD individuals was significantly increased compared to that of obese adults. Particularly, Lachnospiraceae and Ruminococcaceae families were more abundant in ASD adults [30]. Even if obesity is found in adult patients with ASD, the rationale for comparing the microbiota between ASD individuals and obese individuals remains unclear.
The study by Fouquier et al. [31] is intriguing and interesting. In this study, they compared the gut microbiota composition between individuals with ASD and controls in two locations in the USA: Arizona and Colorado. The gut microbiome composition differed between individuals in Arizona and Colorado. Their results suggested that GI symptoms were higher in ASD individuals than in neurotypical individuals in Arizona but not in Colorado. The composition of the gut microbiota appears to be associated with ASD but not with GI symptoms. This result suggests that the relationship between microbiota composition and symptoms is unclear or very complex [31].
Ye et al. [32] published an article in which they compared the gut microbiome of 71 boys with ASD in comparison with 18 neurotypical controls. Significant differences were observed between the two groups. At the genus level, a decrease in the relative abundance of Escherichia, Shigella, Veillonella, Akkermansia, Provindencia, Dialister, Bifidobacterium, Streptococcus, Ruminococcaceae, and several others were observed in the ASD cohort, the abundance of Eisenbergiella, Klebsiella, Faecalibacterium, and Blautia was significantly increased. These results have not been discussed, and no attempt has been made to correlate these data with clinical aspects [32].
In 2021, Laghi et al. [33] published an article in which they realized a metabolomics analysis of the fecal microbiota in ASD patients. In this study, 59 molecules were identified and quantified in several groups of individuals: children with and without GI symptoms, and children with low vs. high autism severity [according to Autism Diagnostic Observation Schedule (ADOS) scores]. No links were identified between the severity of autism, intestinal symptoms, and the relative abundance of the analyzed microorganisms. According to the authors of this study, the results of their study suggest that “fecal metabolome discriminates the severity of autism (SA) and intestinal microorganisms mediate the link between metabolome and SA regardless of GI symptomatology” [33].
Ha et al. [34] investigated the fecal microbiota of Korean children with ASD to determine the gut bacterial profiles associated with ASD. This study was carried out using fecal samples obtained from 54 children with ASD and 38 age-matched control children. The authors reported that the composition of the gut microbiota differed between individuals with ASD and the controls. They also reported that SCFAs are more abundant in children with ASD. These authors reported lower Bacteroides levels and higher Bifidobacterium levels in the ASD group, they also found a significant decrease in the relative abundance of Bacteroidetes and an increase in the abundance of Actinobacteria in ASD patients. The authors concluded that these changes may be associated with functional alterations, including genetic information processing and amino acid metabolism [34].
In a remarkable work by Yap et al. [35], the authors reported the results of a metagenomics study performed on the stool of 247 ASD patients. The authors did not find any major link between gut microbiota composition and disease severity. Instead, they suggested that limited changes in the microbiota could be associated with a less diverse diet in patients with ASD. They concluded that ASD induced both restricted interest and a less-diverse diet, and subsequently, the lack of variety in the food-induced a minor microbiome composition. The authors concluded that “microbiome differences in ASD may reflect dietary preferences that relate to diagnostic features” [35].
Based on the analysis of 25 samples of fecal microbiota harvested from ASD individuals and compared with 20 controls, Ding et al. [36] identified that GI symptoms are strongly associated with symptoms of ASD. Their study also revealed that significant differences exist between individuals with ASD and controls in the composition of the microbiota (at the family, order, genus, and phylum levels). The authors concluded that there were significant differences in Actinobacteria and Firmicutes between the two groups. However, this study did not attempt to explain the potential link between these changes in the microbiota and disease severity. The association with less diverse foods has not been addressed [36].
A case-control study conducted by Xie et al. [37] on autistic and healthy children showed the potential risk of gut microbial dysbiosis in ASD onset by increasing the relative abundances of Actinobacteria, Proteobacteria, and Escherichia-Shigella, and decreasing the relative abundance of Blautia and some Lachnospiraceae. Such gut microbiota imbalance may disturb functional pathways such as amino acid metabolism, cofactor and vitamin metabolism, and the adenosine monophosphate (AMP)-activated protein kinase signaling pathway. Gut microbiota analysis showed increased relative abundance of Fusobacterium, Ruminococcus torques group, and Bacteroides plebeius DSM 17135, and reduced relative abundance of Ruminococcaceae UCG 013, Erysipelotrichaceae UCG 003, Parasutterella, Clostridium sensu stricto 1, Turicibacter, Clostridium spiroforme DSM 1552, and Intestinimonas butyriciproducens in autistic individuals compared to the control group [37].
In 2022, Chen et al. [38] published an intriguing paper. They studied the alpha and beta compositions of the gut microbiota from 81 individuals with ASD in comparison with 31 controls. The two populations were relatively homogeneous, with comparable (not similar) ages, intelligence quotients (IQs), etc. They characterized the composition of the microbiota and found that both groups showed comparable alpha diversity. However, in individuals with ASD, the authors reported a higher weighted UniFrac distance for beta diversity. Fusobacteria, Fusobacteriaceae, Fusobacteriaceae, Fusobacterium, and Prevotellaceae were more abundant in the gut microbiota of ASD patients. Conversely, they observed that Turicibacter, Erysipelotrichaceae UCG 003, Clostridiaceae 1, Clostridium sensu stricto 1, and spiroforme DSM 1552 were less abundant in the microbiota of ASD patients. Analysis of microbiota suggested that in ASD patients, metabolic pathways involving amino acid metabolism [Valine (V), Leucine (L), Isoleucine (I), and Arginine (R)], arachidonic acid, and energy metabolism were increased. Individuals with autism had worse GI symptoms than those with typically developing controls (TDC). The altered taxonomic diversity in ASD is significantly correlated with autistic symptoms, especially when looking at delinquent behaviors and self-dysregulation. However, they did not find any association between the altered microbial composition and gut symptoms in these patients. They concluded that their findings suggested that “altered microbiota are associated with behavioral phenotypes but not GI symptoms in ASD” [38]. This paper reports on a very well-conducted study.
Guidetti et al. [39] published a paper describing a randomized crossover study aimed at evaluating the effects of probiotics on GI symptoms and behavior in children with autism in 2022. They analyzed the results of a double-blind crossover study (probiotic vs. placebo) experiment using individuals aged 2–16 years old who were diagnosed with ASD. The experimental group was administered the drug every day for up to eight months. The mixture contained 10 × 109 colony-forming units (CFU) active fluorescent units (AFU) of Limoslactobacillus fermentum LF10, Ligilactobacillus salivarius LS03, Lactiplantibacillus plantarum LP01, and a mixture of five strains of Bifidobacterium longum. The total number of participants included in the study was 61. The results presented in this paper suggest that the administration of specific probiotics can partially but significantly reduce the severity of behavioral and GI symptoms, which may affect individuals with ASD. They concluded that “the presence of taxa related to Streptococcus thermophilus, Bifidobacterium longum, Limosilactobacillus fermentum, and Ligilactobacillus salivarius species in fecal samples may represent a huge probiotic intestinal colonization in these AD patients”. They concluded that many aspects of the connection between the gut and brain still need to be investigated [39].
In September 2022, Raghavan and collaborators [40] published a paper, which a randomized pilot clinical study was carried out to evaluate the composition and role of the gut microbiota of subjects with ASD after the food consumption of Nichi Glucan. In this study, 18 subjects with ASD were randomly allocated to a control group (6 subjects) or a group (12 subjects) supplemented with Nichi Glucan. The duration of treatment was 90 days. The authors reported alterations in the composition of the intestinal microbiota after Nichi Glucan treatment and noticed that Enterobacteriaceae was drastically decreased in the treated group; in these individuals, the level of Bacteroides also decreased. No real attempt has been made to correlate these observed changes with the disease, either at the intestinal or nervous levels [40].