Gut microbiota effects on developmental benchmarks in neurotypical infants

Introduction

Autism spectrum disorder (ASD) is a disorder characterized by an onset of neurodevelopmental delays around the ages of two to three [1]. Some delays include the loss of motor, communication, and social skills. Infant brain development is primarily categorized via developmental scales, where infants with low-scoring cognitive benchmarks have an increased diagnostic chance for neurodevelopmental disorders like ASD. Thus far, pathologies that arise during critical stages of infant brain development such as ASD and attention deficit hyperactivity disorder (ADHD) have no known causes. While infant trials are scarce, preliminary mice trials have proven associations between neurological pathologies and gastrointestinal microbiota (GIM). However, the findings act as an indirect model of the gut-brain axis for humans, requiring further research [2-3]. Presently, research on the human gut-brain axis connection has found similarities to these mice models. One such similarity includes a significant association between an infant's GIM and their developmental windows [4]. This review will critically examine recent studies which associate GIM with developmental benchmarks in infants. By highlighting these associations, further exploration into the effects of early-life microbiota on neurological development could provide an alternative perspective to treatment and understanding of early onset developmental pathologies. Furthermore, these potential associations could reveal the effects of GIM on other non-neurological pathologies.

Background 

During infancy (defined in this paper as 0-3 years), a person's brain and behavior develops as their gastrointestinal tract is rapidly colonized by bacteria, referred to as gastrointestinal microbiota (GIM) [2, 5-7, 9]. The colonization of the GI tract begins with anaerobic bacteria such as actinobacteria, proteobacteria, and classical skin bacterias [5, 8]. Due to changes in hygiene practices, previous early colonizers like Eschericia coli and strains from the genus Bacteroides have now been reclassified as late colonizers, while Clostridium difficile is now an early colonizer of the GI tract. [8]. Although the shifts in the  microbial colonization of an infant's gut and its effects are not well studied, the study on the impacts of early gut microbiota has gained traction. Within the last five years, studies highlighting the role of specific strains of microbiota and their associations to fine motor development and other critical benchmarks like language and socioemotional have surfaced. One major predictive benchmark for later ASD diagnosis in infants includes the fine motor benchmark, so understanding the associations between GIM and neurodevelopmental benchmarks serves as a possible avenue for intervention in ASD treatment.

Fine Motor Development & Microbiota

Developmental scales separate motor skills in infants into two categories. Gross motor (GM) benchmarks test generalized movement and fine motor (FM) benchmarks examine precision movements such as writing and drawing. Low FM benchmark scores are associated with multiple areas of behavioral and cognitive development. This correlation is not seen for GM benchmark scores. FM scores on benchmark tests are also associated with language and communication development [2, 6]. Poor FM benchmark scores in infants are highly predictive of future neurodevelopmental pathologies [10]. Although correlations between neurodevelopmental pathologies and FM benchmark scores are recognized, causes of poor FM skill development in infants are not known. Thus, studies into an infant’s GIM have been exploring such connections.

Recent infant trials have shown associations between the abundance of Bifidobacteria, Firmicutes, and Bacteroides in the early gut microbiome and its varied effects on the FM benchmark. Acuna et al. (2021) analyzed infant GIM in relation to multiple cognitive benchmarks. The FM benchmark scores established a link between the presence as well as a large quantity of Bifidobacteria and a low population of Bacteroides (a genus of anaerobic bacteria) and above average FM skills in infants (18 months). Using the Bayley Scale (Bayley-III) for infant development, the researchers tested the infants’ language, cognitive, and motor skills and dichotomized the results into above and below median categories. Infant microbiomes were analyzed using 16s rRNA gene sequencing and specific microbiota were identified using operational taxonomic unit (OTU) profiles. Results show that infants scoring above-median in the FM skill category have an abundance of Bifidobacterium, Lachnospiraceae, and Streptococcus in their microbiome in comparison to other infants within each cohort, referred to as beta-diversity (ꞵ–diversity) [5]. The researchers suggest a possible association between poor Bifidobacterium populations and neurological pathologies in infants. 

Using a different developmental scale, Sordillo et al. (2019) found similar associations. When infants were 3-6 months old, fecal samples were collected for gene sequencing to identify gut microbiota populations. At 3 years of age, the infants’ primary caregivers were given the Ages and Stages Questionnaire (ASQ-3), a survey which evaluates their infant's cognitive benchmarks. Sordillo et al. categorized the microbiota they found into four factors: F1, F2, F3, and F4. Of the most significant, F1 represented an abundance or positive loading for Lachnospiraceae and Clorstridales along with a decreased abundance or negative loading for Bacteroides while F3 presented a positive loading for Bacteroides and a negative loading for Bifidobacteria. Results indicated that infants with a ꞵ–diversity characterized by F1 had poorer social and communication scores while infants characterized by F3 had lower FM skill scores [6]. Based on the findings of both Sordillo et al. and Acuna et al., an abundant Bacteriodes population has an association to poorer FM scores in infants ages 18 months to 3 years old. Furthermore, Sordillo et al.'s findings also support Acuna et al.’s findings on Bifidobacterium by highlighting the potential negative effects of a poor Bifidobacterium presence on FM scores in infants (3 years). Although Sordillo et al. and Acuna et al. found corroborating results, the differences in age between the infants studied and fecal samples collected highlight potential limitations. Unlike Acuna et al.’s study which featured fecal samples from 18 month old infants, Sordillo et al.’s data is from an auxiliary study. Using previous data from a VDAART study, the fecal samples came from infants aged 3-6 months while the benchmark results came from the sample population at 3 years old. This indicates that the data shows potential correlations between GIM and fine motor development in infants from ages 3 months up to 3 years old. Additionally, the findings further propose potential associations between Firmicutes and Bacteroides on the language and communication benchmark. 

Language & Communication Benchmark 

The language and communication (LC) benchmark is another test of infant development. The LC benchmark tests for expressive communication in infants using preverbal behavior and receptive communication [2, 6, 11-12]. Expressive communication in infants can be quantified in multiple aspects, including rhythm and neural rhythm tracking. Rhythm is an infant's ability to process cognitive information through time and neural rhythm tracking is a researcher's preferred method to characterize an infant's rhythm deficiencies on a timed axis [9, 11]. Deficits in these categories have been linked to poorer neurogenesis [9]. Sordillo et al. (2019) using the same methodology used to link FM benchmark scores with GIM, found a negative association between F1 and the LC benchmark. Such findings suggest that an abundance of Firmicutes and a decreased abundance of Bacteroides in an infant’s GIM are associated with negative impacts on communication. 

Sordillo et al.’s findings are similar to the results found by Tamana et al. on infants ages 1-2 years old. Tamana et al. (2021), using the Bayley-III Scale for infant motor, language, and cognition development, identified a significant association between Bacteroides and positive language composition scores. The associations found indirectly support Sordillo et al.’s findings where a decreased population of Bacteroides has a negative association with language and communication. Although Sordillo et al. did not highlight any major sex-dependent results, Tamana et al. found sex-dependent results for microbiota and the LC benchmark [12]. These results found positive associations between male infants, the LC benchmark with both Firmicutes and Bacteroide dominant clusters [12]. The results indicate potential significant sex-dependent differences between the GIM and cognitive benchmark scores which were not explored by Sordillo et al. or other studies in this review. Future research should highlight sex as a confounding factor in relation to language and communication benchmark scores for infants. 

Socioemotional Benchmark & Microbiota

The social and emotional (SE) benchmark is characterized by an infant's ability for self-regulation, temperament, and attentional bias [11]. Self-regulation is the infant's ability to engage and disengage from stimuli [11, 13-14]. Temperament is characterized by an infant's surgency (sociability and positive emotionality) [11]. Surgency in infants has a negative association with ASD symptoms and depression prognosis [13-16]. Research highlights associations between Bacteroides, Bifidobacteria, and other early GI colonizers to surgency and regulation, indicating potential correlations between GIM and socioemotional development in infants.

Aatsinki et al. (2019) identified infants with poor temperament using the Infant Behavioral Questionnaire-Revised Short Form (IBQ-R SF). Aatsinki et al. took infant fecal samples at 2.5 months old and administered the questionnaire to caregivers at 6 months old. Similarly, Fox et al. (2021) also tested infants using the IBQ-R SF, but at 12 months of age. While the methodology of the two studies are similar, the findings differed. Aatsinki et al. established three groups with GIM dominated by clusters of either Bacteroides, Veillonella, or Bifidobacteria. Results include a positive association between Bifidobacteria and regulation in infants while the Bacteroides dominant clusters were associated with negative regulation. 

In terms of surgency, Aatsinki et al. found a negative association between Veillonella dominant clusters while Bifidobacteria had a positive association with surgency. Similarly, Fox et al. found Bifidobacteria had a positive association with surgency using fecal samples previously obtained from the infants at 1-3 weeks old but significant findings on regulation were not reported.  Loughman et al. (2020) also studied GIM and temperament in infants employing a different behavioral developmental scale and found additional connections between behavior and the early-gut microbiota. With a child behavioral checklist (CBCL), caregivers were asked to categorize their infant’s temperament at 2 years of age. The CBCL categorized the infant’s temperament into three categories, each highlighting specific components of overall behavioral issues identified by the caretaker. Using this scale and 16s rRNA gene sequencing, Loughman identified a positive correlation: a reduced abundance of Prevotella indicated worse overall behavioral development according to caretakers. 

Further study by Aatsinki et al. (2020) into specific components of temperament highlighted an association between Prevotella and fear bias. Fear bias is identified as a form of attention bias in infants that identifies which emotions they engage with most [11, 14]. Aatsinki et al. found a negative association between Prevotella, Bifidobacterium, and Lactobacillus on fear bias. A negative association between the microbiota and fear bias indicates less engagement with fearful emotionality in infants whose GIM are dominated by those bacterial strains [14]. Implications for these findings suggest a correlation between abundant Prevotella and positive temperament. Bifidobacterium’s association with emotional regulation and reduced fear bias, as found by Aatsinki et al., suggest that Bifidobacterium affects multiple domains of temperament. This multipronged association is not seen in other microbiota besides Prevotella [3, 11, 14]. Overall, this highlights a need for further research into Bifidobacteria and Prevotella’s association with infant temperament. 

Conclusion 

The purpose of this review was to identify current research trends in studies that examine the effects of gastrointestinal microbiota on neurodevelopmental benchmarks in infants. Most studies in this field are still primarily observational. Researchers should explore the many confounding factors between  the infant diet, GIM colonization, and neurological development. Small sample sizes and environmental factors are further confounding variables that may be resolved with an experimental study design. However, while these studies are inconclusive, the potential impact of the gut-brain axis on infant development provides a solid foundation for understanding developmental pathologies like Autism Spectrum Disorder and other neurodegenerative disorders. Further research into the gut-brain axis could investigate beyond neurological implications or provide insights into the relationship between GI tract dysregulation (a common symptom of ASD) and ASD.

About the Author: Tiffany Liu

Tiffany Liu is a third-year undergraduate student pursuing a degree in Neurobiology, Physiology and Behavior at UC Davis. During her sophomore year, she discovered her love for physiology due to its importance for understanding unknown pathologies affecting the body. With this new found passion for physiology, Tiffany’s curiosity for the human body expanded upon the concepts introduced to her during her courses. This led her to the focus of this article. While highlighting the importance of microbiota on physiology and behavior, the article also serves as a reminder to readers that the human body is impossible to simplify into just a few systems. Hopefully readers will recognize there is much more to explore about our physiologies beyond what is known, so it’s imperative to always stay curious. 

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