There is evidence linking the gut microbial community to a range of diseases and disorders, including inflammatory bowel disease (IBD), mood disorders, obesity, autism and psoriatic arthritis
[52][53][54][55][56][57], though further research is needed to ascertain the causal link between the gut microbiota and such diseases. In some of these conditions, interventions that impact the gut microbial community have led to improvement of symptoms, further supporting a role for the microbiota
[58][59][60]. An altered gut microbiota can be caused by environmental factors such as antibiotic use, diet and stress, as well as genetic factors. These changes can impair the ability of the gut microbiota to maintain good health and may allow the growth of potentially pathogenic bacteria (e.g.,
Clostridioides difficile), leading to production of metabolites that may cause a disease state in the host
[61]. The relative abundance of bifidobacteria in faecal samples of normal-body-weight children was found to be higher than in overweight children of the same age bracket (7 years old). In contrast,
Staphylococcus aureus levels were higher in the overweight children than in those with normal body weight
[56]. In addition, a recent study reported an increased abundance of Firmicutes and a reduction in bifidobacteria in the gut microbiota of overweight and obese children
[62]. Furthermore, it has been observed that children who became overweight/obese at age 10 years had significantly higher levels of
Bacteroides fragilis in early infancy (3 to 6 weeks after birth) than those whose body weight remained normal
[63]. These studies thus indicate that the microbial community of children may differ according to BMI, suggesting that the microbiota may have a part to play in weight gain, for example, through energy salvage. However, caution is required when considering causal links between the gut microbiota and obesity, as recent meta-analyses failed to find variations in the taxonomic microbial compositions of obese and lean adults, suggesting that microbiota differences observed in other studies could be related to factors such as diet
[64][65][66]. Several other conditions, including eczema, asthma, inflammatory bowel disease (IBD) and type 1 diabetes, in infants and young children have been linked to differences in the gut microbiota. For instance, some studies show that infants with eczema have a significant reduction in relative abundance of
Bifidobacterium, Blautia, Coprococcus, Eubacterium and
Propionibacterium species
[67] as well as a reduction in intestinal microbial population diversity
[68][69]; although it is worth noting that in healthy infants, there is low microbiota diversity predominated by bifidobacteria. Moreover, an analysis of faecal microbiota of infants (1 to 11 months old) revealed that gut microbiota alteration promotes the disfunction of CD4
+ T cells, which is linked to atopy in children
[70]. Further, many studies have reported the gut microbiota to be altered in IBD
[71][72][73][74], with an increase in Enterobacteriaceae being particularly prominent
[75]. In addition, it has been found that children with prediabetes have higher levels of intestinal Bacteroidetes than healthy controls
[76].