# Microbes in Health and Disease: Human Gut Microbiota
## 4 Associations with Human Diseases  
Dysbiosis of the normal gut microbiota is associated with digestive, metabolic, cardiovascular, immune, and neural diseases, among others [85, 99, 100, 101, 102, 103]. In a cross-sectional study including 355 patients with inflammatory bowel disease (IBD), 412 patients with irritable bowel syndrome (IBS), and 1025 healthy controls, similar dysbiotic profiles of the gut microbiome were observed in two main IBD types ([PERSON]'s disease and ulcerative colitis) and were associated with gut inflammation; for instance, an increase in Bacteroides occurred only in IBD patients, and a decrease in butyrate-producing bacteria and increases in _Actinomyces_ and _Streptococcus_ were observed in IBS patients [104]. A group of special _E. coli_ strains (adherent-invasive _E. coli_, AIEC) was identified in patients with [PERSON]'s disease and is characterized by hypercomtility and increased acetate consumption, which exacerbates inflammation during mucosal injury [105]. A systematic review revealed that patients with IBS generally had lower gut microbiota diversity; increased Firmicutes, Clostridia, and Clostridales; a higher Firmicutes/Bacteroidetes ratio at the phylum level; and decreased Bacteria and Bacteroidales at lower taxonomic levels compared to healthy controls [106]. While several studies have shown that obese individuals have a relatively high Firmicutes/Bacteroidetes ratio, contradictory observations have been reported in other studies. A recent review revealed that the Firmicutes/Bacteroidetes ratio is not a convincing biomarker of obesity and that the obesity-associated gut microbiota pattern is heterogeneous around the world, although the gut microbiota affects the development of obesity [107]. A metagenome study including 218 Chinese subjects with atherosclerotic cardiovascular disease (ACVD) and 187 healthy controls revealed that compared with healthy controls, ACVD patients had increased _Enterobacteriaceae_ and _Streptococcus_[108]. A systematic review of the associations between the gut microbiota and allergic diseases revealed that allergic children had greater numbers of _Bacteroides_; lower numbers of _Akkermansia muciniphala_, _Faecalibacterium prausnitzii_, and _Clostridium_; a higher prevalence of _Bifidobacterium adolescentis_; a lower prevalence of _Bifidobacterium catenulatum_ and _Staphylococcus aureus_; and lower bacterial diversity [109]. Allergic diseases include food allergy and non-food allergies (e.g., eczema, asthma, drug allergy, and allergic rhinitis) and are inflammatory disorders resulting from an abnormal immune response to these innocuous substances (termed as allergens) [110]. In the past years, food allergy has become an increasingly prevalent health problem in children and adults around the world [111] and is caused by various factors (dysbiosis of gut microbiota, impaired integrity of intestinal epithelium, etc.) [112]. The mechanisms are associated with the Th2/Th1 and Th17/Th1 ratio imbalance [111]. Immunoglobulin (IgE)-mediated food allergy is a common pattern; briefly, under certain circumstances (e.g., impaired intestinal epithelium), the allergens trigger the immune response of the T helper 2 (Th2) cells that activate B cells to release IgE antibody [113; 114], the allergen-specific IgE binds to basophils and master cells via the high-affinity surface receptors on the surface, and subsequently, various inflammatory mediators (e.g., histamine) are released in repeated exposure to the allergen [110], increasing the vascular permeability and eventually causing the allergic symptoms (urticaria, diarrhea, etc.) [111; 115]. Food allergy was found to be associated with gut microbiota in human and animal studies [116; 117; 118].