Molecular Microbiology Research 2024, Vol.14, No.1, 10-19 http://microbescipublisher.com/index.php/mmr 13 Moreover, metabolic products of microbiota also play crucial roles in the development of chronic diseases. Metabolites produced by certain microbiota, such as short-chain fatty acids, vitamins, etc., are essential for host health. Their deficiency or excess may affect the metabolic balance of the host, thereby increasing the risk of chronic diseases (Palmu et al., 2020). Microbiota also indirectly influence the occurrence of chronic diseases by affecting the host immune system. Microbiota can train and regulate the host immune system, affecting its response to foreign pathogens and self-cells. When microbiota balance is disrupted, it may lead to an overreactive or inadequate immune response, thereby triggering or exacerbating chronic diseases. 2.2 Interaction between the microbiota and host system and its impact on chronic diseases The interaction between the microbiota and the host immune and metabolic systems constitutes a complex network for human health. The microbiota exerts profound effects on the host immune system through its diversity, the release of metabolites, and direct communication with host cells. Valdes et al. (2018) found that the gut microbiota can stimulate specific immune responses in the host to combat pathogens while also helping to maintain immune system balance and prevent overreaction. However, imbalance in the microbiota may disrupt immune system function, thereby increasing the risk of chronic diseases such as inflammatory bowel disease or autoimmune diseases. Simultaneously, the relationship between the microbiota and the metabolic system is also significant. The microbiota influences host metabolism by participating in the metabolism of nutrients, synthesizing essential vitamins, and producing short-chain fatty acids, among other functions. For example, certain gut bacteria can affect the host's metabolism of carbohydrates, thereby influencing blood sugar levels and the risk of diabetes. This interaction between the microbiota and the metabolic system makes the microbiota a key factor in regulating host metabolic balance. The occurrence and development of chronic diseases are often associated with imbalances in the interaction between the microbiota and the host immune and metabolic systems. When this interaction goes awry, it may lead to abnormal immune responses, metabolic disorders, and ultimately the onset or exacerbation of chronic diseases. 2.3 The impact of chronic diseases on the microbiota and potential feedback mechanisms The impact of chronic diseases on the microbiota is multifaceted and profound. Under the state of chronic illness, the host's internal environment equilibrium is disrupted, directly leading to dysbiosis of the microbiota. For instance, in diabetes, the elevated blood sugar levels alter the osmotic pressure in the gut, affecting the survival and metabolism of microorganisms, resulting in a decrease in beneficial bacteria and an increase in harmful ones. Similarly, cardiovascular diseases and autoimmune diseases also lead to changes in the microbiota, with the inflammatory environment and metabolic abnormalities associated with these diseases negatively impacting the microbiota (Wells et al., 2020). Chronic diseases may also indirectly affect the microbiota by influencing the host immune system. Aberrant immune system responses may cause changes in the composition and function of the microbiota. For example, in inflammatory bowel disease, an overactive immune system leads to decreased diversity in the gut microbiota and an increase in harmful bacteria. Research by Ogunrinola et al. (2020) found that end products released by gut microbiota can affect the coverage of intestinal cells, promoting carcinogenesis or inhibiting tumor occurrence. Apart from colorectal cancer, the gut microbiota also plays a role in extraintestinal cancers (such as hepatocellular carcinoma) through systemic dissemination to other parts of the body (Figure 2). It is worth noting that the relationship between the microbiota and chronic diseases is not unidirectional. The microbiota may also influence the development of chronic diseases through its metabolic products and interactions with host cells, forming feedback mechanisms. For example, metabolites produced by certain
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