*3.1. Early-Life Gut Microbiome*

Although microbes colonize the neonatal gut immediately following birth [52], microbial colonization continues to develop and vary in species abundance until a typical adult-like gut microbiota is established at the age of 2–3 years [53]. A variety of maternal factors and early-life events determine the establishment of the gut microbiome, such as

gestational age, type of delivery, maternal conditions, formula feeding, antibiotic exposure, and ecological factors [52–55].

During pregnancy and lactation, the mother gut microbiota can influence offspring gut microbial structure and composition, which highlights the importance of maternal factors in the establishment of early-life gut microbiome [55]. Several risk factors related to CKD of developmental origins have also been linked to alterations of gut microbiota, such as gestational diabetes [56], maternal obesity [57], prematurity [58], LBW [59], and maternal malnutrition [60]. Additionally, the establishment of the microbiome is highly interconnected with development of the immune system, and CKD has strong immune and inflammatory etiologies [61].

Moreover, several environmental chemicals that pregnant mothers are likely to be exposed to are associated with developmental origins of kidney disease [62]. Among them, exposure to heavy metals, polycyclic aromatic hydrocarbons, and dioxins also affect the gut microbiome, accompanied with the development of adult diseases [63]. All of these studies suggest that the early-life microbial alterations after the CKD-related adverse insults may be involved in the development of kidney disease in later life.
