The gut microbiota includes trillions of microorganisms that colonize the human gastrointestinal tract, including bacteria, archaea, viruses, and fungi. This complex ecosystem plays a critical role in maintaining intestinal and systemic health. Most of these microbes inhabit the large intestine, establishing a relatively stable and diverse community that contributes to gut homeostasis through various metabolic, immunological, and protective mechanisms.Dominant bacterial phyla, such as...

The gut–brain axis is a bidirectional communication system that connects the gastrointestinal tract and the brain. This interaction is mediated through multiple pathways, including the vagus nerve, hormonal signals, immune responses, and chemical messengers produced by gut microbes.Microbial Contributions to Brain FunctionGut microbiota contributes significantly to brain function by producing neuroactive compounds. These include neuroactive compounds that influence neurotransmitters such...

The human respiratory tract, comprising the upper and lower segments, serves as a critical interface with the external environment. The upper respiratory tract (URT)—including the nostrils, sinuses, pharynx, and oropharynx—is heavily colonized by microbes, while the lower respiratory tract (LRT), composed of the larynx, trachea, bronchi, and lungs, was long thought to be sterile. However, recent molecular studies have revealed that the lungs are not devoid of microbes but act more...

The human microbiota begins developing at birth and undergoes continual change as we age. Infancy marks a critical period of microbial sensitivity, offering a “window of opportunity” during which beneficial microbes help mature the immune system. By age three, children typically develop a more stable and diverse microbial community. Newborns acquire microbes from their immediate environment; vaginal delivery favors maternal vaginal microbes, while cesarean births favor microbes from...

Microorganisms colonize various regions of the human body, including the mouth, nasal passages, throat, stomach, intestines, urogenital tract, and skin. The total number of microbial cells is estimated to range from 10¹³ to 10¹⁴—comparable to, or exceeding, the number of human somatic cells. This host–microbiome relationship has led to the conceptualization of humans as supraorganisms, wherein microbial communities perform vital roles in development, immunity,...

The human gastrointestinal (GI) tract is characterized by distinct physicochemical conditions that shape its microbial communities. Among these, the stomach presents a particularly challenging environment for microbial colonization due to its highly acidic pH, ranging from 1 to 3. This extreme acidity effectively limits microbial density. However, certain acid-tolerant microorganisms are capable of surviving in this niche. Notably, Helicobacter pylori can colonize the gastric mucosa,...