Early scientific estimates suggested that prokaryotic cells (primarily bacteria) within the human body outnumbered human cells by a ratio of $${}10:1$$. More recent research indicates the actual ratio of bacteria to nucleated human cells is closer to $${}1:1$$, which still represents a massive microbial presence.

Ratio of Prokaryotes to Human Cells

Humans acquire their initial inoculations of normal flora during natural birth and shortly after birth. This initial colonization is followed by the acquisition of additional microbes from parents, healthcare providers, and other individuals who come in contact with the infant.

Acquisition of the Human Microbiome

The delivery method determines the initial microbial colonization of a newborn. Vaginally delivered infants are predominantly colonized by maternal vaginal *Lactobacillus* spp., which is associated with a lower incidence of diseases. Conversely, infants delivered by cesarean section are more frequently colonized by skin microbiota, including common hospital-acquired pathogens.

Effect of Delivery Mode on Infant Microbiome Colonization

The human microbiome is not static; it continues to evolve over an individual's lifetime as new microbes colonize and others are eliminated. For example, the microbiota of the small intestine can change rapidly, with an estimated half of its microbial inhabitants shifting within a nine-hour period.

Temporal Dynamics of the Human Microbiome

The Human Microbiome Project (HMP), launched by the National Institutes of Health (NIH) in 2008, is a collaborative research initiative designed to generate a comprehensive repository of gene sequences from microbes inhabiting the human body. By compiling metagenomic and clinical data from diverse body sites, the project aims to establish a reference database of the human microbiota, characterize microbial profiles in both healthy and diseased cohorts, and elucidate the relationship between microbial composition and human disease susceptibility.

Human Microbiome Project

The human microbiome refers to the totality of microorganisms, particularly bacteria and other prokaryotes, that live on or within the human body. These microbes populate various anatomical sites—thriving in areas like the mouth, nasal cavity, throat, gastrointestinal tract, vagina, and skin—and their composition varies between individuals, differs across different body regions, and changes over time.

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The microbiome refers to all microorganisms, prokaryotic and eukaryotic, that are living and adapted to a certain environment. It also includes the genetic material of these organisms. 

Microbiome

https://openstax.org/details/books/microbiology/

Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

The Human Microbiome

The microbiota refers to the collection of microorganisms living in a specific environment. In the context of the human body, this refers to the totality of microbes (especially bacteria and other prokaryotes) inhabiting a particular region, such as the mouth, throat, gastrointestinal tract, eye, or vagina.

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