The human mouth hosts thousands of bacterial species whose distribution is determined by oxygen exposure. For example, oxygen-exposed surfaces like the inner cheek have the least diverse microbiota, dominated by *Streptococcus*, whereas oxygen-limited spaces like the crypts of the tongue and interdental spaces support highly diverse microbiota, including anaerobes like *Bacteroides* and *Fusobacterium*.

Diversity and Composition of Oral Microbiota

The predominant bacterial species change dramatically across different anatomical sites of the same individual. For instance, *Streptococcus* is predominant on the inner cheek but decreases in the throat, palatine tonsils, and saliva, where *Fusobacterium* is more abundant. Furthermore, *Fusobacterium* dominates gum plaque, but both *Streptococcus* and *Fusobacterium* are absent in the intestine, where *Bacteroides* becomes the dominant genus.

Microbiota Variation Across Human Body Sites

The resident microbiota exhibits substantial diversity in both species variety and preferences for specific anatomical locations. Different body sites present distinct physical environments, such as varying levels of oxygen exposure, which dictate the specific microbial populations that can colonize them.

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Within the human microbiome, resident microbiota consists of microorganisms that constantly live in or on the body, with their composition subject to alteration by hygiene and diet.

Resident Microbiota

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

Diversity and Anatomical Preference of Resident Microbiota

Resident microbiotas are crucial for human health because they occupy ecological niches throughout the body. By occupying these sites, they prevent pathogenic microorganisms from colonizing these areas and establishing infections.

Protective Role of Resident Microbiota

Antibiotic therapy can disrupt the normal microbiotas of the intestinal and respiratory tracts. This disruption increases the risk of secondary infections and can promote the long-term carriage and shedding of pathogenic microorganisms.

Antibiotic Disruption of Intestinal and Respiratory Microbiotas

Although the human microbiota contains many beneficial microbes, healthy individuals also host serious pathogens in their normal microbial communities. For example, the conjunctiva of the healthy human eye can harbor 24 bacterial genera and multiple pathogenic species, while a healthy mouth often hosts Streptococcus species like Streptococcus pyogenes and Streptococcus pneumoniae. The presence of these organisms raises the question of why some microbes exist commensally in certain hosts but function as lethal pathogens in others.

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