Normal Resident Microbiota and Infection

Definitions and Key Concepts

The human body is host to a diverse community of microorganisms, many of which form mutual or commensal associations. These microbes are collectively known as the normal resident microbiota (also called indigenous microbiota, microflora, normal flora, or commensals).

• Infection: The process by which pathogenic microbes penetrate host defenses, enter tissues, and multiply.

• Pathogen: A microbe that acts as an infectious agent.

• Infectious disease: An infection that causes damage or disruption to tissues and organs.

Resident Microbiota: The Human as a Habitat

Distribution of Microbiota

Internal organs, tissues, and fluids are typically microbe-free. Most areas of the body in contact with the external environment harbor resident microbes.

• Transients: Microbes that occupy the body for only short periods and do not become established.

• Residents: Microbes that become established as part of the normal flora.

Table: Sites That Harbor Normal Resident Microbes

Origins and Stability of Resident Microbiota

• Generally stable, but can fluctuate with health, age, diet, hygiene, hormones, and drug therapy.

• Microbiota often benefit the host by preventing overgrowth of harmful microbes (competitive exclusion).

• Endogenous infections: Occur when normal flora is introduced to a site that was previously sterile.

Table: Sterile (Microbe-Free) Anatomical Sites and Fluids

Colonizers of the Human Skin

Skin Microbiota

The skin is the largest and most accessible organ, hosting two main populations of microbes:

• Transients: Cling to the surface but do not grow there.

• Residents: Stable, predictable, less influenced by hygiene; primarily bacteria (e.g., Staphylococcus, Corynebacterium) and yeasts.

Microbial Residents of the Gastrointestinal Tract

GI Tract Microbiota

The gastrointestinal (GI) tract is a long hollow tube lined by mucous membranes. Microbial distribution varies due to shifting conditions such as pH, oxygen tension, and anatomy.

• Large intestine: High microbial density, pH ~7, low O2; dominated by facultative aerobes (e.g., E. coli) and anaerobes (Clostridium, Bacteroides).

• Stomach: Fewer microbes due to low pH (~2); Helicobacter pylori is commonly found on the stomach wall in ~50% of the world's population.

Table: Major Bacteria Present in the GI Tract

Microbiota of the Mouth

Oral Microbiota

• The most common residents are aerobic Streptococcus species: S. sanguis, S. salivarius, S. mitis.

• Extensive growth results in a thick bacterial layer called dental plaque.

• Dental caries: As dental plaque accumulates, microbes produce organic acids that decalcify tooth enamel, leading to tooth decay.

• S. mutans and S. sanguinis are major contributors to dental caries.

• Infection along the periodontal membrane can cause gingivitis (gum inflammation) and periodontal disease (tissue and bone loss).

Microbiota of the Large Intestine

Intestinal Microbiota

• Complex interactions with the host; 108 to 1011 microbes per gram of feces (≥30% fecal volume).

• Environment favors anaerobic bacteria: Bacteroides, Bifidobacterium, Fusobacterium, Clostridium.

• Many species provide useful byproducts, such as vitamins (e.g., pyridoxine, riboflavin, thiamine) and acids (acetic, butyric, propionic).

• Bacterial enzymes convert disaccharides to monosaccharides and promote steroid metabolism.

• Intestinal bacteria contribute to intestinal odor.

Maintenance and Disruption of Normal Microbiota

• Normal flora create an environment that prevents infections and enhances host defenses.

• Oral antibiotics decrease all microbes in the gut, leading to loss of antibiotic-susceptible bacteria and overgrowth of opportunistic organisms (e.g., Staphylococcus, Proteus, Clostridium difficile, Candida albicans).

• Probiotics: Introduction of known microbes back into the body to restore normal flora.

Additional info:

• These notes are based on textbook-style content and are suitable for college-level Anatomy & Physiology students studying the human microbiome and infection.