Normal Flora of the Human Body

Normal Flora in the Human Body

1. Overview

The human body hosts roughly 10¹⁴ microorganisms—about 10 times the number of human cells—forming a complex ecological community essential for health. (NCBI Source)

2. Major Body Sites & Principal Microbes

Body Site	Common Microbes	Role in Health
Skin	Staphylococcus epidermidis, Propionibacterium acnes	Protects against pathogens; maintains pH
Oral Cavity	Streptococcus spp., Neisseria spp.	Begins digestion; prevents overgrowth of harmful bacteria
GI Tract	Bacteroides, Escherichia coli, Lactobacillus	Vitamin synthesis; nutrient absorption; immune modulation
Genitourinary	Lactobacillus, Corynebacterium, skin staph	Acidifies environment; prevents infections
Respiratory (upper)	Staphylococcus aureus, Streptococcus pneumoniae	Occupies niches so pathogens can’t colonize

3. Functions of Normal Flora

• Colonization resistance: Occupy niches to block pathogens
• Nutrient production: Synthesize vitamins (B, K) and SCFAs
• Immune education: Prime immune system
• Digestion: Break down complex carbs

4. When Normal Flora Turns Pathogenic

• Skin breach → Staph epidermidis → endocarditis in IV drug users
• Antibiotics → GI flora disruption → C. difficile overgrowth
• Immunosuppression → oral Candida → thrush

5. Quick Quiz: How Well Do You Know Your Flora?

1. Which site has the highest density of microbes?
A. Skin
B. GI tract
C. Oral cavity
D. Genitourinary

Answer: B. GI tract

2. Name one beneficial function of gut flora.

Possible answers: Vitamin K production, fermentation of fiber to SCFAs, pathogen resistance.

3. True or False: Antibiotics can cause diseases by disrupting normal flora.

Answer: True

References

• Medical Microbiology, Chapter: Normal Flora – NCBI Bookshelf