Chapter 11

The human microbiome is defined as the sum total of microbes living in or on an individual person.
Sometimes referred to as microbiota, which means the same thing.
Importance of the human microbiome in human health is significant, with implications for various diseases.

Importance of the human microbiome cannot be overstated.
Up to 90% of diseases can be traced back to gut health.
Studies continue to link gut microbiome health to:
- Autism
- Crohn's disease
- Autoimmune diseases
- Susceptibility to cancers and multiple other illnesses.
There is a vast number of microorganisms in and on the human body, with ten times more bacteria than human cells.
Generally, the microbiome is beneficial, offering protection against various pathogens, educating the immune system, and contributing to nutrient processes.

Types of Bacteria:
- Colonized Bacteria:
- Bacteria that live permanently or predominantly within a human host.
- Example: E. Coli, commonly found in the gut.
- Transient Bacteria:
- Bacteria that are only temporarily present in or on the human body.
- They can be eliminated by the immune system or environmental changes.
Most interactions (colonized and transient) are typically positive or neutral; however, some interactions can lead to diseases.
Infection occurs when a microbe invades human tissues and causes harm.

It is hard to conceptualize the number of microorganisms humans interact with daily; e.g., millions of bacterial spores inhaled or touched daily.
Most bacteria encountered are non-pathogenic and do not cause illness.
There are periods of colonization throughout life, starting with exposure at birth.

Each person's microbiome is unique, akin to a fingerprint, influenced by factors like:
- Genetics
- Environment
- Diet
- Medical history
- Age
- Physical sex
- Stress levels.
Microbial diversity exists within individuals due to different body environments:
- Different microbial communities thrive in moist versus dry or oily versus non-oily skin areas.

Propionibacterium acnes is one of the organisms in the Propionibacteriaceae family.
This bacterium causes acne and feeds on skin oils (sebum) produced by the skin.
Its prevalence in oily areas of the skin correlates with acne occurrences.

All healthy individuals harbor potentially dangerous pathogens in low numbers.
The concept of microbial antagonism explains how non-pathogenic microbes prevent pathogenic ones from causing disease.
Changes in the microbiome or the immune system can lead to infections.

Early colonization may begin before birth, although historically it was thought the womb was sterile.
Current studies indicate there may be microbes present in the uterus.
Babies born vaginally receive significant bacterial exposure from the mother during birth, which is crucial for early microbiome development.
Babies born via C-section miss this exposure, potentially leading to a slower development of their immune systems.

Microbes can invade tissues to cause disease via:
- True Pathogens: Cause disease in healthy individuals a high percentage of the time (e.g., influenza).
- Opportunistic Pathogens: Generally cause disease in immunocompromised individuals or when they move to a different body area (e.g., E. Coli causing urinary tract infections when moved from the gut).