Symbiotic Relationships and the Human Microbiota

The Human Microbiome

The collection of microorganisms that colonize the body's surfaces without typically causing disease is known as the microbiome. Depending on the context, this is also referred to as the normal microbiota, normal flora, or indigenous microbiota.

The microbiome is not a static entity; it is established during the birthing process, and much of a person's resident microbiota is settled during the first months of life. It is important to note that certain parts of the body are axenic, meaning they are naturally free of microorganisms. Examples include the alveoli of the lungs and the deeper layers of the skin (the dermis and hypodermis).

Resident vs. Transient Microbiota

Microbiota are generally divided into two functional categories:

1. Resident microbiota: These microbes are a permanent part of the normal flora throughout a person's life. They are mostly commensal in nature. They occupy specific niches, such as the skin, the upper respiratory tract, and the digestive tract.

   

2. Transient microbiota: These microbes occupy the same regions as resident microbiota but only remain in the body for short periods. They cannot persist long-term due to competition from resident microbes, the body's immune defenses, or physiological and chemical changes in the host.

Study Tip: To remember the difference, think of Resident as a "permanent resident" of a house, while Transient is like a "tourist" passing through—they are in the same location, but they don't stay.

Opportunistic Pathogens

While most normal microbiota are harmless, they can occasionally become opportunistic pathogens. These are normal microbes that cause disease under specific, altered circumstances. This shift typically occurs due to:

• Introduction into an unusual site: If normal microbiota are moved to a part of the body where they do not belong (e.g., bacteria from the skin entering a wound), they can cause infection.
• Immune suppression: If the host's immune system is weakened, microbes that were previously kept in check may multiply uncontrollably.
• Changes in microbiota composition: Significant shifts in the balance of the microbiome can allow certain species to dominate.
• Stressful physiological conditions: Changes in the body's environment can create opportunities for pathogens to thrive.

One specific example of a "superinfection" is when the use of antimicrobial drugs inhibits resident microbiota, allowing other, often more resistant, microbes to thrive and cause disease.