Human Microbiota 2025

Microbiota: Refers to the collective microbial populations (bacteria, archaea, fungi, viruses, protozoa) that live in and on a host organism. It is often considered a "Superorganism" due to its extensive presence and influence. For humans, there are approximately $10^{14}$ microbial cells compared to roughly $10^{13}$ human cells.
Microbiome: Encompasses all the genetic material associated with the microbiota of a particular environment or host. It's crucial to distinguish this from the human genome:
- While approximately 99% of the human genome is similar between individuals, less than 20% of the microbial genome is similar, highlighting its vast diversity.
- The human genome contains about 20,000 genes, whereas the collective human microbiome is estimated to possess around 10,000,000 genes. This significantly larger genetic repertoire of the microbiome implies a more extensive range of genetic effects and functions, including the potential for mutations.
Metagenome: Represents all the genetic material found within a collected sample from any natural environment, such as water, soil, or a specific body part. An example of a short DNA sequence within a metagenome could be GATAAATCTGGTCTTATTTCC.

Timeline: Launched in 2007 and concluded in 2014.
Scope: Over 11,700 samples were gathered from 300 volunteers at various time intervals and subsequently analyzed.
Discoveries: Detected approximately 10,000 bacterial and fungal species, along with roughly 3,000 viruses within the human microbiota.
Current Knowledge: Predominantly, our understanding of the human microbiota is still focused on its bacterial population.

Status: Since 2012, the microbiota has been increasingly accepted and referred to as an organ of the human body (as per doi:10.1111/j.1469-0691.2012.03916.x).
Characteristics: It weighs approximately 1.5 kg and consists of a staggering $10^{14}$ cells (100 trillion).
Composition: Includes a diverse array of microorganisms: bacteria, yeast, viruses, and parasites.
Physiology and Pathology: Like any other organ, the microbiota possesses its own physiological functions and can experience pathological changes. Any alteration in its collective population structure can significantly impair individual health.
Evolving Understanding: The comprehension of the normal human microbiota remains an ongoing area of research and is continually transitioning.

Traditional View: Historically, many internal body sites were considered sterile (free of microbes). These included: blood, cerebrospinal fluid (CSF), brain, heart, liver, spleen, kidneys, pancreas, and lungs.
Modern Understanding: Following advancements in modern detection techniques, it has been revealed that these sites are NOT truly sterile. While they may contain very small numbers of microorganisms or their nucleic acids, the previous absolute sterility assumption has been disproven.

Bacterial Focus: Our knowledge is more extensive regarding bacterial members than viruses.
Main Bacterial Phyla: The human microbiota is primarily composed of six major bacterial phyla:
- Actinobacteria
- Firmicutes (often abundant)
- Proteobacteria
- Bacteroidetes (often abundant)
- Cyanobacteria
- Fusobacteria