The human microbiome refers to the aggregate of microorganisms—including bacteria, archaea, fungi, viruses, and protists—that inhabit the human body and the genetic material they contain.[1] These microbial communities reside on the skin, in the digestive tract, the respiratory system, the urogenital tract, and other moist body surfaces.[2] The microbiome plays a critical role in human health, influencing digestion, immune system development, metabolism, and even neurological function through the gut-brain axis.[3]
Research indicates that the microbiome contains significantly more genes than the human genome—estimated at 150 times as many—expanding the functional capabilities of the host organism beyond its own genetic coding.[4] The relationship between humans and their microbiome is typically symbiotic, though it can shift toward pathogenicity under certain conditions known as dysbiosis.[5]
Overview
The study of the human microbiome has accelerated since the launch of the NIH Human Microbiome Project (HMP) in 2007, which aimed to characterize the microbial communities found at various body sites and analyze their role in human health and disease.[6] The microbiome is now considered a "forgotten organ" due to its systemic influence on physiology.[7]
Composition
The human microbiome is a complex ecosystem comprising diverse domains of life. While bacteria constitute the majority of microbial biomass, archaea, fungi, viruses, and protozoa contribute significantly to community dynamics and host interactions.[8]
Bacteria
Bacteria are the most abundant members of the human microbiome. The gut microbiota alone contains an estimated 38 trillion bacteria, surpassing the number of human cells in the body.[9] Major phyla include:
- Firmicutes: Includes genera such as Lactobacillus, Staphylococcus, and Clostridium. Dominant in the gut.
- Bacteroidetes: Includes Bacteroides and Prevotella. Specialized in breaking down complex carbohydrates and glycans.
- Actinobacteria: Includes Bifidobacterium, prevalent in the infant gut and important for immune maturation.
- Proteobacteria: Includes E. coli and Helicobacter. Often less abundant but includes significant pathogens and commensals.
Archaea
Archaea were once thought to be minor components but are now known to be significant, particularly in the gut. The most prevalent genus is Methanobrevibacter, specifically M. smithii, which consumes hydrogen produced by bacterial fermentation and facilitates methanogenesis.[10] This process enhances bacterial metabolic efficiency and may influence energy harvest in the host.
Eukarya & Virome
Eukaryotic microbes include fungi (the mycobiome), such as Candida and Malassezia, and protists. The virome consists of bacteriophages and human viruses. Bacteriophages regulate bacterial populations through predation and horizontal gene transfer, playing a key role in microbial community stability.[11]
Key Ecosystems
The composition of the microbiome varies dramatically across different body sites, adapting to local environmental conditions such as pH, oxygen levels, and nutrient availability.
Gut Microbiome
The gastrointestinal tract hosts the densest and most diverse microbial community. The composition shifts along the alimentary canal: the stomach is sparse due to acidity, the small intestine contains moderate diversity, and the colon is densely populated with anaerobes.[12]
Skin Microbiome
Skin communities are classified by habitat type: dry, intermediate, and sebaceous. Corynebacterium and Staphylococcus dominate dry and intermediate areas, while Propionibacterium thrives in sebaceous regions like the face.[13]
Oral Microbiome
The oral cavity contains over 700 species of bacteria, making it one of the most diverse niches. Dysbiosis here is linked to periodontal disease and potentially systemic conditions like cardiovascular disease.[14]
Physiological Functions
- Metabolism: Fermentation of indigestible dietary fibers into short-chain fatty acids (SCFAs) like butyrate, which nourish colonocytes and regulate inflammation.[15]
- Immunity: Training of the immune system, maintenance of mucosal barriers, and prevention of pathogen colonization via competitive exclusion.[16]
- Vitamin Synthesis: Production of Vitamin K, Biotin, and Vitamin B12.[17]
- Neurotransmission: Microbial synthesis of serotonin, dopamine, and GABA precursors, influencing the gut-brain axis and mental health.[18]
Dysbiosis & Disease
Dysbiosis refers to an ecological imbalance in the microbiome, characterized by loss of diversity, depletion of beneficial taxa, or overgrowth of pathogens. It has been associated with:
- Inflammatory Bowel Disease (IBD)
- Obesity and Type 2 Diabetes
- Autoimmune disorders
- Neurodegenerative diseases (Alzheimer's, Parkinson's)
- Depression and Anxiety
Therapeutic Interventions
Targeting the microbiome is a burgeoning field of medicine. Interventions include:
- Probiotics: Live microbial supplements.
- Prebiotics: Non-digestible substrates that stimulate beneficial microbes.
- Fecal Microbiota Transplantation (FMT): Effective treatment for recurrent C. difficile infection; under investigation for IBD and metabolic syndrome.[19]
- Phage Therapy: Using bacteriophages to target specific pathogens without disrupting commensals.