Normal Microbiota

Normal microbiota: the community of microorganisms that routinely colonise specific body sites in a healthy individual without causing disease.

• Resident microbiota: organisms consistently present at a site over time, re-establishing themselves after disturbance, a stable component of the site's ecology.
• Transient microbiota: organisms present temporarily, acquired from the environment or another body site, failing to establish long-term residence, cleared over hours to weeks.
• Colonisation: the presence and multiplication of microorganisms at a body site without invasion of tissue or production of disease. Distinct from infection (covered in full in the dedicated Colonisation, Infection and Disease subsection).

• Blood, CSF, deep tissues, and (in traditional teaching) the lower respiratory tract are considered free of resident microbiota in health.
• Isolation of an organism from one of these sites is, by traditional teaching, presumed clinically significant rather than incidental colonisation, an important interpretive principle in diagnostic microbiology.
• Modern microbiome sequencing techniques have identified low-biomass microbial DNA signals at some of these sites (including the lower respiratory tract and, controversially, blood and placenta) even in health. This remains an active research area and does not currently change standard clinical interpretation: a positive culture from blood or CSF is still treated as significant until proven otherwise.

• Colonisation resistance: occupation of available adhesion sites and nutrient niches by resident organisms physically and metabolically excludes incoming pathogens.
• Competition for nutrients: resident organisms consume available substrates, limiting resources for pathogens attempting to establish themselves.
• Vitamin production: colonic bacteria synthesise vitamin K and several B vitamins, contributing to host nutritional status.
• Immune system development: microbiota exposure during early life is required for normal maturation of gut-associated lymphoid tissue and calibration of systemic immune responses.
• Metabolic functions: colonic bacteria ferment otherwise indigestible dietary fibre into short-chain fatty acids, an energy source for colonocytes and a contributor to overall host energy metabolism.

• Dysbiosis: an alteration in the normal composition, diversity, or balance of the resident microbial community, disrupting its normal protective functions.

Clinical examples:

• Antibiotic-associated dysbiosis: broad-spectrum antibiotic therapy suppresses normal anaerobic gut flora, removing colonisation resistance and creating an ecological niche for pathogen overgrowth.
• *Clostridioides difficile* overgrowth: the classic consequence of antibiotic-associated dysbiosis. Loss of normal colonic flora allows C. difficile spores (which survive the antibiotic course) to germinate and overgrow, producing toxins that cause colitis, ranging from mild diarrhoea to life-threatening pseudomembranous colitis.
• Candidal overgrowth: suppression of normal bacterial flora (vaginal, oral, or gastrointestinal) by antibiotics removes competitive inhibition, allowing Candida species (normally present in low numbers) to proliferate, producing oral thrush, vulvovaginal candidiasis, or, in severely immunosuppressed or critically ill patients, invasive candidiasis.