Normal Microbiota
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Normal Microbiota of the Human Body 


A symbiont is any microorganism that spends all or a portion of its life associated with another organism of a different species

Symbiosis is the living together in close physical association of two or more different organisms

Ectosymbiosis - organisms remain outside each other

Endosymbiosis - one organism is found within the other

Types of Symbiosis, Functions, and Examples

Commensalism - the microorganism (commensal) benefits, while the host is neither harmed nor helped

Distribution of the Normal Microbiota of the Human Body

Reasons to acquire knowledge of normal human microbiota and its distribution

It provides greater insight into possible infections resulting from injury to these areas

It gives perspective on the possible sources and significance of microorganisms isolated from an infection site

It increases understanding of the causes and consequences of overgrowth of microorganisms normally absent from a specific body site

It aids awareness of the role these indigenous microorganisms play in stimulating the immune response that provides protection against potential pathogens


Resident microbiota multiply on or in the skin

They vary from one part of the body to another

They experience periodic drying in a slightly acidic, hypertonic environment

Transient microbiota are found on the skin for a short time and do not multiply there; they usually die in a few hours

One species, Propionibacterium acnes, is associated with acne vulgaris

Nose and nasopharynx

Nose - just inside the nares;

contains the same organisms as skin, including Staphylococcus epidermidis and S. aureus

Nasopharynx - above the level of the soft palate;

contains nonencapsulated strains of some of the same species that may cause clinical infection; other species also are found

Oropharynx - between the soft palate and upper edge of the epiglottis; houses many different species

Respiratory tract - no normal microbiota due to mucociliary blanket, the enzyme lysozyme in mucus, and the phagocytic action of alveolar macrophages

Oral cavity (mouth) - contains those organisms that survive mechanical removal by adhering to gums (anaerobes) and teeth (aerobes); organisms contribute to the formation of dental plaque, dental caries, gingivitis, and periodontal disease

Eye - aerobic commensals are found on the conjunctiva

External ear - resembles microbiota of the skin with some fungi

Stomach - most microorganisms are killed by acidic conditions unless they pass through very quickly;

the number of microorganisms present increases immediately after a meal, but decreases quickly

Small intestine

Duodenum - few microorganisms present because of stomach acidity and inhibitory action of bile and pancreatic secretions

Jejunum - Enterococcus fecalis, diphtheroids, lactobacilli, and Candida albicans

Ileum - microbiota resemble that of the colon as the pH becomes more alkaline

Large intestine (colon) - largest microbial population of the body

Over 300 different species have been isolated from human feces

Most are anaerobes or facultative organisms growing anaerobically

Ratio of anaerobes to facultatives is approximately 300:1

They are excreted by peristalsis, segmentation, desquamation, and movement of mucus

They are replaced rapidly because of their high reproductive rate

This is a self-balancing (self-regulating) microbial ecology

The balance may change with: stress, altitude, starvation, diet, parasite infection, diarrhea, use of antibiotics or probiotics (microorganisms orally administered that promote health)

Genitourinary tract

Kidneys, ureter, and bladder are normally free of microorganisms

Males - a few microorganisms are found in distal portions of the urethra

Females - complex microbiota in a state of flux due to menstrual cycle; Döderlein’s bacilli are primarily Lactobacillus acidophilus that forms lactic acid and thereby maintains the pH of the vagina and cervical os between 4.4 and 4.6