Human Commensal and Mutual Organisms

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Human Commensal and Mutual Organisms

From the moment of birth and throughout our entire lives, humans share their bodies with a surprising variety of microscopic organisms. Dust mites remove dead skin, amoeba live on their teeth and scavenge food particles, and eyebrow mites live on their eyebrows.

Scientists estimate that the human body has around 7.5 1013 cells. Many of these cells are not of human origin, but represent microscopic commensal and mutual organisms. In commensal interactions, one species of organism benefits and the other is unaffected. In mutual interactions, both organisms benefit.

The term "commensal" is derived from a Latin word mensa, meaning "table." Commensal organisms share their food from a common table. In the case of human commensals, the human host is the table. It is often difficult to identify a relationship between organisms as purely commensal, mutual, or parasitic, as the way in which one organism benefits or harms its host may not be obvious. A commensal or mutual organism may depend on its host for food, shelter, support, transport, or a combination of these factors. The host may receive a variety of benefits, including protection from infection, improved digestion, or cleaner skin.

While in the womb, humans live in a sterile environment, protected by the placenta and the amniotic sac. After birth, humans are introduced and subjected to an array of new organisms. If these organisms find themselves in a suitable ecological niche , whether on the internal or external parts of the human body, they will multiply and form complex communities, or colonies, with their host. The first step in this colonization process requires the microscopic organisms to adhere to their host. If the organisms find a suitable location, they will form long-term, stable, interdependent relationships with other organisms in the same location and the human that harbors them. Since the benefit to one or the other species may be subtle and hard to identify, it is often difficult to distinguish between true commensalism and mutualism . Humans certainly derive considerable benefit from many resident organisms.

A wide variety of microorganisms interact with humans, taking advantage of several microenvironments. Certain parts of the body, such as the solid organs, blood, cerebrospinal fluid, and urine, are normally sterile. However, established microbial populations may be found on the skin and in the lower respiratory tract, mouth, and lower gut. Throughout life, these resident organisms vary in type and number, and individuals can have significant differences in their resident populations. If for any reason the commensals gain access to inappropriate body sites, infections may occur.

Skin and Eyes

The skin is a highly complex organ that provides a variety of ecological niches for colonies of microscopic organisms. It is also the first line of defense against infection. The skin on the head, armpits, groin, hands, and feet has more microscopic organisms than on other places on the body. Bacteria, fungi, and mites form the commensal flora and fauna on the skin. The fungi Malassezia furfur and Candida albicans are found on the skin of some individuals. All humans can act as hosts to skin mites such as Demodex folliculorum and Demonex brevis. It is thought that a large majority of the human population acts as hosts to these mites. These microscopic animals survive on a diet of dead epithelial (skin) cells and sebum, an oil excreted by hair follicles and other skin glands.

The commensal flora and fauna on the skin are dispersed into the environment when washed or sloughed off in some manner. This is important, because 10 to 40 percent of healthy individuals and up to 90 percent of hospital staff carry the bacteria Staphylococcus aureus. This bacteria is one of the most common causes of infection in wounds after surgery, and nearly all babies born in hospitals become colonized within a week.

Human eyes are covered with a specialized skin that is bathed in tears, and only a few microbes can survive these conditions. Corynebacteria such as Corynebacterium xerosis can establish themselves as resident commensals on human cornea.

Digestive Tract

The mouth provides a number of ecological niches where microscopic organisms can colonize. Dental caries (tooth decay) are caused by the interaction between commensal bacteria and sugar in the diet. Streptococcus mutans converts sugar into slime which sticks firmly to the enamel of the teeth, beginning the decay process. The crevices between the gum and teeth also harbor bacteria such as Bacteroides and Fusobacterium that can cause gum disease.

Commensal organisms usually do not colonize the stomach because it is highly acidic, although some acid-tolerant lactobacilli can live there. One bacteria, Helicobacter pylori, has recently been linked with ulcer formation in some people. The normal flora of the intestines, Eschericia coli, Streptococcus, and Bacteroides contribute to the normal functioning of the digestive system. The importance of the role of these organisms becomes more evident when the administration of antibiotics or laxatives kills them. Without these organisms, the digestive system may be colonized by pathogenic bacteria that are resistant to antibiotics.

Respiratory Tract

The respiratory tract is anatomically complex and constantly exposed to microorganisms in the air breathed in. The microflora of our nostrils resembles that of the skin, with colonies of commensal organisms such as micrococci, corynebacteria, staphlococci, and streptococci. Streptococcus pyogenes is part of the commensal flora of the nose in healthy individuals, but may cause tonsillitis and strep throat. The warm, moist environment of the upper respiratory tract provides a haven for commensal bacteria including Streptococcus, Moraxella, Neisseria, and Haemophplus, species. The lower respiratory tract is generally free from microorganisms, mainly because of the efficient action of the cilia that line the tract.

Roles of Commensal and Mutual Organisms

Nineteenth-century French microbiologist Louis Pasteur believed that animals cannot exist without a population of commensal and mutual organisms, and early experiments to raise germ-free animals met with failure. All germ-free animals have weak, poorly developed immune systems. This suggests that the roles of normal microscopic organisms are very important.

Commensal organisms play a significant role in preventing infections. This may be simply because they deny the invading organism access to the target site, or because the benign organism actively produces substances that inhibit the growth of, or even kill, other organisms.

Commensal flora and fauna may also "switch roles" and become an important source of infection for the human host. Infections caused by microscopic organisms derived from commensal organisms are known as endogenous infections. These infections range from minor conditions, such as boils, to life-threatening infections. For example, streptococcal bacteria from the mouth or skin can gain access to the bloodstream and cause bacterial endocarditis, an infection of the interior of the heart.

Commensal and mutual organisms are in a constantly changing dynamic equilibrium with their human host. Strains, or groups of these organisms, are constantly being replaced and displaced by other strains. In this way, the commensal organism adapts to changes that occur in the host.

see also Interspecies Interactions.

Leslie Hutchinson


Dixon, Bernard. Power Unseen: How Microbes Rule the World. London, Freeman Press,1994.

Heritage, J. E., G. V. Evans, and R. A. Killington. Microbiology in Action: Studies in Biology. Cambridge, U.K.: Cambridge University Press, 1999.

Postgate, John. Microbes and Man, 3rd ed. Cambridge, U.K.: Cambridge University Press, 1992.

Internet Resources

Medical Microbiology on the World Wide Web. Ed. Ayana Gibbs. University of Leeds. <>.