Normal flora of Skin, Eye and Respiratory tract

Skin

The epidermis, or outer surface layer of the skin, covers a connective tissue layer, called the dermis. Together they form an affective barrier against most microorganisms, which is why the skin is frequently called the “first line of defense”. For instance, the outermost layer of the epidermis is a layer of dead, nonnucleated horny cells that is constantly in contact with microorganisms from the surrounding environment. Although cuts, abrasions, or burns can allow microbes to penetrate this layer, it is normally impermeable to microorganisms. The skin varies widely in structure (for example, a thickness of the epidermis) and function, depending on its location on the body. These differences determine the types and numbers of microorganisms that occur on each site, the location of common microorganisms that effective that either grows on or contaminate the skin.

Constant exposure to the environment means the skin harbors many transient microbes. However, the skin surface is hostile to the survival and growth of many nonresident bacteria. For instance, Streptococcus pyrogens do not survive for more than a few hours on the skin, whereas it may survive for weeks in room dust. Several factors are responsible for discouraging microbial growth on the skin:

1. Dryness: the relatively dry surface of the skin is inhibitory to microbial growth. When allowed to dry, many bacteria enter a dominant condition; some species die in a matter of hours. Some sites of the skin are moister than others, including the armpit region, the skin between the thighs. These regions have higher numbers of normal flora organisms (about 106 bacteria per square centimeter) than do the drier areas of skin (about 102 to 104 bacteria per square centimeter). When the skin in moist areas becomes damp and soft, certain fungi easily produce skin infections. An example is a fungus that causes athlete’s foot.
2. Low pH: skin has a normal pH between 3 and 5 (higher in moist regions), due in part to organic acids such as lactic acid produced by normal skin microorganisms. This low pH can inhibit the growth of many kinds of microorganisms.
3. Inhibitory substances: Several bactericidal or bacteriostatic compounds occur on the skin. For example, sweat glands secrete lysozyme, an enzyme that destroys the peptidoglycan of bacterial cell walls. Sebaceous glands secrete complex lipids, which may be partially degraded by bacteria such as propionibacterium acnes to produce long-chain fatty acids, such as oleic acid, that are highly inhibitory to other bacteria.
4. Despite these formidable antimicrobial factors, some bacteria not only survive on the skin but even grow, forming the normal flora. The secretions of the sweat glands and sebaceous glands provide water can serve as nutrients for these microorganisms. Most of these bacteria are species of Staphylococcus-mainly, S. epidermis but Micrococcus, Corynebacterium, and Propionibacterium also occurs. Fungi (e.g., Candida spp. and Pityrosporum spp.) are often present in skin folds. Acid-fast nonpathogenic mycobacteria occur in areas rich in sebaceous secretions, such as in the genitalia and external ear. In the deep sebaceous glands, anaerobic bacteria such as Propionibacterium acnes thrive on the lipids present in the gland secretions. Part of the normal flora, P.acnes is usually harmless; however, it has been associated with acne vulgaris, a disease of the sebaceous glands of the skin. Because of their deep location, the numbers of these propionibacteria are little affected by washing or by disinfecting solutions.

Eye

Lining the eyelids and covering the eyeball is a delicate membrane called the conjunctiva. It is a continuation of the skin at the margin of the eyelids and is the mucous membrane that is continually washed by a flow of tears, which tends to remove microorganisms. Moreover, lysozyme, an antimicrobial substance, is secreted in tears. Consequently, the conjunctival flora is sparse. The main organisms found are Staphylococcus epidermis, Staphylococcus aureus, Corynebacterium spp., Streptococcus pneumonia, Neisseria spp., Moraxella spp., and Haemophilus parainfluenza; other microorganisms may be isolated occasionally.

Respiratory tract

The Respiratory tract includes the mouth, tonsils, nasopharynx, throat, trachea (windpipe), tracheal branches (bronchi), and lungs. Although these moist areas might appear to be prime sites for microorganisms, they actually are rather difficult to colonize.

Upper respiratory tract

The upper respiratory tract is that portion above the larynx, the organ of the voice situated between the trachea and the base of the tongue. The mucous membranes of the upper respiratory tract are moister than skin; nevertheless, they can create problems for microorganisms passes along the tortuous nasal passages and then into the nasopharynx (that part of the pharynx that lies above the level of the soft palate), and many microorganisms will stick to the thin, moist layer of highly viscous mucus that overlies the epithelial surfaces. Because of the rhythmic beating of cilia on the surface of the epithelial cells lining the nasopharynx, the mucus layer continuously flows downward toward the oropharynx. The trapped bacteria are eventually swallowed and may be destroyed by hydrochloric acid in the stomach. In addition to this mechanical removal of bacteria, the enzyme lysozyme in nasal mucus kills bacteria.

Despite these factors, the nose and nasopharynx are inhabited by numerous microorganisms as part of their normal flora. This is possible because of the microbial ability to adhere to the epithelial cell layer of the mucous membranes, thereby avoiding being swept away by the flow of mucus. The bacteria most frequently and most consistently found in the nose are Staphylococcus epidermis and Staphylococcus aureus; in the nasopharynx avirulent strains of Streptococcus pneumonia and other? Hemolytic streptococci predominate. But species of the genera and micrococcus are also common.

Lower respiratory tract

The mucous membrane surface of the trachea and the bronchi do not have a normal flora, because of the efficient mechanical removal of microorganisms by an upward, cilia-driven flow of mucus. The few bacteria that do manage to traverse the air passages all the way to the air sacs of the lungs are usually engulfed and destroyed by phagocytic body cells called macrophages.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.