Distribution of Normal Flora in The Body
المؤلف:
Cornelissen, C. N., Harvey, R. A., & Fisher, B. D
المصدر:
Lippincott Illustrated Reviews Microbiology
الجزء والصفحة:
3rd edition , p8-9
2025-05-31
1041
The most common sites of the body inhabited by normal flora are, as might be expected, those in contact or communication with the outside world, namely, the skin, eye, and mouth as well as the upper respiratory, gastrointestinal, and urogenital tracts.
A. Skin
Skin can acquire any bacteria that happen to be in the immediate environment, but this transient flora either dies or is removable by washing. Nevertheless, the skin supports a permanent bacterial population (resident flora), residing in multiple layers of the skin (Figure 1). The resident flora regenerate even after vigorous scrubbing.
Fig1. A. Examples of bacteria that inhabit the skin. B. Arm of individual who injects drugs by “skin popping.”
1. Estimate of the skin microbiome using classical culture techniques: Staphylococcus epidermidis and other coagulase-negative staphylococci that reside in the outer layers of the skin appear to account for some 90 percent of the skin aerobes. Anaerobic organisms, such as Propionibacterium acnes, reside in deeper skin layers, hair follicles, and sweat and sebaceous glands. Skin inhabitants are generally harmless, although S. epidermidis can attach to and colonize plastic catheters and medical devices that penetrate the skin, sometimes resulting in serious blood stream infections.
2. Estimate of the skin microbiome using molecular sequencing techniques: The estimate of the number of species present on skin bacteria has been radically changed by the use of the 16S ribosomal RNA gene sequence to identify bacterial species present on skin samples directly from their genetic mate rial. Previously, such identification had depended upon micro biological culture, upon which many varieties of bacteria did not grow and so were not detected. Staphylococcus epidermidis and Staphylococcus aureus were thought from culture-based research to be dominant. However DNA analysis research finds that, while common, these species make up only 5 percent of skin bacteria. The skin apparently provides a rich and diverse habitat for bacteria.
B. Eye
The conjunctiva of the eye is colonized primarily by S. epidermidis, followed by S. aureus, aerobic corynebacteria (diphtheroids), and Streptococcus pneumoniae. Other organisms that normally inhabit the skin are also present but at a lower frequency (Figure 2). Tears, which contain the antimicrobial enzyme lysozyme, help limit the bacterial population of the conjunctiva.
Fig2. Examples of bacteria that inhabit the conjunctival sac. [Note: Tears, which contain the antimicrobial enzyme lysozyme, help limit the bacterial population of the conjunctiva.]
C. Mouth and nose
The mouth and nose harbor many microorganisms, both aerobic and anaerobic (Figure 3). Among the most common are diphtheroids (aerobic Corynebacterium species), S. aureus, and S. epidermidis. In addition, the teeth and surrounding gingival tissue are colonized by their own particular species, such as Streptococcus mutans. [Note: S. mutans can enter the bloodstream following dental surgery and colonize damaged or prosthetic heart valves, leading to potentially fatal infective endocarditis.] Some normal residents of the nasopharynx can also cause disease. For example, S. pneumoniae, found in the nasopharynx of many healthy individuals, can cause acute bacterial pneumonia, especially in older adults and those whose resistance is impaired. [Note: Pneumonia is frequently preceded by an upper or middle respiratory viral infection, which predisposes the individual to S. pneumoniae infection of the pulmonary parenchyma.]
Fig3. Examples of bacteria that inhabit the mouth.
D. Intestinal tract
In an adult, the density of microorganisms in the stomach is relatively low (103 to 105 per gram of contents) due to gastric enzymes and acidic pH. The density of organisms increases along the alimentary canal, reaching 108 to 1010 bacteria per gram of contents in the ileum and 1011 per gram of contents in the large intestine. Some 20 percent of the fecal mass consists of many different species of bacteria, more than 99 percent of which are anaerobes (Figure 4). Bacteroides species constitute a significant percentage of bacteria in the large intestine. Escherichia coli, a facultatively anaerobic organism, constitutes less than 0.1 percent of the total population of bacteria in the intestinal tract. However, this endogenous E. coli is a major cause of urinary tract infections.
Fig4. Examples of bacteria that inhabit the gastrointestinal tract.
E. Urogenital tract
The low pH of the adult vagina is maintained by the presence of Lactobacillus species, which are the primary components of normal flora. If the Lactobacillus population in the vagina is decreased (for example, by antibiotic therapy), the pH rises, and potential pathogens can overgrow. The most common example of such over growth is the yeast-like fungus, Candida albicans , which itself is a minor member of the normal flora of the vagina, mouth, and small intestine. The urine in the kidney and bladder is sterile but can become contaminated in the lower urethra by the same organisms that inhabit the outer layer of the skin and perineum (Figure 5).
Fig5. Examples of bacteria that inhabit the vagina.
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