NORMAL FLORA IN THE BODY

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

The resident flora regenerate even after vigorous

scrubbing.

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 bloodstream

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 material.

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

Tears, which contain the antimicrobial enzyme lysozyme, help limit

the bacterial population of the conjunctiva

c.mouth 

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.]

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

 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.

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 overgrowth

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 )..