Learn facts about normal flora

normal flora

Normal Flora

Microbes are everywhere. They populate the air, the water, the soil, and have even evolved intimate relationships with plants and animals. Without microbes, life on earth would cease. This is due mainly to the essential roles microbes play in the systems that support life on earth, such as nutrient cycling and photosynthesis.

Prior to birth, most humans are essentially sterile microbiologically.  There are viruses and bacteria that can cross the placental barrier, but these occurrences are rare.  Our first exposure to large numbers of microorganisms is occasioned by our short trip down the birth canal, although this exposure is delayed a bit for the nearly 30% of babies now born by Cesarean section.  This initial exposure to the rich microbial population of the mother's birth canal rarely causes problems although it does result in the inoculation of the newborn skin, nasopharynx and gut with a variety of microorganisms, not all of which persist in the tissues of the baby.  Handling, cuddling, cleansing and nursing contribute more representatives to the cast of microbial species gaining a foot-hold in the new-borne.

The diet of the new borne plays a decisive role in the microbial population of the gut with the flora of breast-fed infants being dominated by organisms such as Bifidobacteriumbifidus.  Transition to cow's milk or milk based formula encourages the growth of gas producing coliforms, a transition commonly noted by parents as an olfactory change when diaper changing is necessary. Continued dietary changes take place as the diet becomes increasingly complex eventually taking on the characteristics of the adult gut with a predominance of such organisms as Bacteroides spp, Fusobacterium spp, anaerobic cocci, Enterococci, yeasts and protozoa.

Microbes that colonize the human body during birth or shortly thereafter, remaining throughout life, are referred to as Normal flora . Normal flora can be found in many sites of the human body including the skin (especially the moist areas, such as the groin and between the toes), respiratory tract (particularly the nose), urinary tract, and the digestive tract (primarily the mouth and the colon). On the other hand, areas of the body such as the brain, the circulatory system and the lungs are intended to remain sterile (microbe free).

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The human body provides many unique environments for different bacterial communities to live. scientists refer to the human body as the host. A positive host-microbe relationship is usually described as either mutualistic or commensalistic. In mutualism both the host and the microbe benefit. Which is in contract to commensalisms, where one partner of the relationship benefits (usually the microbe) and the other partner (usually the host) is neither benefited nor harmed. "Normal flora"

normal flora
sites of normal flora in the body

Beneficial Effects of the Normal Flora

The overall beneficial effects of microbes are summarized below.

  1. The normal flora synthesize and excrete vitamins in excess of their own needs, which can be absorbed as nutrients by their host. For example, in humans, enteric bacteria secrete Vitamin K and Vitamin B12, and lactic acid bacteria produce certain B-vitamins. Germ-free animals may be deficient in Vitamin K to the extent that it is necessary to supplement their diets. "Normal flora"


  1. The normal flora may antagonize other bacteria through the production of substances which inhibit or kill non indigenous species. The intestinal bacteria produce a variety of substances ranging from relatively nonspecific fatty acids and peroxides to highly specific bacteriocins, which inhibit or kill other bacteria. "Normal flora"


  1. The normal flora prevent colonization by pathogens by competing for attachment sites or for essential nutrients. This is thought to be their most important beneficial effect, which has been demonstrated in the oral cavity, the intestine, the skin, and the vaginal epithelium.  In some experiments, germ-free animals can be infected by 10 Salmonella bacteria, while the infectious dose for conventional animals is near 106


  1. The normal flora stimulate the production of natural antibodies. Since the normal flora behave as antigens in an animal, they induce an immunological response, in particular, an antibody-mediated immune (AMI) response. Low levels of antibodies produced against components of the normal flora are known to cross react with certain related pathogens, and thereby prevent infection or invasion.  Antibodies produced against antigenic components of the normal flora are sometimes referred to as "natural" antibodies, and such antibodies are lacking in germ-free animals. "Normal flora"


  1. The normal flora stimulate the development of certain tissues, i.e., the caecum and certain lymphatic tissues (Peyer's patches) in the GI tract. The caecum of germ-free animals is enlarged, thin-walled, and fluid-filled, compared to that organ in conventional animals. Also, based on the ability to undergo immunological stimulation, the intestinal lymphatic tissues of germ-free animals are poorly-developed compared to conventional animals. "Normal flora"


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Harmful Effects of the Normal Flora

However, life is not always perfect, and in certain situations good-standing members of your normal flora can cause disease or invading pathogens can displace them. The result will be disease. To illustrate some of these scenarios let’s take a closer look at microbial communities found in different areas of the human body.


The Mouth

It’s estimated that 500-600 different kinds of bacteria thrive on mucus and food remnants in the mouth. A predominant member of this community is the Gram positive bacterium Streptococcus mutans. It grows on biofilms on the surface of teeth (plaque) where it consumes sugar and converts it to lactic acid. Lactic acid erodes the enamel on the surface of teeth, which leads to the formation of cavities.


 The Skin

Human skin is not a particularly rich place for microbes to live. The skin surface is relatively dry, slightly acidic and the primary source of nutrition is dead cells. This is an environment that prevents the growth of many microorganisms, but a few have adapted to life on our skin.

Propionibacterium acnes is a Gram positive bacterium that inhabits the skin. P. acnes are anaerobes, so they lives in pores and glands where oxygen levels are lower. As the name implies P. acnes causes the common skin condition called acne.

Another prominent member of the skin flora is Staphylococcus epidermidis. This is a highly adapted Gram positive bacterium that can survive at many sites throughout the body. S. epidermidis can cause life threatening disease in hospital patients when invasive medical devices such as catheters are used. In such cases, S. epidermidis form antibiotic resistant biofilms along the catheter and enter the bloodstream causing systemic infection that can be fatal. Under this scenario S. epidermidis would be considered an opportunistic pathogen, since it remains benign until provided with specific conditions that allow it to cause disease."Normal flora"


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The Nose

The human nose is home to the infamous Gram positive bacterium Staphylococcus aureus, best known for its role in hospitals where it is a major cause of surgical wound and systemic infection. It is often carried in the noses of health care workers and transmitted from patient to patient. Why some people carry S. aureus while others do not, is unknown.


Stomach Acid

What kind of organism would live in a highly acidic (pH 1-2) environment like the stomach? Not surprising there aren’t many organisms that have adapted to life in this environment. One organism that has been discovered living in the human stomach is the Gram negative bacterium called Helicobacter pylori . How can it survive? Well, it creates a less acidic micro environment. The bacteria achieve this by burrowing into the stomach’s mucosal lining to a depth where the pH is essentially neutral. In addition, H. pylori produce an enzyme called urease to convert urea produced by the stomach into ammonia and carbon dioxide. to avoid pH levels that would normally kill it. Here, it may also produce ulcers. H. pylori is the causative agent of gastric ulcers.


Changing the composition of your normal flora has been known to have effects on your digestion, gas etc Maintaining a balance is crucial. Normal flora consists of communities of bacteria that function as microbial ecosystems. If these ecosystems are disrupted the consequences can be unpredictable. Antibiotics, tissue damage, medical procedures, changes in diet, and the introduction of new pathogens are examples of changes that can affect your normal flora.  "Normal flora"



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