Introduction to Normal Flora

Origin of the normal flora

Before the birth of a healthy human, a foetus is free of bacteria.under natural circumstances, the fetus first acquires microorganisms while passing down the birth canal. It acquires them by surface contact, swallowing or inhaling.These microbes are soon joined by other microbes from many sources in the newborn baby surrounding.miroorganisms which find the suitable environment,either on an outer or the inner body surface,quickly multiply and establish themselves.The initial flora may change considerably in composition during the first few days or weeks after birth until a stable fora become established and form the normal flora. Each part of the human body with its special environmental conditions has its own particular mixture of microorganisms. For example, the oral cavity acquires a different natural microbial population than the intestines,Then, In the short time, depending on factors such as the frequency of washing , diet, hygienic particles and living conditions the child will have the same kind of normal flora as an adult in same environmental condition.

Even though an individual a “normal” flora, did often happens that during his or her life there are fluctuations in the composition of this flora due to general health conditions, diet, hormonal activity, age, and many other factors.

Normal flora and the human host

What effect does the establishment of normal flora (colonization) have on the body? Three approaches have been used to answer these questions:

Use of germfree animals: If the colonization of experimental animal by the microorganisms can be prevented, one can compare the property of such germfree animal with those of normal animals. The result can be helpful in understanding the role of normal flora in human beings.

Use of antimicrobial agents: If the balance that occurs between normal flora and human host can be altered by using such-such agents, a various effect may occur that are useful in indicating the role of normal flora in the human body.

Knowledge of certain characteristics of normal flora organisms: The nature of these organisms suggest that normal flora may help to discourage the growth of microorganisms that are not the part of indigenous flora.

Germ-free and xenobiotic life

Pasture did not believe that animals could live in the absence of microorganisms, and in 1897, following his suggestions, an abortive attempt was made to rear germfree chickens. Between 1899 and 1908 , Schottelius, a German, was successful in raising chickens that were bacteria free. However, because his birds did not develop normally and died in about a month, he concluded that intestinal bacteria are essential in the nutrition of vertebrates. In 1912, Cohen at the Pasteur institute raised 17 germfree chickens for 40 days and concluded that vertebrate life is possible in the absence of microorganisms. We now know then when an adequate diet is provided, germfree chickens live long, healthy lives and reproduce as well as normal birds.

Rearing germ-free animals

In 1928 James A. Reyniers at the university of Notre Dame started work on germfree chickens he and his assistant developed techniques and equipment for rearing germfree chickens, rat, mice, and other animals in the absence of any detectable microorganisms for several generations. They emphasized the anatomical and physiological description of these animals and made comparision with conventional nongermfree animals of the same species. As a consequence, of these studies, germfree animals no longer belong to the realm of biological curiosities but havenowadays become practical modles for solving problems of importance in biology and medicine. Animals that are either germ-free or that love in assositionto be with one or more known organisms are known as gnotobiotic.he first germ-free animals reared by Reyniers were chickens obtained by shells of a 20 day old embronated embryonated egg with an effective germicide and placing them in sertile or germ-free containers such as glass churn jars or steel tanklike cages into which filtered(sertile) air is passed from which waste gases are removed. Sertile food and water are placed in the cages prior to introducing the ready to hatch chicks. Periodic bacteriological exdaminatoin of the exhaust air , feathers, excreta, and bofy orifices are made to confirn the absence of microorganisms in the cage or the birds. Germ-free rats, mice, guinea pigs, and other mammals can be obtained by cesarean section of gravid motherrs under sertile operative conditions in special chambers that allow the young animals to be introduced directly into the rearing cage. This babise must be hand-fed hourly for 2 or 3 weeks with specially divised nipples attached to medicine dropper with a formula containing, as nearly as can be determined, all of the components of the natural mothers’ milk. Once established, a colomy of germfree animals can be mantained by reproducing in a germfree conditions.

Germ-free animals versus normal animals

Compared with normal animals, germfree animals exhibit an underdeveloped immune system, make them unusually susceptible to infection if subsequently exposed to pathogenic bacteria. They also require the higher level of B vitamins than do normal animals, and they require vitamin K, which normal animals do not require. These findings indicate that the normal flora may make a significant contribution to the vitamin requirement of the host.

Other uses of germ-free animals

gnotobiotic techniques have been used to assess the effect of particular species of a microorganism to a host. Here, the germfree animal is reared in the presence of one or more known microbial species to determine the effect of those species on growth and development of the animal or on various physiological processes. Similarly, one can inoculate a germ for the animal with a known species of microorganisms or a mixture of known species, to determine their ability to produce disease or cause pathological or immunological changes in the animal. For example, gnotobiotic techniques have helped to elucidate the role of bacteria in the causation of dental caries.

Effect of antimicrobial agents

Suppression of the normal flora by suing antibiotics or other antimicrobial agents indicate that the normal flora may serve as the defense against colonization by potential pathogens. For instance, treatment of the skin of the human with antibacterial agents such as hexachlorophene result in suppression of normal gram-positive flora and promote colonization and infection by Gram-negative bacilli and other microorganisms that cannot establish themselves easily in skin. In another example, hospital patient receiving antibiotics may undergo suppression of the normal flora of the large bowel, leading to pseudomembranous colitis, which is serve disease produced my excessive growth of toxin-producing strains od Clostridium difficult. In human not receiving antibiotic therapy, such strains are ordinarily held in check by normal flora and do not grow in high numbers. Normal flora organisms can prevent the establishment of pathogens by various means such as successful competition for available nutrients or formation of inhibitory metabolic products and antibiotics.

References:

D greenwood, Slack RCB and J Peutherer. Medical microbiology. 2001.

JG College, AG Fraser and BP Marmion. Practical Medical microbiology. Fourteenth Edition. Churchill Livingstone, 1996.

JP Micheal, ECS Chan and NR Krieg. Microbiology. Fifth Edition. Delhi: Mcgraw-hill, 1993.

M Cheesbrugh. Medical laboratory manual for tropical countries. London, 2007.