Germ-free life

Germ-free life

Louis Pasteur did not believe that animals could live in the absence of microorganisms. In 1897, following his suggestion, scientists made an unsuccessful attempt to raise germfree chickens. In Germany between 1899 and 1908, bacteria-free chickens were raised for the first time, but they failed to develop normally and died in about a month. This supported the notion that intestinal bacteria were essential in the nutrition of vertebrates. However, in 1912, 17 germfree chickens were raised at the Pasteur Institute in France, showing that vertebrate life was, in fact, possible in the absence of microorganisms.

In 1928, James A. Reyniers at the university of Notre Dame in the United States began his work on germfree animals. He and his associates developed equipment and techniques for rearing several generations of chickens, rats, mice, and other animals in the absence of microorganisms. They compiled anatomical and physiological descriptions of these animals, including comparisons with conventional no germfree animals of the same species. As a result of these studies, germfree animals no longer belong to the realm of biological curiosities. Instead, they are practical tools for solving problems of importance in biology and medicine. Germfree laboratory units have been designed to raise such animals. Such units made it possible to study the effects of microorganisms added to germfree animals. Animals that are raised in an environment with one or more known microorganisms are said to be gnotobiotic.

The first germfree animals raised by Reyniers were chickens obtained by sterilizing the shells of 20-days old embryonated eggs with a germicide and then placing them in sterile containers such as glass jars or steel, tanklike cages. Sterile air was passed into these containers and waste gasses removed. Sterile food water also was placed in the cages prior to adding the ready-to-hatch chicks, periodic microbiological monitoring of the exhaust air, feathers, excreta, and body orifices confirmed the absence of microorganisms in the cages or on the birds.

Germfree laboratory units are now commercially available. Germfree mammals such as rats, mice, and guinea pigs can be obtained by cesarean section, a surgical incision made through the walls of the abdomen to deliver the offspring. This operation is performed under sterile conditions in a special chamber that allows the young animals to be introduced directly into a sterile rearing cage. These babies must be hand-fed hourly for 2-3 weeks, using a formula containing, as nearly as can be determined, all of the components of the natural mother’s milk (which is avoided because it may contain microorganisms). Once established, a colony of germfree animals can be maintained by natural reproduction under germfree conditions.

Germ animals versus normal animals.

Compared with normal animals, germfree animals have an underdeveloped immune system, making them unusually susceptible to infection if exposed to microorganisms. They lack antibodies to normal flora antigens, which often share similarity to the antigens of disease-causing microorganisms and confer partial protection against these pathogens. Germfree animals are vulnerable not only to pathogenic bacteria but also to many nonpathogenic bacteria. This is because immunologic priming (a building of resistance to antigens) has not occurred under the protection of material antibodies soon after birth.

Germfree animals require higher levels of? Vitamins in their diet than do normal animals; they also require vitamin K, which normal animals do not require in their diet. These findings indicate that the normal flora makes a significant contribution to satisfying the vitamin requirements of the host

Other uses for germfree animals

Germfree animals have been used gnotobiotically to assess the effect of particular species of microorganisms on a host. The animal is reared in the presence of one or more known microbial species, in order to determine the effect of those species on growth and development of the animal or on various physiological processes. Similarly, one can inoculate a germfree animal with one or several known species or microorganisms to determine the microorganisms ability produced disease or cause a pathological or immunological change in the animal. For example, gnotobiotic techniques have helped to understand the role of bacteria in causing dental caries.

Effect of antimicrobial agents on the normal flora

Experiments designed to suppress the normal flora with antibiotics agents indicate that these microorganisms defend the host against potential pathogens. For instance, treating the skin of humans with antibacterial agents, such as hexachlorophene, suppresses colonization by the normal Gram-positive microbes and promotes growth and chemical infection by Gram-negative rods and other microorganisms that are not normally able to cause infection. Such opportunities microorganisms generally are resistant to the antimicrobial agent used. In another example, hospital patient receiving antibiotics may lose much of the normal flora of the large bowel, leading the excessive growth of toxin-producing strains of an anaerobic spore-forming bacterium, Clostridium difficult, which normally is absent or is only a minor component of the normal flora. In humans not receiving antibiotic therapy, such microorganisms are held in check by the normal flora. The yeast Candida albicans, a minor component of the normal flora, may multiply dramatically following antibiotic therapy (since it is a eukaryote, it is resistant to antibacterial antibiotics). Candida albicans can cause diarrhea and superficial fungal infections in the mouth, vagina, or anal area.

Thus the normal flora can prevent the establishment of pathogens by various means, including successful competition for available nutrients or formation of inhibitory metabolic products. This protective role of the normal flora is well illustrated by the beneficial effect of breastfeeding infants. The bacteria acquired from the mother’s milk in the colon of the breastfed infant create an environment antagonistic to enteric pathogens. This protective effect is augmented by ingested maternal antibodies those pathogens.

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.