Microbiology and It’s Component | What is Microbiology | Importance & Facts of Microbiology

Microbiology can be described as the science of all living organisms which are only visible under the microscope. Although medical microbiology deals with the causative agents of the infectious diseases of man , his reaction to such infections,  the way in which they produce disease and the methods of their diagnosis.


The disease is transferred by touch, from man to man , and has been well known since earlier times . Varo and Columella had postulated that diseases were given by invisible organisms which they referred to as ‘Animalia minuta’. As microbes are invisible to the naked eye , conclusive knowledge about them had to wait until microscopes were made. The credit for observation and illustration of bacteria goes to Antony van Leewenhoek , a dealer from Delft , Holland , whose hobby was crushing lenses.  He had described three major morphological forms of bacteria.  The importance of all these forms was not recognized then. In the year 1678 , Robert Hook developed compound microscope and confirm Leewenhoek’s observation.

Needham and Irish priest in 1745 believed the spontaneous generation of microorganisms from the decomposing organic matter.  He had manifested his experiments that microbes grew in closed flasks of previously heated decomposable fluids. This view was contradicted by Spallanzani in 1769 who found no such generation of microorganisms after long heating of the putrescible fluids. He proved that organic contents of flasks supported the microbial growth

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when open to air. Later on , Pasteur proved in the end  that all forms of life , even microbes,  appeared only from their life and not de novo.


Scientific development of microbiology was escorted in by Louis Pasteur, perfection on microbiological studies by Robert Koch , the institution of antiseptic surgery by Lord Lister and the contributions of Paul Ehrlich in Chemotherapy. 

LOUIS PASTEUR (1822-1895) 

He was an instructed chemist of France. His studies on fermentation led him to take attention in microbiology.  He confirmed that fermentation was led by microbial agents. He further then noted that different types of fermentations were linked with different kinds of microbiology.  He is also known as the father of microbiology. 


These are stated below as follows.

  1. Enlargement of methods and procedure of bacteriology.
  2. Prove convincingly that all forms of life , even microbes arise only from their like and not de novo.
  3. Pasteur also had to go through a challenge from Pouchet , who was an advocate of spontaneous generation theory of microbes. Pasteur denied the view by explaining the ubiquity of microorganisms in the air by his experiments performed in the swan necked bottle.
  4. Establishment of sterilization methods and development of steam steriliser , autoclave and hot air oven.
  5. Studies on anthrax, chicken cholera and hydrophobia. At the time of learning on rabies, though Pasteur could not be isolated from any microorganism from man and dog , he mentioned that the causative agent of rabies was too small to be seen by microscope.
  6. Live vaccine.  He established attenuated live vaccines for prophylactic usage. An accidental examination of chicken cholera bacillus cultures left for a number of weeks lost their pathogenicity but kept their ability to protect the birds against the following infection by them which led to the concept of weakness and gestation of live vaccines. He attenuated the anthrax bacillus by incubation at high temperature and showed that inoculation of such bacilli in animals induced specific sheilding against anthrax. He convincingly explained the protective role of anthrax vaccine in a public experiment where vaccinated sheep, goats and cows were dared with a virulent anthrax bacillus culture. All the vaccinated animals remained alive while an equal number of unvaccinated control animals died. Pasteur stamped the term vaccine for such prophylactic preparations to commemorate the first of such devising,   namely cowpox, employed by Jenner for protection hostile to smallpox.


He was a Teacher of Surgery in Glasgow Royal Infirmary. He placed in Pasteur’s toil and handled antiseptic techniques in surgery affecting a fortunate decline in mortality and morbidity due to surgical sepsis. It was a landmark in the advancement of surgical practice from the time of laudable pus to present day aseptic techniques.  His antiseptic surgery incorporated the use of carbolic acid. He is known as the father of antiseptic surgery.


Robert Koch was a German general expounder. He is also known as the father of bacteriology. His contributions are as follows. 

  1. Improved bacteriological procedures and introduced several ways for isolation of pure strains of bacteria.
  2. Initiate methods of obtaining bacteria in pure cultures using solid media.
  3. Initiate staining procedures.
  4. Locate the anthrax bacillus , tubercle bacillus and the cholera vibrios.
  5. KOCH’S POSTULATES: According to Koch’s postulates,  a  microorganism can be received as the causative representative of an infectious disease only if the following conditions are satisfied.

i. The organism should be constantly related with the lesions of the disease. 

ii. It should be feasable to isolate the organism in pure culture from the lesions of the disease.

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iii. The isolated organism when inoculated in suitable laboratory animals should implement a similar disease.

iv. It should be attainable to re isolate the organism in pure culture from the lesions found in the experimental animals.

Koch’s Phenomenon

Robert Koch observed that guinea pigs were so far infected with tubercle bacillus answered with an exaggerated inflammatory response when injected with the tubercle bacillus or its protein. This hypersensitivity reaction is called Koch’s Phenomenon.


The presence of viruses became obvious during the closing years of the nineteenth century , when many contagious diseases had been shown to be caused by bacteria. But there remained a vast number of infectious diseases for which no bacterial cause could be made. During the studies on rabies, Pasteur thought that the causative agent could be too small to be seen under microscope.

The presence of such ultramicroscopic microbes was shown when Ivanovsky reproduced mosaic disease in tobacco plants by using the juice of diseased plants to the healthy leaves. The first human disease manifested to be of viral origin was the yellow fever. It was discovered by Walter Reed in 1992 in Cuba. Landsteiner and Popper in 1909 demonstrated that poliomyelitis was because of a filterable virus. Goodpasteur in 1930 made techniques of viral cultivation in chick embryos.

The use of tissue culture increased the scope of virological techniques considerably.  Though the larger viruses could be viewed under light microscope after proper staining, detailed morphological study of viruses became possible only with the launching of electron microscope by Ruska in 1934.

The chances of virus causing cancer was first put forth by Ellerman and Bang in 1908 in fowl leukemia.  Rous in 1911 Separated a virus causing fowl sarcoma. Many viruses have since been isolated which aim tumors in animals and birds.

The discovery of viral and cellular oncogenes was a breakthrough for possible mechanisms of viral oncogenesis. The positive proof of viral causation of human malignancy was put  when the human t cell leukemia virus was isolated in 1980.


Von- Behring and Kitasato in 1890 produced antibodies in guineapigs’s serum by injecting a sequence of sublethal amount of tetanus toxin. The antibody so produced could counteract the tetanus toxin notably. The tetanus and diphtheria antitoxins were the first ever known antibodies. Pfeiffer in 1893 introduced live Vibrio cholera into guinea pigs that had been previously inoculated with killed cultures of vibrios. The vibrios were shown to go lysis due to bactericidal effects. Bordet in 1895 again did the same  experiment and demonstrated that two components of serum took part in lytic reaction , the first being heat stable (antibody) and the second being heat labile ( complement). The single antibody sensitised the vibrios and the lytic reaction was due to complement.  Later on agglutinin, precipitin, and complement fixing antibodies were explained in serum.

Metchinoff in 1883 explained that phagocytic reaction was the prime defence mechanism against microbial attack of tissues and thus the cellular data of immunity was established. In 1967, Burnet proposed the concept of immunological surveillance according to which the primary function of the immune system is to protect the integrity of the body, seeking and killing all the foreign antigens. Malignancy was seen as a failure of immunological surveillance.  The scope of immunity was enlarged to include natural defense against cancer.

A number of scientists in medicine and physiology have been awarded Nobel Prizes for their contributions given to microbiology.


An antigen is a substance which,  when introduced into a body evokes an immune response to produce a specific antibody with which it reacts in an observable manner. 

Types of Antigen

They may be classified as

1. Complete antigen

2. Haptens/ Incomplete Antigen : Complex haptens ; Simple haptens

1. Complete Antigen

These are substances which can induce antibody formation by themselves and can react specifically with these antibodies.  

2. Haptens

Haptens are substances unable to induce antibody formation on their own but can become immunogenic when covalently linked to proteins , called carrier proteins. However, haptens can react specifically with induced antibodies.

Complex haptens can combine with specific antibodies to form precipitates e.g capsular polysaccharide of pneumococcus.

Simple haptens combine with specific antibodies but no precipitate is produced. 

Antigenic determinant (epitope) is the smallest unit of antigenicity. Antigen possesses a number of these determinants. Each type of determinant induces a specific antibody formation. 


The exact basis of antigenicity is not clear but a number of factors have been implicated which make s substance antigenic. These factors are as follows: 

1. Foreignness

An antigen must be foreign to the individual (non-self) to induce an immune response. The immune system does not normally mount an immune response against his own antigens (self). 

2. Size

Larger molecules are highly antigenic whereas low molecular weights are either non antigenic or weakly antigenic. 

3. Chemical Nature

Most naturally occurring antigens are either proteins or polysaccharides. Proteins are more effective antigens.

 4.Susceptibility to tissue enzymes

Substances which can be metabolised and are susceptible to the action of tissue enzymes behave as antigens. The hidden antigenic determinants can be identified only when a molecule is broken down. 

Substances which cannot be metabolised in the body are not antigenic. 

5. Species Specificity

Tissues of all individuals in a species possess species specific antigens. Some degree of cross reaction exists between antigens from related species.  

6. Iso Specificity

Iso Specificity depends on antigens which may be found in some but not all members of a species. A species may be grouped according to the presence of different antigens in its members.  These are genetically determined. Blood grouping is one example depending on human erythrocyte antigens (antigens) .

7. Autospecificity

Self antigens are generally non antigenic , but there are exceptions. These exceptions are lens protein and sperm. These are not recognized as self antigens. Lens protein normally confined within its capsule doesn’t come in contact with the immune system,  therefore,  it is not recognized. Similarly,  antigens that are absent during the embryonic life and develop later are also not recognized by the immune system. When these antigens are released into tissues following injury, antibodies are produced against them. 

8. Organ Specificity

Organ specific antigens are confined to a particular organ. Some organs like the brain , kidney and lens protein of one species share specificity with that of another species . As brain specific antigens are shared by man and sheep , anti rabies vaccines may cause neuroparalytic complications in man. The Anti Rabies vaccine contains a large amount of partially denatured brain tissue. In man , the sheep brain tissue induces immune response causing damage to their recipient’s nervous tissue. 

9. Heterophile Specificity 

The same or closely related antigens present in different tissues of more than one species are known as heterophile antigens. Antibodies to these antigens produced by one species cross react with antigens of other species. Examples include : 

i. Forss man Antigen

It is a lipoprotein-polysaccharide complex widely present in man , animals , birds and bacteria. 

ii. Paul – Bunnell Test

In infectious mononucleosis,  heterophile antibodies appear in the serum of the patient.  These antibodies agglutinate sheep erythrocytes because Epstein Barr virus shares antigen with sheep erythrocytes. 

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